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greater than the number of degrees of temperature which the sulphurets can bear without decomposition, minus 440°. In a similar manner might be determined the affinity of iodine, bromine, and chlorine for the few mietals from which they can be separated by heat, provided the temperatures at which the decompositions take place could be more accurately determined : so likewise might be estimated the affinity of mercury and arsenic for certain other metals, that of ammonia for the more tixed acids, such as the boracic and phosphoric acids, and that of carbonic acid for most salifiable bases. The greater number of bases part with their combined carbonic acid at a low red heat, lime at a somewhat higher temperature, strontia at a still higher, baryta only in the strongest wind-furnace, potash and soda not at all. Accordingly, the last two bases must have the strongest affinity for carbonic acid; and the fact of lime, baryta, and strontia taking carbonic acid from them when a considerable quantity of water is present, must be explained by the greater affinity of water for caustic potash and soda (page 128). Nitrate of copper is decomposed at a lower heat than nitrate of silver, whence it follows that the latter oxide has the greater affinity for nitric acid. Lastly, since many oxidized compounds, such as peroxide of manganese, chromic acid, antimonic acid, and arsenie acid give up part of their oxygen at a high temperature, and the noble metals give up the whole of it, and since the compounds of hydrogen with carbon, phosphorus, and sulphur, and those of nitrogen with chlorine and iodine are decomposed at various temperatures,—the affinities by which these compounds are held together may be at least comparatively determined;—but no exact numbers can be assigned to them, because the boiling points of oxygen, hydrogen, and nitrogen are unknown. This method of determining magnitudes of affinity deserves closer examination.
b. Decompositions in which Ponderable Bodies are alone concerned.
a. By Simple Affinity. If we find that the compound A B is decomposed by C with formation of A C, and that similarly the compound AC is decomposed by D with formation of A D, &c., we conclude that A has the greatest affinity for D, the next for C, and the smallest for B. In this manner, A may be tested with respect to all the substances with which it can combine. If, then, we place A at the head, and below it all the substances capable of uniting with it, in the order in which their affinity for a diminishes, we obtain the Column of Affinity of A. And if we proceed in the same manner with other bodies, simple and compound, assigning a column to each, and collecting all these columns into a general table, we shall obtain a Table of Affinity, Tabula A finitatum.
The first table, which was very imperfect, was drawn up by Geoffroy; he was followed by Gellert, Rüdiger, Limbourg, Marherr, De Fourcy, Demachy, Erxleben, Weigel, Wiegler, and Bergman.
A few examples will suffice to illustrate this method. Carbonate of lime treated with hydrochloric acid yields hydrochlorate of lime and carbonic acid; hydrochlorate of lime is resolved by sulphuric acid into sulphate of lime and free hydrochloric acid; and when oxalic acid is added to a solution of sulphate of lime in water, oxalate of lime is thrown down, while free sulphuric acid remains in the water. Hence in the column headed Lime, the four acids above mentioned succeed one another in the following order: oxalic, sulphuric, hydrochloric, carbonic. From an aqueous solution of sulphate of alumina ammonia precipitates the alumina, producing sulphate of ammonia; this salt is converted by lime into sul
phate of lime and free ammonia; the sulphate of lime treated with solution of potash is resolved into sulphate of potash and free lime; and, lastly, the aqueous solution of sulphate of potash gives with baryta-water a precipitate of sulphate of baryta, while free potash remains in solution. Consequently in the sulphuric acid column, the four bases here considered would stand in the order: baryta, potash, lime, ammonia, alumina.
Simple and sure as this method may appear, and well as it may be adapted to furnish available materials for the determination of relative magnitudes of affinity, it is still far from being unexceptionable, and demands the greatest caution in its application. The influence of cohesion, elasticity, and the affinity of the solvent, are especially deserving of the most careful attention. For example, that oxalic acid added to an aqueous solution of sulphate of lime precipitates oxalate of lime, might be explained on the hypothesis that the cohesion of the latter salt is greater than that of the former; and that at the same time the affinity of water for sulphuric acid is greater than for oxalic acid; if such be the case, the affinity of sulphuric acid for lime may still be greater than that of oxalic acid. It has also been suggested that hydrochloric acid may expel carbonic acid from carbonate of lime, not in consequence of greater affinity, but because carbonic acid is more elastic i. e., has greater affinity for heat than hydrochloric acid has; but this supposition is negatived by the experiment described on page 130, in which the decomposition was found to take place under a pressure sufficient to liquify the carbonic acid set free. Again it has been shown (p. 129) that e. g., boracic acid decomposes sulphate of soda at a red heat, whilst the opposite effect takes place in the cold. Generally, the various cases of reciprocal affinity (page 125 .... 133) show that it is important to examine the action of bodies under variously altered circumstances, and in drawing conclusions respecting magnitude of affinity from decompositions which result from simple affinity, never to neglect the circumstances which, as was shown in discussing the theory of reciprocal affinity, may invert the result and enable the weaker affinity to gain the victory. One of these circumstances, viz., difference of temperature, was long ago noticed by Bergman. In his table, the Affinitates elective via humida are distinguished from those via sicca, accordingly as the decompositions take place at ordinary temperatures or at a red heat. This mode of distinction is not indeed unexceptionable, since opposite results often take place at different degrees of incandescence: thus, for example, at a red heat potassium takes oxygen from iron, while at a white heat iron takes oxygen from potassium. At the same time such distinctions oblige us to admit that tables of affinity do not always give the relative magnitudes of that force, but merely the results of decomposition under certain circumstances: hence these tables are by many chemists called Tables of Precipitation, or more correctly, Tables of Decomposition.
It must also not be forgotten that a body C sometimes takes from the body B only a part of the body A. When soda precipitates oxide of lead from a solution of chloride of lead in water—which may be regarded as hydrochlorate of oxide of lead—this does not exactly prove that the affinity of soda for hydrochloric acid is greater than that of oxide of lead; for the precipitate is a compound of 4 atoms of oxide of lead with one atom of hydrochloric acid; and the same compound is on the other hand produced, with separation of soda, when oxide of lead in excess is digested with a solution of hydrochlorate of soda. From this it follows that 4 atoms of oxide of lead have a greater affinity for 1 atom of hydrochloric acid than 1 atom of soda has; but that 1 atom of soda has a greater affinity for hydrochloric acid than 1 atom of oxide of lead, and consequently abstracts from the latter of the acid with which it is combined. Hence, in precipitations of this kind, it must be carefully examined whether the precipitate contains the substance B in a state of purity or still in combination with part of the body A.
Lastly, on bringing together A B and C, we very often obtain, not AC and B, but A C and B C. If, for example, in order to determine whether arsenic or sulphur has the greater affinity for oxygen, we heat together arsenious acid and sulphur, we obtain indeed sulphurous acid, but the separated arsenic combines with another portion of the sulphur and forms sulphuret of arsenic (Sch. 100). In this case we cannot conclude that oxygen
has a greater affinity for sulphur than for arsenic, but only that the affinity of sulphur for oxygen + that of sulphur for arsenic is greater than that of arsenic for oxygen;—the affinity of heat for sulphurous acid must likewise be taken into account.
B. By Double Affinity. Guyton-Morveau supposed that when two salts decompose one another, the sum of the two decomposing affinities must be greater than that of the two quiescent affinities. According to the decompositions which the salts of certain acids and bases exhibit one with another, he endeavoured to assign such magnitudes to their affinities that calculation should agree with observed results. He thus found by trial the following numbers:
According to this table, sulphate of soda and hydrochlorate of baryta must decompose each other, because 66 + 31 (= 97) is greater than 58 + 36 (94); similarly with carbonate of potash and acetate of lime, since 12 + 26 > 19 + 9, and so on. But in many cases in which decomposition takes place the sums are equal; e. I., with sulphate of potash and nitrate of baryta (62 + 62 = 66 + 58); and with sulphate of potash and hydrochlorate of baryta (62 + 36 = 66 + 32). In others the sum of the latent is even greater than that of the separating affinities, so that the calculation directly contradicts the experimental result; e. g., in the case of nitrate of baryta and sulphate of soda (62 + 58 > 50 + 66); so likewise nitrate of baryta is decomposed by sulphate of ammonia, sulphate of lime, sulphate of magnesia or carbonate of soda, and sulphate of magnesia by carbonate of ammonia, although calculation would lead to the contrary result. Moreover, Guyton-Morveau assigns to the affinity of nitric and hydrochloric acid for baryta larger numbers than to the affinities of the same acids for potash, although potash separates baryta both from the nitrate and hydrochlorate of that base. Generally, it is easy to see that it would be useless trouble to attempt to rectify the preceding numbers and adapt them to all decompositions of these salts by donble affinity,—since these results depend, not alone on the sum of the magnitudes of affinity, but also on cohesion, temperature, and the nature of the solvent.
B. Attempts have been made to determine the relative strength of affinity of two bodies from their force of adhesion. Guyton-Morveau regarded adhesion as a commencing affinity; supposing that heterogeneous substances attract each other in masses before the attraction between their individual atoms comes into play and forms chemical compounds. The greater therefore the affinity between two bodies, the greater should also be their adhesion, and the magnitude of the former should also be determinable from that of the latter. Morveau suspended a metal disc one inch in diameter from one of the arms of a balance, and counterpoised it by weights in the opposite scale; he then placed under the disc a glass filled with mercury, so that the surface of the mercury just came in contact with the lower surface of the disc,—and ascertained what additional weight required to be laid in the opposite scale-pan in order to separate the disc from the mercury. In this manner he found that the following weights were necessary: gold 446 grains, silver 429, tin 418, lead 397, bismuth 372, zinc 204, copper 142, antimony 126, iron 115, cobalt 8. This is almost exactly the order of facility in which these metals combine with mercury, and so far experiments appear to accord with the preceding view. But it has not yet been shown that the magnitudes of adhesion and affinity are in direct proportion one to the other. Although the affinity of sulphur for mercury is much greater than that of either of the metals just named, still a disc of sulphur would adhere to it with far less force than either of the metallic discs did. Moreover, the fact of mercury combining with gold more easily than with zinc does not show that gold has the greater affinity for the mercury: for facility of combination is one thing, intimacy another. Besides, Morveau's method does not give even the force of adhesion; for a certain quantity of mercury remains attached to the plates, and on separation the mercury itself is torn asunder, and the force determined is in reality its cohesion. Finally both on this ground, and because many substances on coming in contact immediately enter into chemical combination,—and the film of the new compound of the two bodies whose adhesion is to be measured is really that which suffers disruption,-an exact determination of the force of adhesion in the most numerous and important cases is impossible.
C. The strength of affinity is sometimes estimated by the time in which combination takes place. Wenzel (Von der Verwandtschaft
, p: 28) exposed metal cylinders of equal height and diameter, and covered all over, with the exception of one of the terminal surfaces, with varnish -to the action of different acids at the same temperature and for equal intervals of time, and estimated the force of affinity by the quantity of inetal dissolved. These experiments however prove nothing, first because in the solution of metals in acids various affinities come into play, viz., the affinity of the metal for oxygen, which has to be taken sometimes from the acid sometimes from the water,—that of the oxide of the metal for the acid—and that of the salt for water; secondly, because Wenzel sometimes used concentrated, sometimes dilute acids, according to the condition of the metal —and thirdly, because a given surface of different metals exposes a different number of atoms to the action of a solvent, according to the atomic weights and densities of the metals. But even experiments in which these sources of error were eliminated would lead to nothing, because the important influence which cohesion, specific gravity,
&c. exert on the rapidity of the combination could not well be taken into account.
D. Strength of affinity has been estimated from the quantities in which bodies combine.
Berthollet laid down the following hypothesis: The smaller the quantity of a body B required to neutralize another body A, that is to say to balance its opposite properties, the more completely opposite must B be to A, and the greater must therefore be their mutual attraction. If for example a certain quantity of an acid is neutralized by 1 part of the base B, but requires 2 parts of the base C and 3 of the base D, the affinities of A to B, C, and D = 3:11:1; in short, the force of affinity is inversely as the quantity required for neutralization. A similar relation exists with regard to the affinity of a base for several acids; that acid of which the smallest quantity suffices for the neutralization of the base, will have, of all acids, the greatest affinity for the base. This view of the matter is contradictory to the order of affinity found from decompositions by double affinity. For example, 40 parts of sulphuric acid are neutralized by 76.6 baryta, 52 strontia, 47.2 potash, 31.2 soda, 28.5 lime, 2014 magnesia and 17 ammonia; and 285 lime are neutralized by 40 sulphuric acid, 54 nitric acid, 36o4 hydrochloric ric acid, 127 hydriodic acid, 32 sulphurous acid, and 22 carbonic acid. The bodies here follow in the order in which they separate each other, so that for example baryta takes sulphuric acid from all other bases aud sulphuric acid takes lime from all other acids. The law here manifested —that, for the most part, those substances of which the smallest quantities are required to neutralize a third body—and which should therefore have the greatest affinity for that body—are separated by those which combine with the same body in greater proportion, and should therefore have less affinity for it—is explained by Berthollet from the influence of cohesion and elasticity. According to that philosopher ammonia has, of all the bases here enumerated, the greatest affinity for sulphuric acid, for it is the base of which the smallest quantity is required to neutralize that acid: that it should however be separated from sulphuric acid by all other bases arises from its tendency to assume the gaseous form. From the same cause the highly elastic substance carbonic acid, wbich of all acids has the greatest affinity for lime, is separated from that base by hydrochloric acid: (the incorrectness of this explanation is manifest from the experiment described p. 130). That baryta should take sulphuric acid from all other bases, although according to Berthollet's view its affinity for that acid must be the smallest, is explained by the great cohesion of sulphate of baryta: and that potash should separate lime and magnesia is supposed to result from the great cohesion of those earths, &c. It is certainly worthy of remark that when an acid is brought in contact with two salifiable bases, the least soluble body is always obtained: if one salt is less soluble than another, it is formed; if one base is less soluble than another, it is precipitated. Thus baryta takes sulphuric acid from strontia and forms with it an insoluble salt; strontia withdraws sulphuric acid from potash, and potash from soda,—sulphate of strontia being less woluble than sulphato of potash, and sulphate of potash than sulphate of boda. Soda takes lime from sulphuric acid, and the lime which separates in the least soluble body; lime takes sulphuric acid from magnesia, and magnesia is loss soluble than lime. Ammonia alone forms an exception: it gives up sulphuric acid to lime, and the lime is thereby converted into thu moro solublo sulphate of lime. When a base comes in contact with