196 TROPICAL AGRICULTURE 



be readily coagulated by gentle heat. The same applies to the 

 highly resinous latex of Euphorbia lorifolia of Hawaii. Among 

 the chemicals which have been used in hastening the coagula- 

 tion of latex we may mention acetic, citric, tannic, and formic 

 acids, various salts of sodium and magnesium, alkaline liquids, 

 and alcohol and acetone. The two last named reagents are 

 used only in laboratory experiments, being too expensive for 

 field practice. 



In tapping rubber trees for obtaining the latex, almost every 

 conceivable manner of wounding the bark of trees has been 

 tried. The tapping systems vary somewhat, according to the 

 species of tree, and brief mention is made of perfected methods 

 in connection with the discussion of the important kinds of 

 rubber trees. If all of the kinds of tapping tools were brought 

 together they would constitute a quite formidable arsenal of 

 instruments, including hatchet-like structures, all possible 

 shapes and sizes of knives, shaves, paring utensils, and revolv- 

 ing wheel prickers. The cuts commonly made in the bark in 

 a tapping operation also vary greatly in length and arrange- 

 ment. The usual systems of tapping include long, longitudinal 

 incisions into which short, oblique incisions may run, various 

 lengths of V-shaped cuts, spiral and semi-spiral cuts about 

 the trunk of the tree, the herringbone and half-herringbone 

 system of tapping, horizontal incisions, simple shallow pricks 

 with a revolving wheel like that in a riding spur, etc. 



For many years the rubber growers have been periodically 

 worried with the bugaboo of artificial rubber. Synthetic rub- 

 ber has been produced in many laboratories in England and 

 in Continental Europe. At a recent rubber exposition in 

 London an automobile was exhibited with tires made entirely 

 of artificial rubber. It was reported, however, that these tires 

 cost about $4,000. Many patents have been taken out covering 

 processes for manufacturing artificial rubber in the United 

 States, England, Russia, France, and Germany. Some of these 

 processes are strictly secret, while others have been published 



