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Along each row the polarity preserves the same direction, but the polarity of each row is opposite to that of each contiguous parallel row. This description applies equally to all three axes. The whole group (Fig. 3) consists of the quartettes of Fig. 2 piled alongside of and also on top of one another. In this way we arrive at what I take to be the simplest possible type of cubic crystal.

In this grouping each molecule has the alignment giving maximum stability, and it seems fair to assume that it will take that alignment when the crystal grain is formed under conditions of complete freedom, as in solidifying from the liquid state. As a rule, the actual process of crystalbuilding goes on dendritically; branches shoot out, and from them other branches proceed at right angles, leaving interstices to be filled in later. We have, therefore, to

conceive of the molecules as piling themselves preferably in rows rather than in blocks, though ultimately the block form is arrived at. In this position of maximum stability each molecule has its six poles touching poles of contrary

name.

Now comes a point of particular importance. Imagine two neighbouring molecules in the same block to be turned round, each through one right angle, in opposite senses. They will now each have five poles touching five poles of contrary name, but the sixth pole will touch a pole of the same name as itself. They are still stably situated, but much less stably than in the original configuration, and they will revert to that configuration if set swinging through an angle sufficient to exceed the limited range within which they are stable in the new position.

Similarly we may imagine a group of three, four, or more molecules, each to be turned through a right angle, thereby constituting a small group with more or less stability, but always with less than would be found if the normal configuration had been preserved. The little group in question may be made up of molecules in a row, or it

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FIG. 3.

may be a quartette or block, or take such a form as a Tor L. A sufficient disturbance tends to resolve it into agreement with the normal tactics of the molecules which build up the rest of the grain.

It is conjecturally possible that small groups of this

kind, possessing little stability, may be formed during the process of crystallisation, so that here and there in the grain we may have a tiny patch of dissenters keeping one another in countenance, but out of complete harmony with their environment.

If this happens at all during crystallisation, it would seem less likely to happen in free crystallisation from a liquid state than in the more constrained process that occurs when a metal already in the solid state recrystallises at a temperature far below its melting-point. Though rare or absent in the first case, it might occur frequently in the second. There are differences in the appearance of crysta grains under the microscope in metal as cast and in metal as recrystallised in the solid state, of which this may be the explanation. It may also explain a difference pointed out by Rosenhain,' that the slip lines in cast metal are straight and regular, whereas in wrought iron and other metals which have recrystallised in the solid they rarely take a straight course across the crystal, but proceed in jagged, irregular steps. These may be due to the presence here and there of small planes of weakness, resulting from the existence of what I have called dissenting groups. Again, these groups, possessing, as they do, less stability than their normal neighbours, may be conjectured to differ from the normal parts of the grain in respect of electrolytic quality, and to be more readily attached by an etching reagent. Hence, perhaps, the conspicuous isolated geometrical pits that appear on etching a polished surface of wrought iron.

It will help in making clear these points, and others that are to follow, if we study the action of a model formed by grouping a number of polarised "molecules" in one plane, supporting them on fixed centres, about which they are free to turn. In the model before you the centres are

uniformly spaced in rectangular rows, and the "molecules" are + shaped pieces of hardened steel, strongly therefore, two north poles and two south poles. The third magnetised along each of the crossed axes, each having, axis is omitted in the model, the movement to be studied

with the help of the model being movement in one plane. On placing these "molecules " on their centres they readily take up the position already indicated in Fig. 3. Each one within the group has its four poles in close proximity to four poles of contrary name, and is, therefore, highly stable. If disturbed by being turned through a small angle, and let go, it swings back, transmitting a wave of vibration through the group, which is reflected from the edges, and is finally damped out in the model by pivot friction and air friction. We may assume some damping action (say by the induction of eddy-currents) in the actual solid, of which the model may be taken as a very crude repre

sentation.

By turning two molecules carefully round together, each through one right angle in opposite senses, we set up a dissenting pair, the equilibrium of which has feeble stability. A slight displacement, such as might be produced by the transmission of a vibrational wave, breaks them up, and they swing back to the normal configuration, giving out energy, which is taken up by the rest and is ultimately dissipated. By making the dissenting coterie consist of three or more we can give it additional strength.

An example is shown in Fig. 4, where the three molecules marked a, b, and c are turned round in this way.

Notice that the normal molecule d, adjoining a line of such dissenters, is in a peculiar position. His neighbours present to him three N. poles and one S. pole. He has the choice of conforming to the majority, or of throwing in his lot with the dissenters; and he has a third possible position of equilibrium (very feeble equilibrium) which reached when his two S. poles are turned until the one neighbouring south pole faces just between them. I have laboured these points a little because they seem important when we come to speak of the effects of strain.

Consider now the straining action, which we may imitate in the model by sliding one part of the group past the other part. For this purpose the centres are cemented to two glass plates which can slide parallel to one of the

axes.

1 Rosenhain, "The Plastic Yielding of Iron and Steel," Tour, Trim ant Steel Institute, No. 1 for 1904, p. 335.

At first, when the displacement by sliding is exceedingly small, the strain is a purely elastic one. The molecules adjacent to the plane of sliding pull one another round a little, but without breaking bonds, and if in this stage the strain is removed, by letting the plate slide back to its original position, there is no dissipation of energy. The work done in displacing the molecules is recovered in the return movement. We have here a representation of what happens between each pair of adjoining rows in the elastic straining of a metal. So far the action is within the limit of elasticity; it leaves no permanent effect: it is completely

reversible.

But now let the process of straining be carried further. The opposing molecules try to preserve their rows intact, but a stage is reached when their resistance is overcome; the bonds are broken, and they swing back, unable to exert further opposition to the slip. The limit of elasticity has now been passed. Energy is dissipated; set has been produced; the action is now no longer reversible. The model shows well the general disturbance that is set up in molecules adjoining the plane of slip, which we may take to account for the work that is expended in a metal in producing plastic strain.

Moreover, when the slip on any plane stops and the molecules settle down again, the chances are much against their all taking up the normal orientation which they had before the disturbance. What I have called dissenting groups or unstable coteries are formed as a result of the disturbance. Here and there like poles are found in juxtaposition. Viewed as a whole, the molecular constitution of the metal in the region adjacent to the plane of slip

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FIG. 4

is now uncertain and patchy. It includes parts the stability of which is much less than normal. Individual molecules or small groups in it are very feebly stable; a touch would make them tumble into positions of greater stability.

Observe how all this agrees with what we know about the nature of plastic strain through experiments on iron or other metals. Its beginning is characteristically jerky. Once the critical force is reached, which is enough to start it, there is a big yield, which will not be stopped even by reducing the amount of the straining force.

Again, we know that there is a slow creeping action that continues after the straining force has done its main work. I ascribe this to the gradual breaking up of the more unstable groups which have been formed during the subsidence of disturbance in the earlier stage of the slip.

Further, we know that overstrained iron is very imperfectly elastic until it has had a long rest, or until it has been raised for a short time to a temperature such as that of boiling water. This is to be expected when we recognise the presence of unstable individuals or groups resulting from the overstrain. When the elasticity of the overstrained piece is tested by removing and reapplying the load, some of these tumble into new positions, making inversible movements, which dissipate energy and produce hysteresis in the relation of the strain to the stress although the strain is quasi-elastic. At the ordinary temperature these unstable groups are gradually becoming resolved, no doubt 1 J. Muir. On the Recovery of Iron from Overstrain," Phil. Trans., vol. cxciii. A, 1900.

through the action of the molecular movements that are associated with heat, and hence the slow progressive recovery of perfect, or nearly perfect, elasticity shown by the experiments of Muir. Let the temperature be raised and they disappear much more quickly; in warm surroundings the rest-cure for elastic fatigue does not need to be nearly so long.

Rosenhain has recently shown that after the slip-bands on the surface of an overstrained specimen have been obliterated by polishing, traces of them will reappear on etching if only a short interval of time is allowed to lapse since the overstraining; but if time is given for complete recovery no traces are found. This is in remarkable agreement with the view now put forward, that the layers contiguous to the surface of slip contain for a time comparatively unstable groups. They are consequently different from the normal metal until the unstable groups are resolved, and the temporary difference manifests itself on etching, provided that is done while the difference still

exists.

From the engineer's point of view a much more important matter than this fatigue of elasticity is the fatigue of strength that causes fracture when a straining action is very frequently repeated. Experiments which I made with Mr. Humfrey showed that this action begins with nothing more or less than slight slip on surfaces where the strain is locally sufficient to exceed the limit of elasticity. An alternating stress, which makes the surfaces slip backwards and forwards many thousands, or it may be millions of times alternately, produces an effect which is seen on the polished surface as a development of the slip lines into actual cracks, and this soon leads to rupture.

We have, therefore, to look for an effect equivalent to an interruption of continuity across part or the whole of a surface of slip, an effect progressive in its character, becoming important after a few rubbings to and fro if the movement is violent, but only after very many rubbings if the movement is slight.

That there is a progressive action which spreads more or less into the substance of the grain on each side of the original surface of slip was clearly seen in the experiments referred to. It was found that a slip-band visible on the polished surface of the piece broadened out from a sharply defined line into a comparatively wide band with hazy edges, and this was traced to an actual heaping up of material on each side of the step which constituted the original line.

I think this suggests that under alternating stresses which cause repeated backward and forward slips, these do not occur strictly on the same surface in the successive repetitions, and hence the disturbance spreads to some extent laterally. It may be conjectured that slip on any surface leaves a more or less defective alignment of the molecular centres; that is to say, the rows on one side of the plane of slip cease to lie strictly in line with those on the other side. If this occurs over neighbouring surfaces, as a result of slips or a number of parallel planes region loses its strictly crystalline character, and with it very close together, the metal throughout the affected loses the cohesion which is due to strict alignment.

Mr. G. T. Beilby, in a very suggestive paper, has advanced grounds for believing that portions of a metal may pass from a crystalline to an amorphous formation under the mechanical influence of severe strain, as in the hammering of gold leaf or the drawing of wire, and that . this occurs in the polishing of a metallic surface, and also in the internal rubbing which takes place at a surface of slip within the grain. In both cases he suggests the formation of an altered laver. When a polished metal surface is etched, the altered layer is dissolved away, and the normal structure below it is revealed.

Without accepting all Mr. Beilby's conclusions, I think the idea of an altered and more or less amorphous laver is supported by the considerations I am now putting forward. We have assumed that in normal crystallisation the intermolecular forces lead to a normal piling, in which 1 Journ. Iron and Steel Institute, 190b.

2 Ewing and Humfrey. "The Fracture of Metals under Repeated Alternations of Stress," Phil. Trans., vol. cc. A, 1902.

3 Beilby, "The Hard and Soft States in Metals," Phil. Mag., August,

1904.

each molecule touches six neighbours. But it may be conjectured that some of them may take up pyramidal piling (touching twelve others) under the compulsion of strong forces-such forces, for example, as act on the superficial molecules of a surface that is being polished.

If this also occurs at a surface of slip, it gives us a clue to several known facts. It at least assists in explaining the familiar result that metal is hardened by straining in the sense of being made less plastic. Again, it accounts for the general increase of density which is found to take place in such an operation as wire drawing. Further, if a local increase of density occurs in the interior of a grain through piling of some molecules in the closer manner where repeated slips are going on, the concentration of material at one place requires it to be taken from another; in other words, the closer piling tends to produce a gap or crack in the neighbourhood where it occurs. This is consistent with what we know of the development of cracks through repeated alternations of strain.

Recourse to the model shows that with pyramidal piling the polar axes point in so random a manner that the aggregate may fairly be called amorphous. To illustrate this a group is shown with centres fixed at the corners of equilateral triangles.

It is obvious that any pyramidal piling at a surface of slip tends to bar further slip at that particular surface. Hence not only the augmented hardness due to strain, but the tendency in repeated alternations to lateral spreading of the region on which slip occurs. The hardness due to straining is, of course, removed when we raise the metal to such a temperature that complete recrystallisation occurs, normal piling being then restored in the new grains.

Taking a previously unstrained piece, it is clear that the facility with which slip will occur at any particular surface of slip in any particular grain depends not only on the nature of the metal and on the orientation of the surface in question to the direction of the stress, but also on the amount of support the grain receives from its neighbours in resisting slip there. In other words, for a given orientation of surface the resistance to slip may be said to consist of two parts; one is inherent in the surface itself, and the other is derived from the position of the grain with reference to other grains.

To make this point clear, think of a grain (under stress) in which there is a gliding surface oriented in the most favourable direction for slipping. Slip on this surface can take place only when its yielding compels the neighbours (which are also under stress) to yield with it, and the surfaces in these on which slip is compelled to occur are, on the whole, less favourably situated. Hence the original grain cannot yield until the stress is considerably in excess of that which would suffice to make it yield if it stood alone, or had neighbours equally favourably inclined.

Slip

Apply this consideration to the case of steel, where there are two classes of grains: the ferrite, which is simply iron, and the pearlite, which is a harder structure. on any ferrite grain is resisted partly by the strength of the surface itself, and partly by the impossibility of its yielding without forcing slip to take place on neighbouring (harder) grains. Now suppose the structure is a very gross one, such as Mr. Stead has shown may be found in steel that is seriously overheated. On the large grains of ferrite in overheated steel the resistance to slip will be but little greater than it would be in iron, and, con⚫sequently, under an alternating stress fatigue of strength, leading to rupture, may be produced by a very moderate amount of load. Mr. Stead1 has shown how the effects of overheating can be removed by the simple expedient of raising the steel to a temperature sufficient to cause recrystallisation--a homoeopathic remedy that transforms the gross structure of the overheated metal into an ordinarily fine structure, where no ferrite grain can yield without compelling the yielding of many pearlite grains. Hence we find, as Rogers has demonstrated by experiment, that steel cured by reheating from the grossness of structure previously produced by overheating, has an immensely in1 See especially a paper by J. E. Stead and A. W. Richards on "The Restoration of Dangerously Crystalline Steel by Heat Treatment," Journ. of the Iron and Steel Inst., No. 2, 1903.

2 F. Rogers, "Heat Treatment and Fatigue of Steel," Journ. Iron, and Steel Inst., No. 1, 1905.

creased power to resist the deteriorating effects of often repeated stress.

I trust you will not feel I have abused the license of the Chair in presenting contributions to molecular theory that are for the most part in the nature of speculative sugges tions, thrown out in the hope that they may some time lead to fuller and more definite knowledge. Remote as they may seem to be from the concerns of the workaday engineer, they relate to the matter which it is his business to handle, and to the rationale of properties, without which that matter would be useless to serve him. We have attempted to penetrate into its very heart and substance in order the better to comprehend the qualities and functions on which the practical work of engineering relies. The man whose daily business leads him through familiar tracks in a forest does well to stray from time to time into the shady depths that lie on either hand. The eyes of his imagination will be opened. He will at least learn his own limitations, and, if he is fortunate, he may gain some clearing on a hilltop which commands a wider view than he has ever had before.

SECTION I.

PHYSIOLOGY.

OPENING ADDRESS BY PROF. FRANCIS GOTCH, M.A., D.Sc. F.R.S., WAYNFLETE PROFESSOR OF PHYSIOLOGY IN THE UNIVERSITY OF OXFORD, PRESIDENT OF THE SECTION. "THE investigators who are now working with such earnestness in all parts of the world for the advance of physiology have before them a definite and well-understood purpose, that purpose being to acquire an exact knowledg of the chemical and physical processes of animal life and of the self-acting machinery by which they are regulated for the general good of the organism."

In this admirable and concise manner the late Sir John Burdon-Sanderson described the aims and methods of physiology. The words were spoken in 1881, when the British Association last met in this historic city. At that time the subjects of Anatomy and Physiology formed a subsection of the Section of Biology, and it was presided over by this distinguished man, whose recent death has deprived not only physiology but natural science of one of its most honoured leaders. His continuous work, extending over a period of fifty years, was remarkable from many points of view, but in none more than the extent of its scope. Sanitary science, hygiene, practical medicine, botany, pathology, and physiology have all been illuminated and extended by his researches. His claim for being included among the great names in English science does not rest merely upon his acknowledged eminence as an original and exact investigator, but also upon the influence which, for four decades, he exerted upon other workers in medical science, endowing their investigations with purpose and materially helping to give English physiology and pathology their proper scientific status, Many circumstances contributed to make this influence widely felt: among these were the peculiar charm of his manner, his striking and commanding personality, the genuine enthu siasm with which he followed the work of others, the devotion with which he advocated the use of experimental methods, his scientific achievements, and his extensive knowledge. All these qualities of mind and character marked him as one of those great masters who inspire the work and mould the thought of a generation. It is in tribute to his memory that, as one of his pupils and his successor in the Oxford Chair of Physiology, I utilis this occasion for recalling such fruitful features of bis scientific conceptions as are expressed in the felicitous phrase which I have quoted.

Probably the most important of the many services which Burdon-Sanderson rendered to English medical science was that of helping to direct physiological and pathological inquiry towards its proper goal. It will be admitted by all who knew him intimately that among his most characteristic scientific qualifications were the insight with which he realised the essence of a physiological problem.

1 Address to the Subsection of Anatomy and Physiology, by J. Burdon Sanderson, British Association Report, York, 1881.

and the tenacity with which he kept this essential aspect in view. The faculty which enables the mind to review the varied aspects of complex phenomena and to determine which of these are mere incidents, or external trappings, and which constitute the core of the subject, is one which every scientific worker must possess in a higher or lower degree; it may, indeed, be confidently asserted that scientific training is successful only in so far as it develops a nice and just discrimination of this character. Many attain this capacity after several years of labour and effort; but in the case of rare and gifted individuals its possession comes so early as to seem almost an intuitive endowment. In 1849, during his student days at Edinburgh, Burdon-Sanderson showed by the character of his earliest scientific work that he viewed the proper aim of physiological inquiry as essentially the study of processes. At the present time it may appear superfluous to dwell upon the importance of this standpoint, but fifty-seven years ago this aspect of the subject was rarely, in this country, a stimulating influence in physiological work, whilst, as regards pathology, the point of view taken by Burdon-Sanderson was, even in 1860, probably unique.

The obvious fact that living processes occur in connection with certain definite structural forms transferred attention from the end to one of the means, and thus education and research in physiology and pathology were almost entirely confined to the elucidation of that structural framework in which the essential processes were now displayed and now concealed. Improved methods of microscopic technique revealed the complexity of this structure, and minute anatomy absorbed the interest of the lew physiologists and pathologists who prosecuted researches in this country. Even when attention was directed to the living processes, it was with an unconscious anatomical bias, and detailed descriptions of structural framework were advanced as affording a sufficient scientific explanation of the character of the subtle processes which played within the structure. Yet upon the Continent the great physiologists of that time had long realised that physiological study must ascertain the characters of these processes, and that research conducted along experimental lines could alone advance scientific physiology as distinct from scientific anatomy. In 1852 Burdon-Sanderson went from Edinburgh to Paris to study the methods used in physics and chemistry. Whilst there he came under the inspiring influence of one of these great Continental physiologists, Claude Bernard, and his views as to the proper end of physiological inquiry received from this master ample confirmation. The sentence which I have quoted from the York address sets forth with scientific precision his enlarged conception of living phenomena, for whilst it asserts that the characteristics of processes form the true aim of all physiological investigation, it defines the particular processes which should be investigated as chemical and physical, and it particularises two further aspects of these, the machinery for their coordination described as self-acting, that is automatic, and the raison d'être of their occurrence, which is said to be the welfare of the whole organism. All these various aspects are strikingly exemplified in the progress of physiology in this country and in the researches now being carried on both at home and abroad; their consideration may thus be not inappropriate in a general address such as it is my privilege to deliver to-day.

At the outset it is desirable to refer to certain wide issues which are involved in the statement that the business of the physiologist is "to acquire an exact knowledge of the chemical and physical processes of animal life. The limitation of physiology to ascertainable characters of a chemical and physical type does not commend itself to certain physiologists, physicists and chemists, who have revived under the term "neo-vitalism" the vitalistic conceptions of older writers. They deny that physiological phenomena can ever be adequately described in terms of physics and chemistry, even if these terms are in the future greativ enlarged in consequence of scientific progress. is undoubted that there are many aspects of living phenomena which in the existing state of our knowledge defy exact expression in accordance with chemical and physical conceptions; but the issues raised have a deeper significance than the mere assertion of present ignorance, for those

It

who adopt neo-vitalism are prepared to state not only that certain physiological phenomena are, from the chemical and physical point of view, inexplicable to-day, but that from the nature of things they must for ever remain so. This attitude implies that it is a hopeless business for the physiologist to try by the use of more appropriate methods to remove existing discrepancies between living and nonliving phenomena, and this is accentuated by the use of peculiar nomenclature which, in attributing certain phenomena to vital directive forces, leaves them cloaked with a barren and, from the investigator's point of view, a forbidding qualification.

a

It is of course possible in describing phenomena to employ a new and special terminology, but since many aspects of the phenomena of living processes can be described in accordance with physical and chemical conceptions, the creation of a vitalistic nomenclature duplicates Our terminology. A double terminology is always embarrassing, but it becomes obstructive when it is of such diversity that description in the one can never in any circumstances bear any scientific relation to that in the other. In this connection it is somewhat significant that the one kind, namely vitalistic, is abandoned as soon as the observed phenomena to which it referred have been found to be capable of expression in terms of the other. The reason for this abandonment raises questions of principle, which appear to me to render it impossible for a scientific physiologist seriously to employ vitalistic nomenclature in describing physiological phenomena. Science is not the mere catalogue of a number of observed phenomena; such a miscellaneous encyclopædia may constitute what many people would describe as knowledge; but science is more than this. It is the intellectual arrangement of recognised phenomena in a certain orderly array, and the recognition of any phenomenon is only the first step towards the achievement of this end. The potent element in science is an intellectual one essentially connected with mental grouping along one particular line, that which tends to satisfy our craving for causative explanation. Hence it involves the intellectual recognition of widespread characteristics, so general in their distribution that they are termed fundamental. The most fundamental of such characteristics are those which possess the widest intellectual sphere, and in natural science these are the broad conceptions of matter and motion which form the essential basis of both chemistry and physics. If this grouping is, in regard to any phenomenon, at present impracticable, then this subject-matter cannot justly regarded as forming a part of natural science, though it might be considered as natural knowledge, and in so far as this is the case in physiology it appears to me to be a confession of present scientific ignorance. If, however, it is boldly asserted that the nature of any phenomenon is such that it can never by any possibility be brought into accord with the broad conceptions which I have indicated, then I fail to understand how it can claim to bear any relation to natural science, since, ex hypothesi, it can never take its proper place in the causative chain which man forges as a limited but intelligible explanation of the world in which he lives. Only in so far as physiological phenomena are capable of this particular intellectual treatment and take part in this intellectual construction can we hope to obtain, however dimly, a knowledge of permanent backgrounds among the shifting scenes of the living stage, and thus, by gradually introducing order amidst seeming confusion. claim that gift of prevision which has long been enjoyed by other branches of natural science.

be

Neo-vitalism, like its parent vitalism, is fostered by the imperfect and prejudiced view which man is prone to take in regard to his own material existence. This existence is, for him, the most momentous of all problems, and it is therefore not surprising that he should assume that in physiology, pathology, and, to a lesser degree, in biology, events are dealt with of a peculiarly mystic character, since many of these events form the basis of his sensory experience and occur in a material which he regards with a special proprietary interest. He is reluctant to believe that those phenomena which constitute the material part of his existence can be intellectually regarded as processes of a physicochemical type, differing

a

This

on

causative explanation, would be immediately abandoned if the phenomena were subsequently found to be explicable physical and chemical conceptions. Biotic energy appears to me as only an intellectual compromise, an abortive attempt to clothe the naked form of vitalism in a decent scientific dress; but, although partially clothed, it offers, like neo-vitalism, no new method for physiological investigation, and must, in consequence, remain barren, never contributing towards physiological achievement. To what extent its adoption may be an intellectual solace is a question which does not fall within the scope of physiology. Certain physiological phenomena are especially brought forward as necessitating the assumption of vitalistic or biotic conceptions; among these are the phenomena of nervous activities, the formation and activities of enzymes, and the passage of substances through will be dealt with later; but as regards the diffusion of gases or substances in solution through cellular membranes a few general considerations may be advanced now.' The passage of substances into and through non-living mem branes is modified in regard to both the velocity and the selective character of the passage by a large number of factors, among which are nature of substance, pressure, osmotic index, temperature, and the structural, electrolytic, and chemical characters of the membrane. Tissue membranes, whether animal or vegetable, possess a complicated particulate structure, and it is obvious that experi ments must be carried out extensively on dead tissue membranes in order to determine how far the general particulate arrangement may modify the rate and character of the passage. In this respect our present information is not sufficiently extensive to warrant any definite general statement, and such experimental evidence as exists opens up difficult problems in molecular physics which still await solution; moreover, the presence of electrolytes, by assist ing adsorption, appears to modify the apparent rate and character of the total passage, and further experiments are necessary on this point. But in the living membrane, especially when it is composed of cellular units, the whole question is additionally complicated by the great probability that the cells are the seat of chemical processes the nature of which is imperfectly known; such processes constitute the metabolism of the cells. It would, therefore, be somewhat surprising if the phenomena of the passage of substances through such cellular membranes were in strict accord with the passage of similar substances through non-living membranes which have not the same particulate framework and are not the possible seat of similar chemical processes. The statement, therefore, that any discrepancy between the two classes of phenomena necessitates the assumption of a peculiar vital directive force disregards the circumstance that between the conditions in the one case and those in the other lies a large and little explored field; moreover, such a statement implies, without any warrant, that any physico-chemical explanation must necessarily be insufficient in the case of the living membrane, although it is realised that there may be active chemical processes of the operations of which we have at present little exact knowledge.

only in complexity from those exhibited in the non-living world, and impelled by this reluctance he fabricates for them, out of his own conceit, a special and exclusive realm. The logical pressure of physical and chemical conceptions forbids the postulation, by either the public or the neo-vitalist, of such an incongruous entity as vital chemical element capable of blending with the familiar chemical elements recognised in the material world; yet the physiological processes of life are in popular estimation still held to be due to peculiar forces blending with those of the material world, but so essentially different that they can only be described as "vital." The neovitalistic school of men of science, without adopting this popular view in its entirety, retains the same term for such physiological characteristics of cell processes as, with our present limited knowledge and with our present inadequate methods of investigation, seem to be in disagree-living membranes. The question of the nervous activities ment with present chemical and physical conceptions. This disagreement is accentuated by the assumption of directive vital forces, and since these cannot be ranged alongside those of chemistry and physics, transcendental phenomena may be always expected to occur the orderly array of which as part of natural science is not merely a futile but on a priori grounds an absolutely impossible task. In order to justify this description as representing the views of some neo-vitalists, I will quote a few sentences from the presidential address delivered in 1898 by Prof. Japp in the Chemical Section of this Association. address dealt with the formation of the optically active substances found in vegetable and animal tissues or their extracts. It asserts that "the absolute origin of compounds of one-sided symmetry to be found in the living world is a mystery as profound as the origin of life itself." In regard to this it may be remarked that the absolute origin of anything, living or non-living, is a mystery which science does not attempt to solve, relative not absolute causation being the object of scientific grouping, hence this assertion does not necessarily imply any fundamental distinction between the two classes of phenomena. But there is more than appears upon the surface, for the whole argument leads up to the sweeping statement that "no fortuitous concourse of atoms, even with all eternity for them to clash and combine in, could compass this feat of the formation of the first optically active organic compound." It is thus inferred that because the manner of such formation cannot be accounted for in the present condition of scientific knowledge, its scientific causation is from the nature of things unknowable. However, although unknowable in the strictly scientific sense, the intellectual craving for causative explanation of some sort urges Prof. Japp to say, "I see no escape from the conclusion that at the moment when life arose a directive force came into play. There is here introduced a grandiose term for life which is viewed as involving directive forces; the term, however, adds nothing to our physiological knowledge, is not in itself explanatory, and not only offers no new method of physiological investigation, but brands as useless all the methods derived from physics and chemistry, past, present, and future. In a recent work Prof. Moore has attempted to set forth a conception which shall be vitalistic in essence, and yet not so completely out of touch with the principles of natural science.1 He regards living cells as transformers of energy and thus leaves them absolutely dependent upon its receipt; the transformed mode which is achieved by the cells is, however, one which cannot be interpreted in terms of the familiar modes presented in the non-living world. He terms the transformed mode "biotic energy, and the distinction between this and "vital directive force" appears to be its absolute dependence upon the other modes for its appearance. It thus does not run counter to the law of the conservation of energy, and warrants, in the opinion of some, the confident expectation that it will be found capable of precise scientific expression. I confess that I am unable to share this confidence. The introduction of the conception entails the same double terminology to which I have referred, and I feel convinced that the assumption, in the case of any given physiological phenomena, of biotic energy as a

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1 See article by B. Moore in "Recent Advances in Physiology and Biochemistry." Edited by L. Hill, F.R.S. (London: Arnold, 1906.)

What possible justification is there, therefore, for branding as hopeless all further physical and chemical investigation of certain aspects of the phenomena by attributing these to vital directive forces? The gaps and imperfections of the paleontological record were triumphantly vaunted by the opponents of evolution; and now that the work of successive years has convincingly contributed towards the filling up of these gaps not only has this objection collapsed, but the hypothesis of special creations which it supported has been involved in its fall. There are indica tions that the discrepancies in diffusion phenomena through widely different structures may be knit by the results of experiment on intermediate modifications. It may be many

1 The conception of Ostwald as to the action of catalytic substances s extremely suggestive in connection with the activities of enzymes, both intracellular and extracellular. It is possible that the changes brought about by enzymes may, with the growth of our knowledge in physical chemistry be shown to b of the same order as those which slowly occur in the absence of enzymes, and that the enzyme itself by facilitating adsorption pheromena may merely act by accelerating the velocity of the special change. See Leathes, "Problems in Animal Metabolism" (London: Murray, 190%).

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