60 SECOND REPORT ON ECONOMIC BIOLOGY. 



the quality of the sulphuric acid, and in this country growers have 

 undoubtedly obtained poor results or burning of the foliage owing to 

 the use of acid containing nitric acid. The writer always demands 

 93 per cent, pure, free from nitric acid or arsenic ; others order com- 

 mercial sulphuric made from brimstone. 



Before discussing the question of the quantities of the chemicals 

 necessary to generate the gas without waste, it will be as well to 

 point out : 



First as regards the water. Unless there is a sufficient quantity 

 of water present, the potassium sulphate produced as a by-product 

 from the reaction of the sulphuric acid upon the cyanide, is not com- 

 pletely dissolved, and it forms a coating on the pieces of cyanide, thus 

 preventing the reaction of the acid. Where large quantities of 

 cyanide are used in a single vessel there should always be an excess 

 of water. Again, if there is an excess of acid the same results obtain 

 and we have partial crystallization of the potassium sulphate, remain- 

 ing as a congealed residue, which ultimately becomes a solid. 



On questioning many who have tried fumigating w r ith cyanide of 

 potassium I find conditions such as the above usually prevail, clearly 

 indicating that the proper quantities of the chemicals are not used, but 

 in the majority of cases burning or scorching of the foliage is the chief 

 complaint. 



QUANTITIES TO BE USED. 



Much of the present uncertainty and poor results are undoubtedly 

 due to the diversity of opinion that exists as to the quantities neces- 

 sary per 100 cubic feet of space. 



Mr. Froggatt (in Hit. March i8th, 1911), states: " Our formula 

 both here and in California, after many experiments, which give the 

 best results and no waste, is i oz. cyanide, i oz. acid, 3 ozs. w r ater, 

 to every 100 cubic feet of space. 



In the Board's Leaflet No. 188, the standard recognized is i, ij 

 and 3!, but this is qualified by the statement : " The proportions of 

 cyanide, sulphuric acid, and water to be used, and the amount of space 

 per unit of cyanide, vary slightly as recommended by different 

 authorities, three different workers recommending i oz. of cyanide of 

 98 per cent, purity to every 200, or 300, or 500 cubic feet of space 

 respectively. 



