INSECTICIDES, FUNGICIDES, ETC. 179 



adhesive than either Burgundy or Bordeaux mixtures. No 

 evidence for such a statement appears to exist, and recent 

 experiments 1 indicate that precisely the opposite is the case. 

 Plots of potatoes were sprayed at the same time with these three 

 substances, and with each of them at three different strengths, 

 the copper content ranging from the equivalent of 2 Ibs. to 5f oz. 

 of the sulphate to 10 gallons : the plots were sampled, by taking 

 a leaf from each plant in them, one week after the spraying, and 

 again, three weeks later. The average relative amounts of copper 

 found on the leaves were 



Burgundy mixture .... 100 

 Bordeaux mixture . . .. .113 

 Bordeaux paste (Bordorite) . . 152 



Thus, in every case the paste left a considerably larger deposit 

 of copper than did Burgundy mixture, and in five cases out of 

 the six, Bordeaux mixture was also superior in this respect to 

 Burgundy mixture, but to a less marked extent than was the 

 paste. 



The ultimate form of the deposit on the leaves is in all cases 

 copper carbonate : with Burgundy mixture, it is the carbonate 

 itself which is applied; with Bordeaux mixture and paste the 

 carbonate is formed in situ from the decomposition of the sulphate, 

 and such a deposit thus formed, would, naturally, adhere better 

 to the leaves than the ready-made carbonate in the Burgundy 

 mixture : hence the results actually found. The reason why 

 Bordeaux mixture appears to be less adhesive than the paste, is 

 that it contains a large excess of lime (soon becoming converted 

 into chalk), the particles of which are comparatively gross, and 

 are easily shaken from the leaves, carrying with them, of course, 

 a certain amount of the copper. 



Several points in connection with making ordinary Bordeaux 

 mixture have been investigated at Woburn. The common source 

 of failure in preparing a satisfactory mixture is, of course, not 

 starting with fresh lime, or not slaking it properly before making 

 it up into a milk with water; but this is merely a question of 

 carelessness in following simple textbook instructions (XI, 50). 

 As the lime used is five times that required for precipitating the 

 copper, there should be little risk of any of the latter remaining 

 in solution ; but the mixture should always be tested ; and the 

 best way of doing so is by adding a drop of potassium 

 ferrocyanide solution to some of the clear liquid obtained by 



1 Gardener's Chroncile, Sept. 28, 1918. 



