294 INSECTICIDES AND FUNGICIDES 



has no advantages whatever over any 38 per cent, solu- 

 tion made by other reputable manufacturers. Formal- 

 dehyde may be used in the liquid form by proper 

 solution or in the gaseous form by liberating it in a closed 

 room. The evolution of formaldehyde may be produced 

 by heating the solution under pressure; by simple evapora- 

 tion from large surfaces; by treatment with potassium per- 

 manganate whereby part of the formaldehyde is oxidized 

 and the heat of oxidation volatilizes the remainder; or 

 by burning the so-called formaldehyde candles. The latter 

 are made of paraform which is a condensation product 

 of formaldehyde made by spontaneous evaporation of the 

 solution. The compound is supposed to be trioxy methylene, 

 (CH 2 0) 3 . On heating this white solid, formaldehyde is 

 evolved. 



(e) Lime-Sulphur Boiled, being an insecticide as well 

 as a fungicide, was discussed under the former head 

 (Section 216 II, c). 



(/) Lime-Sulphur Self Boiled is essentially a mechanical 

 mixture of sulphur and slaked lime. It is prepared 

 by adding, for example, 6 pounds of sulphur, finely 

 ground, to 6 pounds of lime which has just started to 

 slake. The mixture is stirred and more water added 

 until the mass boils, due to the slaking lime. The boiling 

 is continued five or ten minutes and then the remainder 

 of the water — to equal 50 gallons in all — is added. This 

 cools down the mass. The violent boiling has thoroughly 

 mixed the sulphur and slaked lime and formed very little 

 calcium sulphides. But at the strengths applied very little 

 sulphides are desired, or leaf burning results. This is why 

 the boiling is checked by the cold water. The mixture is of 

 course a suspension and must be applied with an agitator. 

 Lead arsenate and some other insecticides have been mixed 

 with the self-boiled lime-sulphur with good results. Some 

 chemical changes take place, due to the calcium sulphides 

 in solution. 



218. Summary. — The spraying of plants for the control 

 of insects and fungi is now one of the most important phases 

 of agriculture, particularly for horticultural crops. For 



