630 



NA TURE 



[October 23, 1890 



To every one, therefore, of the three test questions as to 

 •constancy and regularity, the experimental results give an 

 unhesitating negative. 



In the instances quoted above the depression actually found 

 for alcohol has been doubled in order to simplify the comparison 

 of it with the other substances. Alcohol belongs to that class 

 of bodies which give just half the value in water that the 

 majority do, and of which there are some instances in the case 

 of every solvent yet examined. The explanations which the 

 supporters of the chemical and physical theories give of these 

 half values differ so radically from each other that it is hopeless 

 to attempt to arrive at any agreement as to the nature of solution 

 till this difference is settled. The chemists say that these half 

 values are in all cases the abnormal ones, just as Raoult did 

 originally, and explain them by representing the molecules of 

 the dissolved substances which give them to consist of two 

 fundamental molecules. The physicists give exactly the same 

 explanation in the case of every solvent except water, but in this 

 case they say that the smaller values are the normal ones and 

 the larger the abnormal, the double magnitude of these being 

 caused by the dissociation of the dissolved molecule into its two 

 ions, whereby two molecules or acting units are formed from 

 every one originally added. 



If Raoult's views as to the constancy of the molecular 

 depression can be maintained, the data themselves are conclusive 

 against making this exception in the case of water ; for, since 

 the substances which give the lower values are supposed to act 

 normally, it is evident that, if the values given are in any way 

 abnormal, this abnormality must be due to the solvent. Now 

 the values certainly are abnormal ; they are about i°'03, whereas 

 the normal value for one molecule dissolved in loo molecules of 

 other solvents is o°'63, and the excess can, therefore, only be 

 explained by assuming that the molecules of water are more 

 complex than those of other solvents in the proportion of i -03 

 to 063, or 1 4 to I ; in other words, the water molecules must 

 be i^HjO- This view cannot be reconciled with the atomic 

 theory. 



Indeed the theory of dissociation into ions is altogether 

 unintelligible to the majority of chemists. It seems to be quite 

 irreconcilable with our ideas of the relative stability of various 

 bodies, and with the principle of the conservation of energy. 

 Of course we know that each ion when dissociated is not sujjposed 

 to be permanently dissociated, but to be continually combining 

 with its neighbours and separating again from them as in every 

 other case of dissociation ; but at any particular moment a very 

 large proportion of them is supposed to be free ; a proportion 

 which, according to the very results under discussion, must be very 

 nearly, if not quite, loo per cent, of the whole ; and we have to 

 settle whether it is probable or possible that a decomposition 

 such as this could have been effected by introducing the com- 

 pound into water. And how can we regard it probable that 

 compounds of such stability and compounds formed with such a 

 development of heat as sulphuric or hydrochloric acid should be 

 thus entirely dissociated by water ; still less that these, and all 

 the most stable compounds which we know, should be thus 

 demolished, while all the less stable ones — such as hydrocyanic, 

 sulphurous, boric acids, &c. — remain intact ? How can we 

 admit that the more stable a body is, the more prone it is to be 

 dissociated ? 



And if such a dissociation has occurred it must have been 

 without any absorption of heat, and, consequently, energy 

 must actually have been created. Take one of the simplest 

 instances, that of hydrochloric acid. If anything at all is 

 certain about atoms, it is that the atoms in an elementary 

 molecule are united very firmly together, and that therefore 

 in separating them a very large absorption of heat would 

 occur. To separate 2HCI into 2H and 2CI would absorb far 

 more than the 44,000 cal. which we know are absorbed in 

 separating 2HCI into Hg and Clj. Yet the supporters of the 

 dissociation theory would have us believe that this separation 

 has actually taken place, not only without any absorption of 

 heat, but actually with a development of 34,630 cal. ; that is, 

 that 44,000 + 34,630 + X cal. have been created, and that too 

 through the intervention of the water, which has ex hypothesi 

 no action whatever. 



This difficulty is realized by the supporters of the physical 

 theory, but the way in which they meet it does not appear to me 

 in any way to overcome it. To explain the non-absorption of 

 heat in the dissociation of the salt, they suppose that a charge 

 of electricity combines with the liberated atoms, and, in doing 

 so, evolves an amount of heat exactly equivalent to that ab- 

 NO. 1095, VOL. 42] 



sorbed in the separation of the atoms from each other ; 

 and a later development of the theory is, I believe, that tl> 

 atoms, though separated, are still held together by means ( 

 these charges, so that the net result is the supplanting of lli 

 chemical bond by an electrical bond of a precisely similar value. 

 It appears to me that nothing substantial is gained by such a 

 substitution, and that its occurrence is not merely hypothetical 

 but impossible. Whence come these electric charges, and by 

 what agency are they brought into play ? On what grounds can 

 it be maintained that a charge can combine with matter so as to 

 evolve hear, and that the heat so liberated is always exactly 

 equal to that absorbed in the decomposition of the compound ? 

 If this equality exists, how can we account for the force which 

 develops the one overcoming the equal frrce which develops the 

 other ? and how, again, can we account for the heat developed 

 in the act of dissolving ? If, on the other hand, the heat of the 

 combination of these charges is supposed to be equivalent to the 

 heat of combination of the atoms phis that of the heat of dis- 

 solution, we are met by the objection that the latter is often 

 negative, and that, therefore, the heat of the combination of the 

 charges must often be less than that of the combination of the 

 atoms and molecules, so that the lesser force must be regarded 

 as overcoming the greater. 



That free ions exist in solution is supposed to have been 

 proved by a recent observation of Ostwald's, to the effect that 

 the ions may be separated and brought into different parts of 

 the liquid by the proximity of a charged body. The separation 

 of the ions is, of course, recognized by the subsequent liberation 

 of hydrogen, oxygen, acid, alkali, &c., and it is certain that on 

 allowing these to mix and combine heat will be developed, and 

 the salt solution re-formed ; and thus, by replacing and removing 

 the charged body, it would evidently be possible to produce an 

 unlimited amount of heat. Now, if the charged body has lost 

 none of its charge, and if no mechanical energy has been ex- 

 pended, this heat must have been produced out of nothing, 

 and the whole ground-work of physical science must be false ; 

 whereas, if energy in some form has been expended on the solu- 

 tion, the experiment proves nothing, for there is nothing to show 

 that this energy has not been utilized in bringing about the very 

 dissociation the previous existence of which was in question. 



1 have already shown that the experimental data prove the 

 absence of that constancy and regularity which ought to exist 

 according to the physical theory, and to place the hydrate theory 

 on unassailable grounds it is only necessary to show that devia- 

 tions from constancy and regularity are of a magnitude such as 

 might reasonably be assigned to deviations due to the presence 

 of hydrates. That variations of 260 and 36,000 per cent, in the 

 value of the depression — such as are observed by altering the 

 dissolved substance or the solvent respectively — are amply 

 sufficient to satisfy the most exalted views of the influence of 

 chemical attraction, requires, I think, no demonstration, and we 

 may therefore content our.-elves with examining the deviations 

 observed when the proportions of the solvent are altered — such 

 deviations as are illustrated in Fig. i. 



It cannot be maintained that the energy of the chemical 

 combination of, say, water with sulphuric acid, is the only reason 

 why the temperature of the mixture of the two must be cooled 

 below 0° before any of the latter will crystallize out ; some 

 lowering of the freezing-point will be caused by the mere 

 interposition of the foreign molecules of sulphuric acid be- 

 tween those of the water, and on certain grounds, which I 

 have explained elsewhere,- I estimate this mechanical lowering, 

 as I term it, at o°-56 for each dissolved molecule to 100 of the 

 solvent (a molecule of solvent water being 3H2O), a value 

 which, it may be noted, is not far removed from Raoult's 

 experimental value of o°"63. There is also another source of 

 lowering depending mainly on the heat capacities of the sub- 

 stances concerned, which I term for convenience the physical 

 lowering ; but its value, in the case of weak solutions, is very 

 small, and I need, therefore, say no more about it here. Both 

 these lowering causes would exist whether there were hydrates 

 present or not ; but if these were present we should get a 

 further depression due to their existence. Any given hydrate 

 would have to be decomposed into the next lower one before it 

 could give up any water for crystallization, and a certain amount 

 of resistance would thus be offered to this crystallization, to 

 overcome which the solution would have to ibe further cooled. 



' On the view that hydrates exist in solution, there'ivn difficulty, as I have 

 shown elsewhere, in explaining the absorption of heat during dissolution, 

 without violating the principle of the conservation of energy. 



2 Proc. Chem. See, 1889, p. 149. 



