GAS-LIME : ITS CHANGE AND COMPOSITION. 55 



of our cities. As for any general use for tlie protection of our farm 

 crops, the supply, at the above rate, would be entirely inadequate, it 

 might be welhto economize its use by employing it first against tho 

 insects of our gardens. 



Its change by age and exposure. — When fresh from the works 

 gas-lime cannot be used in any large quantity upon growing crops 

 without destroying them, as the large proportion (nearly one-sixth) of 

 sulphite of lime which it contains is readily washed out by rain and 

 moisture, and when carried into the soil is destructive to nearly all 

 plant growth. Upon exposure to the atmosphere, the sulphite is 

 changed into the harmless sulphate of lime (plaster). This marked 

 change which it undergoes by age and exposure is shown in the fol- 

 lowing analyses by Prof. F. W. Johnston : 



Composition of Gas-lime. 



Nearly Year or 



fresh. two old. 



Water and coal -tar 9.59 9.59 



Carbonate of lime 58.38 56.41 



Hydrate of lime (caustic) 5. 92 



Sulphate of lime (plaster) 2. 78 29.32 



Sulphite and hyposulphite of lime 15.92 .... 



Sulphur 0.92 



Prussian blue 1.80 1.80 



Alumina and oxide of iron 3.40 3.40 



Sand, etc 1.29 1.29 



100.00 101.81 



The length of exposure required to fit it for use upon growing crops 

 may best be learned by experiment. If left until it has parted with 

 nearly all of its sulphurous odor, it will no longer be of special value as an 

 insecticide ; it should, therefore, be tested from time to time and used 

 just as soon as it is found that no harm to the plant follows its appli- 

 cation. In many of the experiments made by English agriculturists 

 and gardeners, it had lain from four to six months. In this condition 

 it may be sprinkled directly upon garden plants, even upon cabbages 

 quite advanced in growth, or scattered over the soil, killing the in- 

 sects without injury to the plants. 



Valuable in preventing insect attack. — In addition to its value 

 as an insecticide and as a fertilizer (the latter directly tending through 

 vigorous growth to lessen the losses from insect attack), it may also 

 claim a high order of value in preventing the attack of injurious in- 

 sects by protecting vegetation from the deposit of their eggs. The 

 operation of this principle will be referred to at some length here- 

 after. 



