78 ORCHARD CATERPILLARS. 



season as to how far it might be serviceable and practicable to use 

 Paris-green and soft-soap together. This matter of soft-soap and Paris- 

 green mixture was a subject of great importance, and I was not able 

 myself to offer any trustworthy opinion as to chemical changes 

 which might take place in such a mixture, or how (if these changes 

 took place) the mixture resulting might affect plant-leafage, I applied 

 for information to our excellent authority, Dr. J. Aug. Voelcker, Con- 

 sulting Chemist of the Eoyal Agricultural Society of England. He 

 kindly gave me the following clear account of the changes which occur 

 to Paris-green when acted on by op. alkali variously applied; some 

 serviceable remarks as to extent of change depending on amount of 

 free alkali, and strength of the soft-soap solution ; and likewise the 

 result of the experiments which he was good enough to make by way 

 of practical test, which information, with his kind permission, I give 

 verbatim : — 



" Aceto-arseuite of copper (Emerald-, or, as now called, Paris- 

 green), whilst it is insoluble in water, is acted upon both by acids and 

 alkalies, with the result that copper is thrown out. With an alkali a 

 blue precipitate of cupric hydrate is first formed, and if this be boiled 

 with the solution it changes to the black cupric, and then to the 

 red oxide. 



" Now, whether this takes place with mixing soft-soap solution and 

 Paris-green depends mainly upon what amount of free alkali is present, 

 and what the strength of the soap solution be. Theoretically there 

 should be some separation, inasmuch as soft-soap always has some 

 alkali. 



" I thought, therefore, the best way was to make a practical test in 

 my laboratory before writing to you, and this I have done, and may now 

 give you the result thus : — If a very strong (28 lbs. soap in 10 gallons 

 of water) solution be used, there is a certain amount of change ; but if 

 a solution of lesser strength, and more like the one recommended for 

 Hop-washing purposes (28 lbs. in 100 gallons) be used, there is no 

 appreciable change." — A. J. V. 



The above gives very clear information available for practical use, 

 as to addition of Paris-green to soft-soap in solution ; but further we 

 needed information on conditions of mixture in solid form, that is, of 

 Paris-green added to soft-soap itself, forming a kind of paste to be dis- 

 solved in water for use. 



Regarding this. Dr. Voelcker further wrote me, on the 17th April, 

 mentioning that, from my letter received that morning, he concluded 

 that a mixture regarding which I wrote was soft-soap and Paris-green, 

 not merely making a solution of the soap, and to that weak solution to 

 add the Paris-green ; and of this he observed : — " Now this is to my 

 mind distinctly open to objection, and I should not countenance it on 



