834 PRINCIPLES OF CHEMISTRY 



substance dissolved can be found. But if r grms. of the solvent and 

 q grms. of the substance dissolved are taken, then there are 100 q/r 

 of the latter per 100 grms. of the former, and this quantity = n X, 



where n is found from the depression and = and X is the mole* 



K 



cular weight of the substance dissolved. Hence X =s - _ ', which 



rd 



gives the molecular weight, naturally only approximately, but still with 

 sufficient accuracy to easily indicate, for instance, \vhether in peroxide 

 of hydrogen the molecule contains HO or H 2 2 or H 3 O 3 , tfcc. (H 2 O 2 is 

 obtained). Moreover, attention should be drawn to the fact that 

 a great many substances taken as solvents give per 100 molecules 

 a depression of about 0*63 n, whilst water gives about 1*05 n t i.e. a 

 larger quantity, as though the molecules of liquid water were more 

 complex than is expressed by the formula H 2 O. 28 A similar pheno- 

 menon which repeats itself in the osmotic pressure, vapour tension 

 of the solvent, &c. (see Chapter I., Notes 19 and 49), i.e. a variation 

 of the constant (k for 100 grms. of the solvent or K for 100 molecules of 

 it), is also observed in passing from indifferent substances to saline (to 

 acids, alkalis and salts) both in aqueous and other solutions as we will 



18 A similar conclusion respecting the molecular weight of liquid water (i.e. that its 

 molecule in a liquid state is more complex than in a gaseous state, or polymerized into 

 HgO4, H^O^ or in general into nB.^0) is frequently met in chemico-physical literature, 

 but as yet there is no basis for its being fully admitted, although it is possible that 

 a polymerization or aggregation of several molecules into one takes place in the pas- 

 sage of water into a liquid or solid state, and that there is a converse depolymerization 

 in the act of evaporation. Recently, particular attention has been drawn to this subject 

 owing to the researches of Eotvos (1886) and Ramsay and Shields (1898) on the variation 

 of the surface tension N with the temperature (N = the capillary constant a- multiplied 

 by the specific gravity and -divided by 2, for example, for water at and 100 the value 

 of a*=15'41 and 12*58 sq. mm., and the surface tension 7*92 and 6*04). Starting from 

 the absolute boiling point (Chapter II., Note 29) and adding 6, as was necessary 

 from all the data obtained, and calling this temperature T, it is found that 

 AS = AT, where S is the- surface of a gram-molecule of the liquid (if Mis its weight 

 in grams, s its sp. gr., then its sp. volume = M /*, and the surface S =1J (M/')*), A the 

 surface tension (determined by experiment at T), and A a constant which is inde- 

 pendent of the composition of the molecule. The equation AS~A;T is in complete 

 agreement with the well-known equation for gases vp = RT (p. 140) which serves for 

 deducing the molecular weight from the vapour density. Ramsay's researches led him 

 to the conclusion that the liquid molecules of CS 2 , ether, benzene, and of many other 

 substances, have the same value as in a state of vapour, whilst with other liquids this is 

 not the case, and that to obtain an accordance, that is, that k shall be a constant, it is 

 necessary to assume the molecular weight in the liquid state to be n times as great. 

 For the fatty alcohols and acids n varies from 1^ to 8, for water from 2J to 4, according 

 to the temperature (at which the depolymerization takes place). Hence, although this 

 subject offers a great theoretical interest, it cannot be regarded as firmly established, 

 the more so since the fundamental obervations are difficult to make and not sufficiently 

 numerous , should, however, further experiments confirm the conclusions arrived at by 

 Professor Ramsay, this will give another method of determining molecular weights. 



