330 PRINCIPLES OP CHEMISTRY 



in the first place, that in the case of solids and liquids, in which the 

 molecules are closer together than in gases and vapours, a greater 

 complexity is to be expected, i.e. a dependence of all the properties not 

 only upon the weight of the molecule but also upon its composition 

 and quality, or upon the properties of the individual chemical atoms 

 forming the molecule ; and, in the second place, that, in the case of a 

 small number of molecules of any substance being disseminated through 

 a mass of another substance for example, in the formation of weak 

 (dilute) solutions (although in this case there is an act of chemical 

 reaction i.e. a combination, decomposition, or substitution) the dis- 

 persed molecules will alter the properties of the medium in which they 

 are dissolved, almost in proportion to the molecular weight and almost 

 independently of their composition. The greater the number of mole- 

 cules disseminated i.e. the stronger the solution the more clearly 

 defined will those properties become which depend upon the composition 

 of the dissolved substance and its relation to the motecules of the 

 solvent, for the distribution of one kind of molecules in the sphere 

 of attraction of others cannot but be influenced by their mutual chemical 

 reaction. These general considerations give a starting point for ex- 

 plaining why, since the appearance of Van't Hoff's memoir (1886), 'The 

 Laws of Chemical Equilibrium in a Diffused Gaseous or Liquid State ' 

 (see Chapter I., Note 19), it has been found more and more that dilute 

 (weak) solutions exhibit such variations of properties as depend wholly 

 upon the weight and number of the molecules and not upon their com- 

 position, and even give the means of determining the weight of mole- 

 cules by studying the variations of the properties of a solvent on the 

 introduction of a small quantity of a substance passing into solution. 

 Although this subject has been already partially considered in the first 

 chapter (in speaking of solutions), and properly belongs to a special 

 (physical) branch of chemistry, we touch upon it here because the 

 meaning and importance of molecular weights are seen in it in a new 

 and peculiar light, and because it gives a method for determining them 

 whenever it is possible to obtain dilute solutions. Among the numerous 

 properties of dilute solutions which have been investigated (for instance, 

 the osmotic pressure, vapour tension, boiling point, internal friction, 

 capillarity, variation with change of temperature, specific heat, electro- 

 conductivity, index of refraction, &c.) we will select one the 'depres- 

 sion' or fall of the temperature of freezing (Raoult's cryoscopic method), 

 not only because this method has been the most studied, but also 

 because it is the most easily carried out and most frequently applied 

 for determining the weight of the molecules of substances in solution, 

 although here, owing to the novelty of the subject there are 



