286 



Potato Disease. 



[AUG., 



powder it (or it can be purchased in powder) and make the 

 solution by stirring it up with a small quantity of hot water, 

 but the easiest way is to tie the sulphate of copper in a piece 

 of canvas and hang it near the top of the water from a stick 

 across the mouth of the vessel. The vessel in which the copper 

 sulphate is dissolved must be of earthenware or wood, a paraffin 

 cask is useful for the purpose, but galvanised iron would be 

 attacked by the solution. 



The lime is to be slaked in another vessel (an ordinary 

 galvanised pail will do), made up into a thin cream with water, 

 and then poured through a sieve or a piece of sacking into the 

 solution of copper sulphate. Now add the rest of the water and 

 stir well up. 



It is essential to have sufficient lime present to precipitate 

 the whole of the copper ; if any remains dissolved it will burn 

 the foliage. Secure freshly burnt iime, and, if possible, obtain 

 white " fat " lime made from mountain limestone or chalk, the 

 kind of lime which is used by plasterers. If only gritty " thin " 

 limes can be obtained it is better to follow one of the other 

 recipes without lime. In any case it is wise to test the mixture 

 • to see there is no copper sulphate present. To do this, obtain 

 from the chemist at the same time as the sulphate of copper 

 an ounce of potassium sulphocyanide. Allow the lime and 

 copper mixture to settle a little and dip out some of the 

 perfectly clear liquid at the top in a teacup ; into this drop one 

 crystal of the sulphocyanide and stir up with a clean stick or 

 spoon. If any copper be present a deep chocolate-red colour 

 will result, which means that more milk of lime must be added 

 to the mixture. 



The mixture should be well stirred up before it is poured 

 into the sprayer, which should have a dasher of some kind 

 to stir the material during spraying. The mixture will keep 

 several days, but the longer it remains unused the more 

 difficult it is to distribute it properly with the sprayer. 



As the ordinary Bordeaux mixture is easily washed off there 

 is an advantage in adding some sticky substance, especially in 

 showery weather. Treacle is perhaps the best, but it is very 

 little good to add treacle directly to the mixture previously 

 described. A better preparation is to take a little more lime, 



