INSECTS ON GREENHOUSE PLANTS 223 



added, it increases the heat, so that the passing off into air of 

 the hydrocyanic acid gas causes the nitric acid also to pass into 

 the air. When this condenses again on the leaves, its action 

 is vigorous and also extremely injurious, causing burning where- 

 ever it touches. 



" For the greatest economy of gas production, then, the acid 

 should be of the grade known as 66 Baume, and in addition 

 it should be entirely free from nitric acid." 



263. Proportion of the ingredients. " Of the water, little 

 need be said. It is, of course, possible that waters containing im- 

 purities might include something which could modify the chem- 

 ical process to some extent, but this will in all probability never 

 be the case." 



The proportions to use of these different materials have been 

 variously given at different times. One writer, who has been 

 widely followed, says, " A half more acid, liquid measurement, 

 than cyanide, and a half more water than acid, are used." An- 

 other suggests a 1-2-3 formula. Recent studies on this point 

 have been based on the principle that when one chemical acts 

 upon another to produce a third, the amount actually used 

 is always the same, and that the presence of more than enough 

 of either simply means a surplus of it left unchanged at the 

 completion of the combination. 



" The actual process of the combination of the chemicals is 

 expressed as follows : 2 KGN + H 2 SO 4 = K 2 SO 4 + 2 HCN. 

 Potassium cyanide + hydrogen sulphate = potassium sulphate 

 + hydrocyanic acid." 



264. The excess of acid. " If we take an ounce by weight of 

 the potassium cyanide, we find that three-quarters of a fluid 

 ounce of sulphuric acid is sufficient to give hydrogen enough to 

 combine with all the cyanogen there is in the potassium cyanide. 

 On the other hand, a little of the hydrocyanic acid may dissolve 

 in the water present, instead of passing off into the air as a gas, 

 and would thus be lost, so far as use goes. The addition of 

 another quarter of an ounce of acid will heat the water more, 

 and tend to drive the gas out, so that for this reason it seems to 

 pay to use a little excess of acid over what is needed for the 

 chemical process, to get more of the gas. 



'* Too much excess of acid, however, produces trouble in a dif- 

 ferent way. The formula given shows that potassium sulphate is 

 formed in addition to the hydrocyanic acid gas. Now, potassium 

 sulphate, though a solid, dissolves in the water present, but if 

 there is much spare sulphuric acid also present, less of the potas- 

 sium sulphate dissolves, and instead it tends to form a crust cov- 



