distribution of the halogens between the silver and the metal of the haloid salt1. 182. From what has been said in the preceding paragraphs (178-181), we see that a changing chemical system may pass through a series of stages some of which are more stable than others. It may indeed be that certain points in the series are so unstable that they are not marked by the production of what we are accustomed to call definite chemical compounds. This view, of well marked chemical compounds being the most stable points in a series of potentially existent substances, has been developed by Mills, starting from the observations of Wurtz on the polyethylenic glycols, which are compounds obtained by the condensation of n molecules of ethylene glycol with the elimination of n - I molecules of water. Mills uses the expression cumulative resolution to mean 'the combination of a substance or mixture 'of substances with itself n times, a particular portion of it 'being lost each time, according to some fixed law.' The general equation representing a process of cumulative resolution is given by Mills in this form, nAaBbCy... - (n - m) AaBbCc ...=Ana - a)+ma B(B-8)+mỏ Cay - c)+mc ; where A.BBC, is the substance which undergoes the change, and A,B,C, is that portion of it which is eliminated at each stage of the process. By giving values to n varying from 1 For a fuller discussion of the influence of mass on chemical changes see post, chap. III. section 1. 2 It should be noted that the expressions 'stability,' 'stable compound,' and the like, are somewhat vague. The conditions under which stability is predicated of a given substance must be stated or implied if the word is to convey any very definite meaning. Thus zinc methide can be gasified without decomposition, but when this compound is brought into contact with water it is violently decomposed, forming zinc hydrate and methane; so also the compound K2O, is decomposed at a red heat, yielding K2O+O3, but it is rapidly acted on by water at ordinary temperatures, forming KOH, O, and H2O2. The fact that some double salts are decomposed in aqueous solutions by the process of diffusion, seems to illustrate the position that certain molecules, or molecular groups, which shew a considerable range of what may perhaps be called chemical stability, are easily broken up when small alterations are made in the physical conditions of their surroundings. 3 See Phil. Mag. (5) 3. 492; or the article 'Cumulative resolution,' in the third supplement of Watts's Dictionary. o to ∞ various formulæ are obtained for the cumulates, or possible products of the change. The theory may be applied to the action of water on bismuthic nitrate, whereby a series of compounds is obtained, each less nitrogenous and more bismuthic than the preceding. Thus, n (BigOg. 3N2O) -(n - 1) N2O2-BignO3n N4n+2010+5; 3 5 by giving various values to n from o to ∞ we obtain the formulæ of all possible substances between Bi,O,. 3N,O, and Bi,O,. 2NO. By repeating this process on Bi̟O. 2NO̟, a series of possible substances is obtained of which the limits are marked by Bi̟,O,. 2N,O, and Bi̟,O,. NO; and lastly by a repetition of the process of cumulative resolution on the last compound, a third series is obtained ranging from Bi̟,O,. NO, to Bi2O,1. 3 183. This theory points to the frequent existence of series of substances forming connecting links between those comparatively stable compounds which can be separated from the materials which have produced them, or from those which are the products of their decomposition. But although we may not be able to separate and obtain in definite form the comparatively unstable members of such series, yet it may be possible to demonstrate the existence of these substances by indirect methods. The substances in question exist only as members of a system; apart from the other members, or from some of the other members, they undergo decomposition. The group of carbon compounds called by Armstrong and Groves Aldehydrols presents us with examples of the phenomenon now under consideration. Aldehydrols are almost certainly produced in the first stage of the oxidation of the primary ethylic alcohols. These alcohols are oxidised only in presence of water; for this and other reasons it is very probable that the process of oxidation is represented by the equations 1 For other applications of the theory see the article in Watts's Dict. (1) C2H2+CH2OH+HOH+0=C„H2+1CH(OH)2+HOH, where CH2+1 CH(OH), is the formula of an aldehydrol'. n Many reactions of the ethylic aldehydes are explained by assuming the existence of aldehydrols; the properties of chloral hydrate point almost with certainty to the formula CCI,. CH(OH), for this compound3. The aldehydrols cannot however exist except as members of a system of which water is a constituent; the system is chemically stable, some of the individual members when separated from the others are very unstable. Another illustration of the existence of a compound only in the presence of others is furnished by Traube's preparation of cupric iodide (CuI) in aqueous solution. An aqueous solution of this compound, the supposed non-existence of which has been often noticed as peculiar, can be prepared according to Traube by mixing dilute solutions of cupric sulphate and potassium iodide, or by acting on cuprous iodide (Cu) with iodine in the presence of a large quantity of warm water1. 184. Looking back on the conception of molecular structure which was reached in book I. (par. 64 et seq.), and applying to it the further knowledge we now have, we should be inclined to say that the function performed by a given atom, or group of atoms, in this molecule or in that cannot be known except by the study of many systems wherein the given individual occurs. Before we have a knowledge of the chemical properties of hydrogen, for instance, we must study the behaviour of this element, under varying conditions, in its compounds with metals, with nonmetals, with negative and with positive groups of atoms, &c. It might indeed be asserted that it is not correct to say that the molecule of 1 See Armstrong and Groves, loc. cit. 1. 417–418. 2 Ibid. loc. cit. 717-718. See also p. 681 (formation of acetal from ethaldehyde). 3 Compare the properties and formation of chloral hydrate (ibid. pp. 743-744) with the properties of chloral alcoholate (ibid. p. 429). • Ber. 17. 1064. M. C. 25 water contains hydrogen or oxygen, just as it is not correct to say that the molecule of sulphuric acid contains the atomic groups SO, or H,O. Mills' goes so far as to affirm that water is not represented by the formula HO, inasmuch as it is a homogeneous substance with its own properties; to this we might, I think, reply that one of the distinctive properties of water is implied in the formula H,O, the property namely of being decomposable into H,+O, and of being formed by the combination of H, and O. Every chemical substance ought to be regarded in its relations to other substances; but each is also a distinct individual. A full chemical knowledge of any substance implies a knowledge of all the possible reactions which would occur in any system of which that substance may form a member. The whole history of dualism warns us against asserting that the properties of any chemical substance are independent of those other substances with which it is or may be associated; but at the same time, all modern research confirms the fundamental conception of the element as a distinct form of matter which impresses its own likeness on all the compounds of which it forms a constituent. SECTION II. Chemical Equilibrium. 185. Thus we come back to the conception of every chemically stable system as being in a condition of equilibrium, which is the result of the actions of various forces, some of which are what we usually call chemical, and others physical; if one of these forces is increased the equilibrium is overthrown and the system undergoes chemical change. The methods used in attempts to solve the general problem of chemical equilibrium may be divided into two classes, (1) those which are based on applications of the molecular theory, and more especially on the kinetic theory of gases; (2) those which are essentially thermodynamical. 1 Phil. Mag. (5) 1. 1. 186. Several years ago Williamson' put forward a somewhat vague view, to the effect that the amount of chemical action between two substances may be measured by the relative velocities of the atomic interchanges taking place between the molecules of these substances. Arguing from the reactions of substitution among carbon compounds, especially the substitution of H in H,SO by CH2n+1 groups and vice versa, Williamson concluded, that if chemically similar atoms continually change places in reacting molecules, much more likely is it that chemically identical atoms will undergo intermolecular change. 'We are thus 'forced to admit that in an aggregate of molecules of any 'compound there is an exchange continually going on be'tween the elements which are contained in it.' In a drop of an aqueous solution of hydrochloric acid, for instance, each 'atom of hydrogen is constantly changing places with other 'atoms of hydrogen'; when a solution of copper sulphate is added to hydrochloric acid, then the interchange of copper for copper, and of hydrogen for hydrogen, proceeds as before, but in addition to this 'the hydrogen does not merely move 'from one atom of chlorine to another, but in its turn also 'replaces an atom of copper, forming chloride of copper and 'sulphuric acid.' When one product is insoluble it is removed, and so almost the whole of one of the original substances is decomposed. 187. Pfaundler' has developed a hypothesis somewhat similar to that put forward by Williamson. Pfaundler's hypothesis is indeed grounded on the second law of thermodynamics, but its development proceeds on the lines of the molecular theory. Let there be two gases, AB and CD, formed from A, B, C and D, with the evolution of less heat than that which accompanies the formation, from the same constituents, of two other gases, AD and BC; then the change from AD, BC back to AB, CD will necessarily be attended with absorption 1 C. S. Journal, 4. 110–112. (See also do. 4. 229; also Phil. Mag. (3) 37. 350.) 2 Fogg. Ann. Jubelbd. 182: and do. 131. 55 et seq. (especially pp. 66—71). |