628 



NATURE 



[October 23, 1890 



present as to the possible complexity of liquid molecules. It is 

 interesting to note that a similar complexity of molecular group- 

 ing must be admitted if we accept Raoult's original statement 

 that one molecule of any substance dissolved in loo molecules 

 of a solvent lowers the freezing-point of this latter by about 

 o°"63 ; for, if this be so, we must assign to the molecules of the 

 various substances entered in the second column of Table I. the 

 magnitude there indicated when they are dissolved in the solvent 

 named in the first column, for it requires that proportion of 

 these bodies to lower the freezing-point of loo molecules of the 

 solvent by o'*63 ; and, amongst these few instances which I 

 have collected from my own determinations, we find molecular 

 aggregates containing as many as 200 of the fundamental mole- 

 cules, and even this number, I may mention, probably under- 

 states the complexity to a very considerable extent ; for the 

 depression in this and some of the other cases had to be 

 estimated from that observed with solutions containing as much 

 as 10 gram molecular proportions to icxj of the solvent, and 

 the molecular depression increased rapidly with the strength of 

 the solution : loooHjO would probably be a low estimate of the 

 complexity of the molecules of water when dissolved in a large 

 excess of the hexhydrate of calcium chloride, a complexity com- 

 parable with that of the hydrates, which my other work has 

 indicated, and that too in the case of that very substance which 

 these hydrates contain — water. 



Table I. — Molecular Weights of Substances in Various 

 Solvents. ^ 



Solvent. Dissolved substance producing 



o°'63 depression. 2 



iooH2S04H,0 32H2O 



63H2SO4 



100H2SO44H2O 8H2O 



15H2SO4 



ioo(CaN03)24H20 90H2O 



42Ca(N03)» 



looCaCIgeHaO 210H2O 



., 63CaCl2 



Now as to the question of how far the theory of osmotic 

 pressure, and the results on which it is based, are antagonistic 

 to the hydrate theory : and let me first define clearly the position 

 which I take in this matter. I do not for one moment call in 

 •question any of Raoult's classical work, which is now so 

 familiar to us, nor do I question that these results reveal the 

 existence of a depression of the freezing-point which is ap- 

 proximately and generally constant ; and I consequently admit 

 that we can generally obtain an approximately correct value for 

 the molecular weight of the substance by observing the depression 

 which it causes ; nor, again, do I wish to question the correct- 

 ness of the mathematical relationship which van't Hoff and 

 Arrhenius have shown to exist between osmotic pressure, the 

 lowering of the freezing-point, and other properties, provided 

 vve accept the fundamental assumptions on which their calcula- 

 tions are based — the truly gaseous nature of dissolved matter, 

 and the dissociation of salts into their ions. But what I do 

 question is that the facts of the case warrant such assumptions, 

 and that the constancy and regularity of the results are so 

 rigorous as to justify the conclusion that the solvent has no 

 action on the dissolved substance, and that there are no irregu- 

 larities such as would be caused by the presence of hydrates. 



According to the osmotic pressure theory, the dissolved 

 matter, so long, at any rate, as it is not present in greater 

 quantity than it would be in the same volume of its gas, if it 

 were gasified under normal conditions, is really in the gaseous 

 condition, and obeys all those laws which apply to gases. Accord- 

 ing to the hydrate theory this will be but partially true. That 

 the dissolved substance is in a condition comparable with that 

 of a gas, in so far as the separation of its own particles from each 

 other is concerned, must be admitted— indeed, I arrived in- 

 dependently at this same conclusion from a study of thermo- 

 chemical data ; but inasmuch as there is present the solvent, 

 which we believe is not an inactive medium, its molecules cannot 

 have the same freedom as if they were truly gaseous, and will, 

 therefore, obey the laws of gases imperfectly only. 



It will be well to confine our attention to but one of those 

 properties connected with osmotic pressure, and to select for 



I Other instances of high molecular weights are mentioned by Brown and 

 Moms (Chem. Soc. Trans., 1888), and Gladstone and Hibbert iPhil. Ma^., 

 1889, vol. ii. p. 38). •^ 



Determined from the freezing-points of very weak solutions. 



NO. 1095, VOL. 42] 



that purpose the one which has been most fully investigated— 

 the lowering of the freezing-point of a solvent : and the tests 

 which may be applied to assertain whether in producing this 

 lowering the dissolved substance behaves as a perfect gas or not, 

 may be grouped under three principal headings :— 



1. Is the molecular depression {i.e. that produced as calculated 

 for one molecule dissolved in 100 molecules) constant, indepen- 

 dent of the nature of the solvent ? 



2. Is it independent of the strength of the solution, so long as 

 this strength does not exceed the limits (" gas" strength) above 

 mentioned ? (Boyle's law.) 



3. Is it independent of the nature of the dissolved substance ? 

 (Avogadro's law.) 



In the Phil. Mag., 1890, vol. i. p. 495, will be found 

 instances of the variation in the molecular depression which 

 maybe noticed by altering the solvent (see also Table I. above). 

 With water in six different solvents it varied between 1°'0'J2 and 

 o°'oo3 ; with sulphuric acid in four different solvents, between 

 2°'I5 and o°'oi ; with calcium chloride in two different solvents, 

 from 2° 773 to o°'Oi ; and with calcium nitrate in two solvents, 

 from 2°'5 to o°'Oi5 ; while many instances may be collected 

 from Raoult's data showing that the same substance which acts 

 normally in one solvent may act abnormally (give only half the 

 usual depression) in another. Such variations are so great — 

 from TOO to 35,600 per cent.— that there can be no doubt but 

 that the solvent is not that inert medium which the supporters 

 of the physical theory would have it to be, but that it has a very 

 great influence on the results obtained. It must be noted, 

 however, that this objection, though applying to Raoult's original 

 views, does not, or, at any rate, may not, apply to van't Hoff's 

 theory, for according to this theory the nature of the solvent has 

 an influence in determining the lowering of the freezing-point, 



W, in van't Hoff's equation, 5/ 



0-02T- 



representing the heat 



of fusion of the solvent. But the lowering is according to this 

 equation independent of the nature or the amount of the 

 dissolved substance, so that the two following objections will 

 apply to van't Hoff's theory as well as to Raoult's statement. 



Secondly, as to the influence of the strength of the solution. 

 It is remarkable that, although the osmotic pressure theory depends 

 on the behaviour of solutions below a certain strength, no attempt 

 whatever has been made by its supporters to obtain any data 

 respecting such solutions. The data on which their views were 

 founded referred to solutions considerably stronger than the 

 requisite "gas" strength, and though, no doubt, it was conve- 

 nient to work with data which afforded a ready excuse for any 

 awkard irregularities which might be met with, such data must 

 lack the conclusiveness which is so eminently desirable. The 

 few data which I have accumulated as to solutions of an "ideal" 

 strength can leave no doubt that, even in their case, the 

 depression is not a constant independent of the strength. 



A solution of sulphuric acid containing 008H2SO4, 100H2O 

 would be of a strength comparable with the gas from the acid if 

 it could be gasified at normal pressure and temperature, and the 

 molecular depression should be constant for all solutions below 

 this strength : it should be represented by a horizontal line such 

 as AB in Fig. i, whereas the observed deviations from constancy 

 are very great, being represented by the lines marked H2SO4 ; 

 and, moreover, these deviations are by no means regular, and 

 cannot therefore be attributed to imperfect gasification ; they 

 possess none of the characteristics of the deviations of gases from 

 Boyle's law. The determinations on which these results are 

 based are very numerous ; there are about sixty experimental 

 points on the portion here shown, and the mean error of each 

 point as determined in two different ways was only o°'ooo5, 

 a quantity represented by one-tenth of one of the divisions of 

 the paper ; the deviations from regularity amount to thirteen 

 times this quantity, and to as much as 16 per cent, of the total 

 depression measured. 



The other lines in Fig. i represent the deviations from 

 regularity in the case of calcium chloride, calcium nitrate, and 

 alcohol respectively, and these, though they are smaller than in 

 the case of sulphuric acid, are far too great to be attributed to 

 experimental error ; and the fact that they occur sometimes in 

 one direction, sometimes in the other, precludes the possibility 

 of attributing them to any constant source of error in the instru- 

 ments used or in the method adopted. 



Remembering that these are the only data which we have at 

 present respecting very weak solutions, we must conclude that 

 the hypothesis that such solutions exhibit perfect regularity is 



