40 



Messrs. Gladstone and Hibbert on the Molecular 



paper (Chem. Soc. Journ., July 1888, p. 679). That in solution 

 (6) was obtained from Para rubber, by dissolving it in ether, 

 and precipitating the etherial solution with alcohol. Solution 

 (c) was prepared from (b) by evaporation in a current of 

 hydrogen. The greater depression observed can hardly be 

 ascribed solely to the greater strength of the solution, since 

 that would only give a proportionate effect. We are inclined 

 to think it possible that there was a lowering of the molecular 

 weight during a three days' gentle heating which was 

 incidentally necessary. The observation, in fact, seems in 

 harmony with other alterations in physical properties which 

 we have sometimes noticed. 



This very high molecular weight for caoutchouc strengthens 

 a previous impression of ours that caoutchouc belongs to the 

 class of substances known as colloids. The impression arose 

 from the fact that caoutchouc is a substance showing not the 

 least tendency to crystallize, which cannot be distilled with- 

 out decomposition, which is subject to great alteration of 

 properties by the action of heat, which is converted into an 

 insoluble modification by small quantities of certain reagents, 

 and which dissolves in its solvents in an extremely sluggish 

 manner. 



Graham, in his classic memoir on the subject * of Colloids , 

 observed that "the equivalent of a colloid appears to be 

 always high;" and he also suggested that the colloid molecule 

 may be " constituted by the grouping together of a number 

 of crystalloid molecules. 3i 



It seemed worth while therefore to examine bodies com- 

 monly regarded as colloidal by Eaoult's method. The follow- 

 ing table gives the results obtained with aqueous solutions of 

 organic colloids, the molecular weights being reckoned for 

 the ordinary value for T given by Raoult in the case of 

 water : — 



Substance. 



Weight in 100 

 grams of Solvent. 



Depression. 



Molecular Weight. 

 T=19. 



Gum arabic 



Ditto purified ... 

 Caramel 



31-6 

 14«0 



8-76 

 225 

 20 



o 



0-3 

 0165 

 0105 

 0-245 



Scarcely observable. 



2001 

 1612 

 1585 

 1745 



Extremely high. 





Albumen 





* Phil. Trans. 1801, pp. 183-224. 



