APPENDICES. 147 



ascending vapour carried with it some of the unchanged cyanide solution, 

 since it was clearly apparent that the gas was more injurious to the 

 foliage when generated rapidly than when it was produced more slowly. 

 Profiting by this discovery, I next tried acting with the acid upon the 

 dry, finely pulverized cyanide, and the result proved that the gas thus 

 produced was less injurious to the foliage than when generated in the 

 usual way. It still injured the leaves to a certain extent, due, as it 

 appears, to the fact that the ascending gas carried with it some of the 

 fine particles of the cyanide and lodged them upon the leaves. My next 

 step was to use the cyanide in large pieces instead of pulverizing it, and 

 the gas thus produced did not injure the tenderest leaves of orange trees, 

 even when confined in it for an hour. The proportion of ingredients 

 used was about two fluid ounces of sulphuric acid to each ounce of the 

 potassium cyanide. 



Muriatic acid may be used instead of the sulphuric, but it is not as 

 strong, besides costing more. Only the best grade of the cyanide, such 

 as that commonly used by photographers, can be used for this purpose, 

 since the cold acid will not act upon the poorest grade, which is 

 commonly used for mining purposes ; and this remark is equally true in 

 regard to both of the processes described below. 



The Dry Gas Process. I have already alluded above to the fact that 

 the drier the gas the less injurious was the effect upon the tree confined 

 in it ; and it occurred to me that the gas might be generated in the 

 usual way, by acting with sulphuric acid upon potassium cyanide 

 dissolved in water, and afterwards be dried by passing it through some 

 medium that would deprive it of its moisture. Knowing the great 

 avidity of sulphuric acid for moisture, I determined to use it as a drier 

 for the gas, and several tests which I have made with this gas dried in 

 this way proved that it does not injure the foliage of orange trees con- 

 fined in it, while it is just as fatal to the scale insects as is the moist 

 gas. The density of the acid through which the gas had passed was 

 lowered about one degree, as indicated by the hydrometer ; but this 

 would not prevent its use for generating the gas. 



The cyanide is dissolved by boiling in water for a few minutes, using 

 1 gallon of water for each 5 Ibs. of cyanide. It is desirable to use as 

 little water as possible for this purpose, but the quantity could not be 

 very much reduced from that given above. I have tried to dissolve 

 5 Ibs. of the cyanide in half a gallon of water, but all of the cyanide had 

 not dissolved after half-an -hour's boiling. For every ounce of the cyanide 

 solution use half-an-ounce of sulphuric acid ; but it is always desirable 

 to add some of the acid to the prescribed dose, in order that there may 

 be an excess of the acid. No evil results will follow if double the proper 

 quantity of the acid were to be used ; whereas if less than the proper 

 quantity were used, the whole of the gas would not be evolved from the 

 cyanide solution, hence the advisability of always using an excess of the 

 acid. 



In generating the gas the acid should flow upon the cyanide solution 

 in a very fine stream. When they came in contact, violent action at 



K 2 



