XVII. MISCELLANEOUS PKODUCTS USED IN AGRICULTURE 373 



supervision of the Yorkshire College and the East and West Hidings 

 Joint Agricultural Council in 1899, seem to show that, even where no 

 charlock or runch may be present, the cereal crop is benefited by 

 spraying. 1 



For severe cases of fungoid diseases, in vines, etc., a strong solution 

 of ferrous sulphate, to which about 1 per cent of free sulphuric acid 

 has been added, is said to be very effective. 



Ferrous sulphate, like copper sulphate, is a plant poison, and its 

 success as a fungicide (indeed, probably that of all substances used in 

 that capacity) is probably due to the fact that the fungi are more sus- 

 ceptible to its action, because of their thinner walls, than the higher 

 plants. 



Mercuric Chloride, HgCl 2 , corrosive sublimate. This well-known 

 poisonous substance is one of the best disinfectants. 



Since it is practically non-volatile at ordinary temperatures, it can 

 only be applied in solution and must come into actual contact with 

 the infected material. A solution of 1 part in 10,000 is sufficient to 

 kill many micro-organisms, though some spores, e.g., those of anthrax, 

 require a 1 per cent solution. According to Lingard, 2 a solution 

 of 1 part in 960 destroys the tubercular bacillus in from four 

 to eight hours. Mercuric chloride combines with albuminoid sub- 

 stances to form insoluble compounds, and this fact sometimes inter- 

 feres with its success as a disinfectant of matter containing proteids. 

 It is said that in such cases the addition of a mineral acid, e.g., hydro- 

 chloric acid, or even tartaric acid, to the solution greatly increases its 

 effectiveness. Later experiments 3 throw some doubt on this point. 

 Mercuric chloride is a heavy crystalline substance. Its solubility in 

 water is greatly affected by temperature. 100 parts of water dis- 

 solve 



5-73 parts of the salt at 



6-57 10 



7-39 20 



53-96 100 



It is also soluble in alcohol, ether and glycerol. The salt melts 

 at 288 and volatilises at 303 ; its vapour is very poisonous. 



Mercuric chloride is largely employed in surgery as an antiseptic, 

 solutions containing from 1 in 1000 to 1 in 10,000 being used. 



Mercuric iodide, HgI 2 , and cyanide, Hg(CN) 2 , are also employed 

 as disinfectants. 



Mercury salts and mercury vapour .are very poisonous to plants, 

 mercury, even at the ordinary temperatures, giving off into the air 

 sufficient vapour to kill many plants. 4 



1 This effect may be either a direct manurial one, an indirect one by promoting 

 the disintegration of the minerals in the soil, or by increasing the chlorophyll 

 production, or, lastly, be due to the destructive action of the salt upon fungoid 

 pests. 



2 Quoted by Blyth, A Manual of Public Health, 1890. 



3 Clark, Jour. Chem. Soc., 1901, Abstracts, ii. 526. 

 4 Dafert, Jour. Chem. Soc., 1901, Abstracts, ii. 269. 



