PROPORTION OF THE RUBBERS. 55 



"The latex of Apocynum differs from that of Asclepias in that it coagulates 

 spontaneously, even if it is kept in closed containers. The spontaneously 

 coagulated latex gives: 



Per cent. 



Liquid portion 67 . 58 



Cheese (wet) 32.42 



"The liquid is white, slightly acid, and [of an] acrid odor. This hquid failed to 

 coagulate after addition of more acid. Slight excess of alkali increased its 

 viscosity, changed its color from white to brownish yellow, but did not 

 coagulate '^r precipitate it. Boiling had no effect. Excess of acetone gave 

 a finely divided precipitate, the particles of which were not cohesive. Evapo- 

 ration of the mixture, after washing with water and treatment with boiling 

 acetone, gave a small quantity of black, soft rubber, destitute of strength. 

 The cheese was composed of: 



Per cent. 



Water 33.46 



Rubber 3 .99 



Resin 62.95 



"Working up this cheese of the plant in the usual manner with solvents, 

 straining through gauze to remove dirt, evaporation with low heat, the excess 

 of solvent, adding an excess of precipitant, washing the precipitant and dissi- 

 pating the precipitating agent, gave a good grade of rubber. 



"The rubber obtained in this manner is black, firm, not tacky, odorless, and 

 strong. In quality it is much better than the product obtained from its 

 neighbor, milkweed." 



In drawing conclusions from the above ]|;emarks it should be remem- 

 bered that no thorough test has as yet been made of the rubber either 

 of Asclepias or of Apocynum, and that considerable variation may be 

 expected in the rubbers prepared from different species. The collec- 

 tion of material in quantity for the purpose of extracting enough of the 

 rubber to permit of extensive tests is now being undertaken. Several 

 of the more important species will be studied in this manner. 



