24 



quantity of water can be combined with the soap; with a good hard 

 soap, for instance, 3 on can still have a soft soap, a gelatinous mixture, 

 after having added ten or fifteen times the weight of the soap in water. 

 But such a soft soap or soap-and-water mixture can not be used with 

 any hope of good results in following out formulas which call for so 

 many i:>ounds of soap. The result of our experiments was that home- 

 manufactured soap is much more expensive than soap bought in com- 

 paratively large quantities from regular makers, at a price, say, of 4 to 

 6 cents per pound. These mainifacturers have the advantage of 

 purchasing ingredients in very large amounts, and the comi)etition is 

 such that the price of the soap to the consumer is but a slight increase 

 over the cost of the ingredients. When the soap is purchased in 

 1 pound lots, put up in neat druggists' pai)ers, four or five prices are of 

 course asked. 



ARSENATE OF LEAD. 



This insecticide, introduced by the Gypsy Moth Commission, is so 

 promising in many respects that it is well worth a place in the list of 

 standard mixtures. It has in general been very satisfactory in its 

 effects on insects, and is totally without action on i)lants at any strength 

 whatever, even when applied almost as a sirup. The chemical nature 

 of this substance has been investigated by Mr. K. P. McElroy, of the 

 Division of Chemistry, and his report, hereto appended, shows in gen- 

 eral the nature of the insecticide, and particularly the probable action 

 of an excess of lead acetate, which, at first thought, might not be 

 deemed essential to the insecticide value. Mr. McElroy says: 



111 accordauce with your request, I have looked up the matter of the iuteraotion 

 of lead acetate and sodium arsenate. In the presence of au excess of the former, 

 such as you employ, the reaction proceeds as follows: 



(Pb (C.H202)2 ■^B.iO)i + (Na-^HAsO, UR.^O)^ = Pb3(A804),! + (NaC2H30.;)4 + (HCzHsOaJa + (H20)3 



Expressed otherwise, 1,137 parts by weight of crystallized lead acetate (contain- 

 ing 14.25 per cent of water) react with 804 parts of crystallized sodium arsenate 

 (53.73 per cent of water), both being dissolved in water to form 899 parts of lead 

 arsenate, 328 parts of sodium acetate, 120 parts of acetic acid, and 594 parts of 

 water. The lead acetate, being wholly insoluble in water, separates out, but the 

 other bodies remain dissolved. 



Whether lead arsenate suspended in a solution of the above substances plus an 

 excess of lead acetate would prove more advantageous as an insecticide than would 

 the equivalent amounts of lead arsenate and lead acetate used separately I am not 

 prepared to say. That is matter for direct experiment. As a solution of lead acetate 

 dries down to a varnish-like film, it may prove advantageous in a mechanical way 

 in keeping the arsenate on the leaf. 



CYANIDE OF POTASSIUM. 



The use of this substance in generating hydrocyanic acid gas for the 

 treatment of scale insects has been introduced in the East, and has 

 been particularly experimented with at Charlottesville, Va. Some 

 additional experiments were al^o made on various scale insects on the 

 Department grounds. The action of the gas was very unsatisfactory. 



