410 



Paris green, having a definite cheniical composition, slionld be prac- 

 tically unitbrui in the percentage of arsenic. An excess of arsenic is, 

 therefore, as mnch an indication of inipnrity as a deficiency. The 

 great advantage in the use of Paris green conies from the fact of its 

 Insolubility in water. Wlierever tliere is a great excess of arsenic 

 this occnrs in a free soluble form, and as it is a very decided source of 

 danger to plants it is highly objectionable. It is a comparatively easy 

 matter to increase the percentage of arsenic, and if the Paris green be 

 improperly made it frequently happens that unchanged arsenious oxide 

 occurs throughout the mixture in the form of small particles coated 

 with the Paris green, and capable of being dissolved in water and exert- 

 ing an injurious action on plants. This fact has been lost sight of in 

 many of the analyses and regulations on the subject, as, for instance, 

 in the laws of Louisiana bearing on this point. The law referred to 

 requires the manufacturer or dealer to guarantee the percentage of 

 arsenic claimed, and provides severe penalties lor any sales of insecti- 

 cides contaiuing a less percentage of arsenic than is indicated on the 

 official label. By the provisions of this law this arsenical is separated 

 into two classes, one containing 50 per cent or more of arsenic, to be 

 labeled "strictly pure," and the other containing less, to be labeled 

 "impure." As a result of this and similar provisions Paris green was 

 occasionall}" adulterated in the sense of being made to contain an excess 

 of arsenic, this working no hardship to the manufacturer, since white 

 arsenic costs only 3 or 4 cents per r)ound, while Paris green sells whole- 

 sale for 20 cents or more per pound. For mutual protection, now, how- 

 ever, the leading manufacturers of this article have combined to prevent 

 adulteratious, and as it now comes from original hands it may generally 

 be relied on as being properly compounded. Out of sixteen analyses 

 by experiment station chemists to which I have access the percentage 

 of arsenious oxide in Paris green ranges from 53 to 63 per cent, the 

 samples averaging 58 per cent, which is the proportion of arsenious 

 oxide indicated bj^ the chemical fornuila. 



Tiie last step in the process of manufacture of Paris green is the com- 

 bination with acetic acid. What value this may have on the substance 

 as an insecticide is not at once a])parent, and at our request the manu- 

 facturer was good enough to make 100 pounds of arsenite of copper 

 prepared from the sulphate of copper as above but omitting the acetic 

 acid. This is not a crystalline product but an impalpable powder, and 

 in this respect is far superior to any other powder insecticide known to 

 me. It is much finer than Loudon purple, and remains in suspension 

 almost perfectly, not settling completely until after the lapse of twenty- 

 four hours. It is the color of Paris green, but is not of as bright a tint. 

 The Department Chemist reports that the arsenic in this compound is 

 practically insoluble in water, but is soluble in acetic acid and would, 

 therefore, likely yield to weak vegetable acids or other solvents, such 

 as the gastric juices of insects. The percentage of arsenic is practi- 

 cally the same as that contained in Paris green. Experiments to be 



