66 



NA TURE 



{_Nov. 1 8, 1886 



than the sum of those of its components, and would of itself go 

 far to prove that a solution did in reality contain such a com- 

 pound. But a rise of temperature would also undoubtedly have 

 another and opposing effect, for, being inimical to the com- 

 plexity of these hydrates, they would be more dissociated at 

 higher than at lower temperatures, and Iience the heat of 

 dissolution would not be so great as it should be according to 

 the various specific heats. This is precisely what Dr. Tilden 

 has proved to be the case [Pyoc Roy. Soc. 1S85, 401). 



There is, however, another action which I believe accompanies 

 every act of dissolution resulting in the absorption of heat. 



The heat absorbed by a large number of salts in dissolving 

 cannot be freely accounted for by the mere physical change of 

 the solid into the liquid salt. Thus, the heat of dissolution of 

 potassium nitrate is - S500 cal., and that of sodium nitrate 

 - 5000 cal., whereas the heat of fusion of these sails at the s.ame 

 temperature is but - 1 300 and —2300 cal. respectively. There 

 must be some otlier heat-absorbing action besides the fusion of 

 the salt. The amount of heat thus absorbed increases also with 

 the dilution of the liquid. Moreover, we cannot, I think, account 

 for the manner in which heat is evolved in one case and absorbed 

 in another, or the way in which an absorption of heat sometimes 

 gives place to an evolution, as the temperature or other con- 

 ditions are changed, but by admitting the constant co-existence 

 of two actions producing opposite thermal effects, and being 

 influenced to different extents by an alteration of circumstances.^ 

 On the theory which I am here advocating, this absorption of 

 heat receives a ready explanation. Whatever be the complexity 

 of the molecular aggregates of a liquid, those of a solid will be 

 still more complex. Fusion would, therefore, entail their simpli- 

 fication ; it would be but a chemical decomposition absorbing 

 heat ; this simplification would be pushed much further, how- 

 ever, when the salt is dissolved instead of being merely fused, 

 for the particles of the liquid act chemically (ex hyp.) on those 

 of the solid and combine with them themselves ; the cold 

 absorbed on dissolution would exceed that absorbed on fusion, 

 and would, moreover, be increased by increasing the amount of 

 the solvent. This accords fully with the facts observed. 



All the phenomena attending dissolution are, therefore, I 

 contend, accounted for by a full recognition of the real com- 

 plexity of the units of matter, and by taking the more liberal 

 view of chemical combination which is inculcated by a study of 

 minerals and other substances. Every act of dissolution involves 

 two actions. The chemical decomposition of the more complex 

 aggregates of the solid into a simpler form, absorbing heat, and 

 a chemical combination of these with the liquid, evolving heat ; 

 the only quantity which we can at present measure is the algebraic 

 sum of these two. 



Mr. Durham next gave a short statement of his own theory 

 of solution : — 



When, for example, common s.alt (NaCl) is placed in water, 

 all the atoms act upon e.ach other. The sodium of the salt acts 

 upon the oxygen of the water, .and the chlorine of the salt upon 

 the hydrogen of the water ; and the result is a definite com- 

 pound, which we call a solution. The heat of formation of the 

 acid is neutralised by the heat of formation of the oxide. If they 

 be not equal, the difference is the heat of the solution ; if they 

 be equal, the heat is of course nil. If the former be the greater, 

 the heat of the solution is negative ; if the latter, it is positive. 

 Solution arises from chemical aflinity, and takes place inversely 

 as the attraction between the positive element and the oxygen — 

 and the negative element and the hydrogen— of water. But 

 chemical affinity is itself physical ; the atoms a;e physical, and 

 all forces which act upon them must be physical forces. In a 

 chemical mixture every atom is acting upon every other atom, 

 but si:ch action can be nothing else than physical ; .and we are 

 therefore led to the conclusion that there is really no difference 

 between chemical and physical action, and, consequently, that 

 the alternative between the two does not exist. 



In the course of the discussion, and preceding the reading of 

 Mr. Pickering's paper, the following remarks were made : — 



Dr. Armstrong said that, from the summary given by Prof. 

 Tilden, it appeared that the two important questions for dis- 



' b. l""'y "f 'h= thermal results attending the dilution of salt-solutions, 

 established by Thorasen ('■ Thermochcm.,," iii., especially plate iv., and also 

 the curves given by formic and acetic acids and by potassium and sidiuni 

 hydrates), impresses very forcibly the co-existence of these 1 

 althoiigh Xhomsen himself does not seem to have noticed it. 



cussion were — (i) Does water of crystallisation exist in solution 

 combined with the salt as it did prior to dissolution ? and 

 (2) What distinction is to be drawn between chemical com- 

 bination and mechanical association or adhesion ? In short, are 

 the phenomena of dissolution of a chemical or of a mechanical 

 character? But Prof. Tilden had made an important omission, 

 inasmuch as he had nit discussed the possible simplification of 

 the molecules on dissolution : in discussing the evidence afforded 

 by the various phenomena, everything turned upon the question 

 whether the crystal molecules are of the composition represented 

 by our ordinary formula;, or are more or less complex. 



As regards the first question, Prof. Tilden appeared to differ 

 from Dr. Nicol, and to think that water of crystallisation did 

 exist in solution. (Prof. Tilden, interposing, desired to explain 

 that what he had said was that it was impossible, in the case of 

 any solution, to say that one portion of the water is in com- 

 bination with the salt and that .another is not ; all the phenomena 

 of dissolution and diUition being continuous, no point can be 

 found at which such a distinction can be set up. He believed 

 that the salt was attached to all the water present without 

 exception.) 



Dr. Armstrong, resuming, said that much of the evidence 

 appeared, he thought, to favour the conclusion that in certain cases 

 water of crystallisation did exist in solution ; e.g. the difference 

 in colour between many hydrated and dehydrated salts taken in 

 conjunction with the colour of their solutions. Again, many 

 dehydrated salts dissolved much less readily than the correspond- 

 ing hydrated s.alts : instances of this kind were not common 

 among inorganic salts, but were often met with among organic 

 salts, and the speaker cited calcium butyrate and certain naph- 

 thalene- and napthol-sidphonates as examples. Dextrose, again, 

 ordinarily crystallises with two molecules of water, but if de- 

 hydrated and carefully dissolved in water at a low temperature 

 it may be crystallised out from the solution in the anhy.lrous 

 state. T. Thomsen's recent experiments, however, appeared to 

 show that when two substances were dissolved in w.ater they ap- 

 propriated the water in the proportions in which they were pre- 

 sent, thus favouring a purely mechanical interpretation of the 

 phenomena of dissolution : but, on the other hand, it was to be 

 noted that in the case of citric and sulphuric acids, for example, 

 Thomsen's results were in accord with this conclusion only when 

 it was assumed that the citric acid was present as the dihydrate, 

 .and sulphuric acid as the monohydrate, HoSOj.OH^. In fine, 

 the speaker was of opinion that while the question could not be 

 regarded as settled, yet there was a considerable amount of evi- 

 dence that the water was not evenly distributed, but was, in 

 some cases at least, in part directly combined with the dis-olved 

 substance. Dr. Nicol had deduced an ingenious argument from J. 

 Thomsen's observations on heats of neutralisation, .^s a criticism 

 of Dr. Nicol's argument from the existence of neutralisation con- 

 stants he would venture to say "Put not your faith in constants." 

 If the views which he held — views which probably were at pre- 

 sent peculiar to himself — were correct, the quantities in question 

 ought to have a constant value. According to Helmholtz, .all 

 atoms hold a positive or negative electrical charge, a single 

 charge being associated with a monad, two with a dyad, and so 

 on. If when combination takes place these charges exactly 

 neutralised each other, all compounds would be neutral and satu- 

 rated ; but actually this is not the Case : in point of fact, there is 

 no such thing as a saturated compound. Helmholtz seems to 

 think that the charges may be held by different atoms with 

 different degrees of force, but the speaker took a somewhat 

 different view, and thought that probably when two atoms com- 

 bined, in consequence perhaps of peculiarities of structure, their 

 charges were not completely used up ; the resulting molecules 

 therefore possessed a certain residual charge or affinity, and were 

 consequently in a position to enter into combination with other 

 molecules. Thus w'ater, he thought, was not a saturated com- 

 pound ; its oxygen atom was still possessed of residual affinity. 

 The same was true of sulphuric acid. Consequently the two 

 could combine together to form a hydrate. On neutralising a 

 dilute solution of alkali by a dilu'e solution of acid, a stable 

 condition is finally attained, and it is to be .assumed that the 

 affinities are fully satisfied, or very nearly so — that the charges 

 practically neutralise each other : hence it may be expected that 

 the heat of neutralisation will have nearly a constant value pro- 

 vided there be no disturbance such as the separation of a pre- 

 cipitate would produce. But the value of each of the several 

 processes which go to make up the heat of neutralisation are 

 entirely imknown to us, and in the .absence of such knowledge it 



