14 RUBBER-CONTENT OF NORTH AMERICAN PLANTS. 



fats, resins, and similar substances. It is reported in the following 

 tables, under the heading of "acetone extract." Since rubber itself is 

 partially soluble in boiling acetone when fats are present, it is possible 

 that small amounts are included in this extract, but since the parts 

 of the plants examined presumably contain but small amounts of fats, 

 it is believed that the error due to this is negligible. 



(6) Extraction with 10 c. c. of boiling benzene for 3 hours, prefer- 

 ably on an electric plate. The material remaining in the thimble 

 after the extraction with acetone as just described was thoroughly 

 dried, the thimble with its contents was placed in a clean flask, and 

 subjected to boiling benzene. At the close of the extraction the ben- 

 zene was distilled off and the residue dried in a water-jacketed oven, 

 cooled in a desiccator, and weighed. This residue was taken to be 

 rubber and is so reported in the following tables, since all other com- 

 pounds soluble in benzene had been previously removed by the acetone. 

 In some cases, especially those in which the percentage of the benzene 

 extract ran very low, the residue would have the consistency of thick 

 sirup and lacked elasticity. It seems not unlikely that in some species 

 which carry less than 1 per cent, as given in the tables, the material is 

 not really rubber, but other substances that escaped extraction by 

 acetone, perhaps mixed with more or less fat. On the other hand, 

 when larger amounts of benzene extract were obtained, the material 

 would possess elasticity, resilience, and other properties of crude com- 

 mercial rubber. Whether or not the substance is in reality pure rubber 

 depends somewhat upon the definition of that word, but since the 

 methods of analysis used are those generally employed by rubber 

 chemists, and in consideration of the physical properties of the product, 

 there is every reason to believe that the percentage obtained in each 

 case represents approximately the amount of crude rubber present, 

 and it is so reported in the tables of analyses. 



In regard to the accuracy of the results, it may be stated that, in 

 view of the partial solubility of rubber in acetone when fats are present, 

 and because of the difficulty in attaining perfection in the extraction 

 from plant tissues, the reported percentages of rubber may be too low. 

 It is to be noted, in comparing the results with those given by rubber 

 chemists, that the latter often report the percentage of crude rubber 

 extracted by some process that carries out with it a larger or smaller 

 amount of impurities, these sometimes constituting as high as 30 per 

 cent of the yield. Even by the method used in the present investi- 

 gation, a varying amount of impurity doubtless finds its way through 

 to the benzene extract, so that the result is not a pure hydrocarbon. 

 However, the percentages reported are believed to'approximate those 

 obtained by commercial processes, and in any event they may be 

 used for purposes of comparison between the species and forms. 



Duplicate analyses were run of most samples, and when unusually 

 high results were obtained they were checked by a larger series of 

 examinations. The close agreement in the results obtained adds to 

 the confidence in the method employed. 



