62 



THE GYPSY MOTH APPENDIX. 



the composition of the commercial salt is expressed as a 

 formula, the water present may be written as (H 2 O) n , where 

 n = the number of molecules of water. 



The most economical grades of arsenate of soda for the 

 preparation of arsenate of lead are those containing the 

 highest percentage of arsenic pentoxide, As 2 O 5 , and the least 

 amounts of chlorine and other foreign substances. A high 

 percentage of arsenic acid alone is not necessarily indicative 

 of the purity or value of the salt. The great variation in 

 the composition of the different grades of arsenate of soda 

 is exhibited in Table No. 3, and is probably due to different 

 processes of manufacture, and possibly, in the case of the 

 first sample, to adulteration with salt (NaCl). 



Table No. 3. 



Sample No. 1 is known to the trade as "50 per cent, 

 arsenate soda," and is sold at a lower price than the " 65 per 

 cent, arsenate soda" represented by sample No. 2. For the 

 preparation of arsenate of lead it will suffice to determine 

 the amounts of arsenic pentoxide and chlorine present in 

 the arsenate of soda. Determinations made from four repre- 

 sentative samples of arsenate of soda follow : 



