226 ELEMENTARY AGRICULTURAL CHEMLSTRY 



horse perhaps 30 grains, for a cow 10 or 15 grains, for a dog 

 about 1 grain. A uthorities, however, differ very greatly as 

 to the lethal doses. In readily soluble form — e.g., sodium 

 arsenite — it is much more powerful than as the oxide. As anti- 

 dotes, emetics should be given, followed by a dose of recently 

 precipitated ferric hydrate, prepared, as required, by adding 

 ammonia or carbonate of soda to a solution of ferric chloride 

 (" perchloride of iron "). Milk, eggs, olive oil, and barley- 

 water are also useful. 



Other poisonous substances sometimes used as insecticides 

 are : 



Carbolic Acid, phenol, CgHgOH (see p. 2 14) . This is a ^violent 

 poison, both to animals and plants, and as an insecticide has 

 to be used with care, in order to avoid injury to vegetation. 



On dormant fruit trees a wash containing about 1 lb. of the 

 crude acid, 2 or 3 lb. of soft soap, and 2 gallons of wateir is 

 sometimes used to destroy boring insects. A solution of car- 

 bolic acid is also sometimes used for preventing the attacks of 

 insects — e.g., the warble fly on cattle. 



On account of its poisonous action on plants, it is probably 

 not safe to use solutions stronger than 0*5 or at the most 

 1 per cfint. It should not be allowed to touch the foliage. 



Alkaline Sulphides. — These are very effective insecticides, 

 but are also poisonous to the roots of plants and corrosive to 

 foliage. 



" Sulphide of potash," or " liver of sulphur," is really a mix- 

 ture of sulphide and polysulphides of potassium, and is used 

 in solution for spraying trees at a strength of from 2 to 4 per 

 cent. More largely used is the sulphide of calcium, generally 

 prepared, as required, by boiling lime and sulphur with water. 

 The resulting yellow liquid contains in solution a mixture of 

 various sulphides of calcium, and often some free lime. 



This " lime and sulphur " wash or dip is largely used both 

 by the horticulturist for the destruction of scale and other 

 insects on trees and by the sheep-farmer for killing insect 

 parasites, especially scab, on his animals. 



