6 



sulphate or a 3 per cent solution of copper nitrate. Copper 

 sulphate may be added with a slight advantage to a solution of 

 arsenite of soda, since, when thus used, it increases the solubil- 

 ity and burning effect of the arsenite. 



Ammonia. Ammonia has also been used as a spray for the 

 destruction of weeds but cannot be recommended for general use 

 on account of its slight burning power and the disagreeable 

 fumes which annoy the laborers. 



Arsenite of soda. This chemical is usually prepared by boil- 

 ing together common white arsenic and washing soda or sal soda. 

 Theoretically these chemicals combine in such a proportion that 

 if about 1% pounds of soda be used for each pound of white 

 arsenic, the proper combination will take place to render all of 

 the arsenic soluble by the formation of arsenite of soda. In 

 practice a considerably larger proportion of soda has been used, 

 up to 4 pounds per pound of white arsenic. It seems unneces- 

 sary, however, to use more than 2 pounds of soda per pound of 

 white arsenic. In preparing arsenite of soda we have had good 

 results from boiling a mixture of one pound white arsenic and 

 two pounds sal soda per gallon of water for from 15 to 20 min- 

 utes. The exact length of time to continue the boiling can not 

 be stated beforehand with any certainty but the boiling should 

 be continued until the solution becomes clear. The clearing of 

 the solution indicates that the proper chemical combination has 

 taken place. In Australia, where arsenite of soda has been 

 widely used for killing weeds, the common practice is to buy the 

 arsenite of soda as such on the market, but in the spraying thus 

 far done in Hawaii with arsenite of soda, it has been prepared 

 by boiling white arsenic and sal soda as just described. The 

 stock solution obtained by boiling together the sal soda and white 

 arsenic is to be diluted before spraying with 15 to 24 parts of 

 water, depending on the hardiness of the weeds to be destroyed. 



Arsenate of soda. This chemical is about equally soluble 

 with arsenite and where obtainable on the market at a reason- 

 able price may be substituted for the latter. In experiments 

 with miscellaneous weeds, Jones 1 found arsenate of soda very 



