FUNGICIDES 293 



for use in making Bordeaux unless the proportion of lime 

 is increased. Calcium carbonate does not form a compound 

 with copper sulphate. 



The active agent in killing fungi is copper sulphate, but 

 the use of copper sulphate even in dilute solution is attended 

 with so much danger to foliage that Bordeaux, which is a 

 suspension of an insoluble copper salt in water, is far pre- 

 ferable. The bluish-white particles are distributed over the 

 surface of leaves, and under the action of water and carbon 

 dioxide very small quantities of copper sulphate are formed, 

 sufficient to kill fungous spores and germ tubes. Calcium 

 carbonate is also formed. 



Under certain climatic conditions, such as long periods of 

 damp weather, but no rain, enough copper sulphate may be 

 formed to burn the leaves. Even under these conditions, 

 however, the presence of an excess of lime prevents this 

 danger. The formulas call for more lime than is necessary 

 to form the compound with copper. 



Bordeaux may be mixed with arsenicals— Paris green and 

 lead arsenate — to good advantage so as to have both an 

 insecticide and a fungicide in the same spray. Paris green is 

 benefited by this mixing because the excess of lime neutralizes 

 the free arsenous acid so readily formed in plain Paris green 

 applications. 



(c) Corrosive Sublimate, Mercuric Chloride, HgCl 2 , 

 is a very powerful fungicide and antiseptic. Furthermore, 

 it is very poisonous to human beings and should be used 

 with great care. A solution of 1 part sublimate to one 

 thousand parts of water is a common strength to employ. 

 Since mercuric chloride corrodes metal the solution should 

 be made up in a wooden pail preferably. Sometimes it is 

 purchased in tablets mixed with ammonium chloride which 

 increases the solubility of the mercuric chloride by the forma- 

 tion of a double mercurammonium chloride, HgCl 2 .(XHiCl) L >. 



(d) Formaldehyde is a gas, CH 2 0, at ordinary tempera- 

 tures. It dissolves in water readily, and in commerce is 

 sold in the form of a solution containing approximately 

 38 per cent, by weight. Formalin is the trade name 

 of such a solution made by one German firm only. It 



