GENEEAL AND PHYSICAL CHEMISTRY NEENST. 247 



place in the first rank, but it has been left for the new epoch, by the 

 aid of the direct and indirect methods of measuring osmotic pressure, 

 to throw an equal light on the molecular weight of dissolved sub- 

 stances. In the determination of the molecalar weight of liquids the 

 value of the temperature coefficient of surface tension has been of 

 the greatest assistance; the heat of vaporization, the critical con- 

 stants, the curves of vapor tension, and a series of other values, also 

 furnish more or less sure means to the same end. 



All these methods, with complete accord, lead to the conclusion 

 that most substances, for instance the saturated hydrocarbons, pos- 

 sess the same molecular weight in the liquid state as in the gaseous 

 condition, but that a number of substances, like the alcohols and 

 especially water, are more or less highly polymerized in the liquid 

 condition. But what is the degree of polymerization in each particu- 

 lar case? ^Miat equilibrium is established in these pure liquids? 

 These interesting questions unfortunately still evade any exact or 

 quantitative solution. 



As the most important results of these new methods of determin- 

 ing molecular weights, especially of the osmotic method, we may 

 mention first the ingenious theory of the existence of colloidal solu- 

 tions as an intermediate stage between true solutions and mechanical 

 suspensions, and then the more definite conception of the ion to which 

 we shall return later. 



A second series of properties are those designated under the name 

 of additive properties. Any such property of a combination is the 

 resultant sum of the properties of its constituents. To all appear- 

 ances this is a very simple rule, but at the same time it is impossible to 

 form any idea from it as to the size and structure of the molecule. 

 Among these properties we may note, beside the molecular volumes 

 of liquid organic compounds, molecular refraction, magnetic rota- 

 tion, heat of combustion and the critical coefficient. 



A third series of properties depends not only on the nature of the 

 atoms which exist in the molecule, but also on their arrangement in 

 the molecular structure ; accordingly they are designated constitutive 

 properties. Thus the molecular refraction of hydrocarbons depends 

 not only on the number of hydrogen and carbon atoms present, but 

 also on the existence or nonexistence of multiple bonds between the 

 carbon atoms. 



From this we derive a very important result ; we may attribute to 

 the double bond a determinate amount of refraction and take into 

 account with a high degree of approximation, the influence of its 

 constitution, by a return to the additive method. 



Frequently certain properties appear only with a particular group- 

 ing of atoms. In such case it is a qualitative rule of the highest 

 88292— SM 1908 17 



