168 



acid and the corrosive sublimate are, on the evaporation of 

 the spirit, left as a very fine covering of poison on the 

 entire treated surface. The poisoning has to be repeated 

 from time to time, according to climate and the abundance 

 of insects. Books thus treated are quite safe to handle. 



Corrosive sublimate, used for disinfecting seed for 

 planting, protects it from subsequent insect attack by 

 means of the fine deposit of poison on the surface of each 

 seed. 



Corrosive sublimate also has a strong effect as a repel- 

 lent for ants. The legs of a table or safe washed with any 

 solution or mixture of this poison are quite safe from 

 attacks as long as the poison remains. Ants apparently 

 will not cross any surface treated with corrosive sublimate. 



Boric or boracic acid. This is a specific as an insecti- 

 cide for cockroaches. Although not a poison in the ordinary 

 way, this substance seems to have a particularly deadly 

 effect on these insects, and it is of course especially valu- 

 able for use in dwelling houses, since cats, dogs, fowls and 

 even human beings are not injuriously affected by it if it 

 should be eaten by them. 



Equal parts of boracic acid and sweet chocolate, flour, 

 molasses or sugar, put down in places frequented by cock- 

 roaches, will be effective in reducing their numbers. When 

 one mixture has been used for a short time, a change to 

 another of those suggested will be found of advantage. 



Phosphorus. This substance is often used in the pre- 

 paration of rat poison and in poison, for crabs. Hat poison 

 containing phosphorus is eaten to some extent by cockroach- 

 es, apparently with effect. 



The insecticides mentioned already are mineral poisons. 

 Several vegetable substances are useful as insecticides, the 

 chief of which are pyrethrum, tobacco and hellebore. Vege- 

 table insecticides lose their power when exposed to the air 

 for any time, and on account of this tendency, are likely to 

 deteriorate in the tropics. Pyrethrum is composed of the 

 finely ground dried flower heads of a plant of the Chrysan- 

 themum group Pyrethrum roseum. Pyrethrum, Dalma- 

 tian and Persian insect powder, and Buhach are names 

 given to this, and to other products from closely related 

 plants. 



