54 RUBBER-CONTENT OF NORTH AMERICAN PLANTS. 



unfavorable conditions obtaining at the place just noted. The experi- 

 ment should be repeated on some of the robust stands in the alkaline 

 districts of the San Joaquin Valley and more time given the third 

 crop to fully mature its stems and leaves. 



X. PROPERTIES OF THE RUBBERS. 



The product obtained from Asclepias and Apocynum by the methods 

 used during these studies is true rubber, with only a small admixture 

 of resins, fats, or other substances such as are sometimes obtained 

 from plants and called by this name. The reasons for this conclusion 

 have been given in connection with the discussion of the chemical 

 methods employed (p. 14). Samples have not been prepared by 

 mechanical processes, but the accumulations of the benzene extract 

 resulting from the chemical analyses have been preserved for some of 

 the species. The largest accumulation (4 grams) is from Asclepias 

 sullivanti. This sample is nearly black in color and possesses elasticity 

 and some resilience. It emits the characteristic odor of rubber when 

 it is burned. During the 4 months since its preparation the surface 

 has undergone slow oxidation, as indicated by the formation of resin- 

 like granules. No attempt has been made to vulcanize the sample. 



The properties of a rubber prepared from some unknown species 

 of milkweed have been reported by A. T. Saunders (1910). The sam- 

 ple, which weighed about 2 grams, was permitted to stand for 10 years 

 before examination, after which period it was found to contain but 

 20 per cent of hydrocarbon, the remainder being vegetable matter and 

 resin-like products of oxidation. It yielded to the usual solvents of 

 rubber, resisted the same reagents, as far as tested, possessed elasticity, 

 and was susceptible of cure. When rolled into a slender thread and 

 subjected to the acid cure it was extensible to five times its length and 

 returned sharply upon release. Fox (1911) reported caoutchouc 

 obtained from Asclepias syriaca as flabby, lacking in strength and firm- 

 ness, and high hi gravity. It responded to the sulphur, chloride, and 

 bromine tests. 



The few and rather desultory examinations made thus far of Asclepias 

 rubber would indicate that it is of low grade, not to be compared with 

 fine Para, with imported plantation rubber, nor with chrysil. It would 

 doubtless find use for mixing with better grades and in those manufac- 

 tures where great elasticity is not demanded. It is possible, however, 

 that other species will yield a better product than that obtained by Fox 

 from A. syriaca. The rubber from A. sullivanti, for example, seems 

 to be better than the sample with which he worked. 



Rubber prepared from Apocynum (Indian hemp and dogbane) is 

 probably superior to that from Asclepias. Fox (1912) reports his 

 experience in the preparation of rubber from A. androscemifolium as 

 follows : 



