April 1907,] 



199 



Saps and Exudations, 



RESEARCHES ON THE COAGULATION OF THE LATEX. 



With the view of defining the conditions under which coagulation takes 

 place and the influence of certain factors on the properties of pure rubber, as well as 

 to study the reaction of vulcanization, V. Henri has carried out a series of experi- 

 ments with latex from the Hevea brasiliensis, which are fully described in "Le 

 Caoutchouc et la Gutta Percha." The latex was slightly alkaline, had a specific 

 gravity of 973, contained 8'7 grams of solids per 100 c.c, and its specific electric 

 conductability at 25 deg. was - 0033, equivalent to that of a solution of sodium 

 chloride of a strength of 25 grams per 100 c.c. As will be seen later the concentra- 

 tion and nature of the salts in a latex influence its coagulation. 



The microscopic examination of the latex revealed the presence of a large 

 number of globules, some with a diameter of nearly 0'002 millimetres, others smaller, 

 the latter exhibiting extremely intense and persistent Brownian movements. It is 

 interesting to note that the number of globules in a latex, ascertained in the same 

 manner as the enumeration of corpuscles in blood, indicates the richness of the latex. 

 The operation is simple. The most suitable diluent in this case was found to be a 20 

 per cent solution of sodium chloride, which arrest the Brownian movements, with- 

 out precipitating or coagulating the latex. An average of fifty million globules per 

 cubic millimetre was obtained. 



Regarding the coagulation of the latex there exists a series of bodies which 

 readily cause coagulation in some, but have no effect on others. The reason of this 

 has not yet been discovered. The coagulation of the latex has been compared with 

 the coagulation of albuminoids, it even being surmised that these bodies are essential 

 to the process. Auother view is held by the author, namely, that the latex is a sus- 

 pension of very fine particles in aqueous liquid more or less rich in saline or organic 

 bodies. When, then, coagulation occurs the rubber globules unite. Further, recent 

 researches have shown that the same general law governs both the precipitation of 

 colloidal solutions and the agglomeration of fine suspensions. A comparison between 

 the two is therefore possible, so that the same methods of investigation are equally 

 applicable to both. 



Now electrolytes cause the precipitation of colloids, therefore in studying 

 this precipitation it is essential to work with solutions as pure as possible. It is the 

 same with the coagulation of the latex of rubber ; other bodies dissolved therein must 

 first be removed. Accordingly, the latex was dialyzed, when after fifteen days it 

 was found to contain no appreciable quantity of salts, since its electric conduct- 

 ability was very similar tojthat of distilled water and the depression of its freezing 

 point, which previously mounted to - 22 deg., was less than 0-01 deg. 



On adding different reagents to the latex one of three things may occur :— 



1. There is no reaction. 



2. Isolated flakes varying in size are formed which either rise or sink, but do 

 not unite, being readily separated by stirring. This may be termed the agglutination 

 of the latex. 



3. A network ol long threads encircling all the globules of the latex is 

 observed. On stirring, the threads reunite, forming a solid elastic coagulum. This 

 is the true coagulation of the latex. 



The effect of a large number of reagents on the dialysed latex was studied, 

 both individually and mixed, with the following results. 



Methyl, ethyl, and amyl alcohols produced no reaction. Hitherto alcohol has 

 been considered a coagulant, but its action evidently is due to salts present in the 

 latex. Sodium potassium and ammonium salts also have no effect. Salts of calcium, 

 barium and magnesium in sufficient quantities cause agglutination. Salts ot 



