410 
Paris green, having a definite chemical composition, should be prac- 
tically uniform in the percentage of arsenic. An excess of arsenic is, 
therefore, as much an indication of impurity as a deficiency. The 
great advantage in the use of Paris green comes from the fact of its 
insolubility in water. Wherever there is a great excess of arsenic 
this occurs 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 for any sales of insecti- 
cides containing 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,' 7 and the other containing less, to be labeled 
"impure." As a result of this and similar provisions Paris green was 
occasionally 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 pound, 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 
adulterations, 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 by the chemical formula. 
The 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 apparent, 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 sufterior to any other powder insecticide known to 
me. It is much finer than London 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 
