10 Oct., 1918.] Copper Fungicides for Vine Diseases. 595 



lime added the higher the basicity of the copper sulphate which is pre- 

 cipitated. Sulphate of lime is, of course, also precipitated, but being 

 an inert substance it need not be taken into consideration. 



If one takes a solution of 10 lbs. copper sulphate in 50 gallons of 



water, for example, and lime milk be added progressively, busic sulphates 



Avill be precipitated, with compositions approximately as follows: — * 



3.5CuO, SO3 — so long as the quantity of lime is less than 1.685 lbs. 



4CU0, SO3 Tetracupric sulphate — when the quantity of lime used 



reaches 1.685 lbs. 

 5CuO, SO3 Pentacupric sulphate — ^with 1.8 lbs. lime. 

 lOCuO, SO3 Decacupric sulphate — with 2.02 lbs. lime. 

 5CuO, CaO, SO3 Double sulphate of copper and lime — with 2.25 

 lbs. lime. 



If the quantity of lime be still further increased, double sulphates 

 of lime and copper are formed richer in lime than the one mentioned 

 above; finally, double hydrates of copper and lime make their appear- 

 ance. 



These salts are not precipitated in a state of purity for each quantity 

 of lime used; it is really a mixture of several of them, the one w'hich 

 predominates being as set out above. 



To Pickering! is due the credit of having first investigated the 

 chemistry of Bordeaux Mixture. More recent French investigators are 

 in agreement with his conclusions, so far as the composition of the 

 different substances contained in the final mixture is concerned; as re- 

 gards the fungicide value of each, or rather their preventive power, 

 there is less concordance. Pickering was of opinion that Tetracupric 

 sulphate is the most valuable of the different basic copper sulphates, 

 and that every endeavour should be made to obtain a precipitate in 

 which this salt would predominate. This would mean a slightly acid 

 mixture; in practice Bordeaux Mixture is nearly always alkaline. 



He recommended Woburn or Lime-water Bordeaux, made with lime 

 water instead of milk of lime. Owing to the slight solubility of lime, 

 Bordeaux made in this way must necessarily be far weaker in copper 

 than the standard 2 per cent, strength. Pickering held that, owing to 

 the greater efficacy of Tetracupric sulphate, the weaker mixture would 

 have equal fungicide power to Bordeaux prepared in the usual way, a 

 contention which has not been borne out in practice. Lime-water 

 Bordeaux, after extensive trial, has been found very inferior to 2 per 

 cent. Bordeaux made with milk of lime; its use cannot, therefore, be 

 recommended. J 



It is true that what is called Woburn Paste has been placed on the 

 market in England. This consists largely of Tetricupric sulphate, and 

 merely requires mixing with water before use; it can therefore be used 

 in any strength desired. Though a good fungicide, it does not appear 

 superior in any way to ordinary Bordeaux. The greater solubility of 

 Terticupric sulphate seems to be a defect rather than an advantage; 



* L. Sicard — Progres Agricole, 20th September. 1914. 



• See reports of the Woburn Experimental Fruit Farm (Sth-llth) bv the Duke of Bedford and 

 Spencer TJ. Pickering. 



t It has even been recommended to reduce the proportion of copper sulphate as low as 10 ozs. 

 to 50 gallons of spray mixture. See article on " Lime-water Bordeaux " in Journal for November, 1910, 

 by Mr. D. McAIpine. 



