REPORT OF COMMITTEE ON PUBLICATION 



157 



Table VI. Relative adhesiveness of 2 per cent Bordeaux mixtures prepared 

 with various ratios cupric sulphate to calcic oxide. 



Strength in 



Copper 



Sulphate 



Per Cent 



2 



Ratio 

 Cupric Sulphate Adhesiveness 



Calcic Oxide 



1:0.2 approx. 



1:0.3 approx. 



1:0.5 



1:0.2 approx. 



1:0.5 



1:0.3 approx. 



1.1 



1:1.5 



Relative 

 Numbers 



64.7 



93.5 

 100 



64.7 



67.8 

 100 



90.4 



56.7 



Autore 



Gastine 



Guillon & Gouirand 

 <t 



Kelhofer 



The data presented clearly show that the ratio, cupric sulphate to calcic 

 oxide, used in the preparation of Bordeaux mixture has a marked effect on 

 the adhesiveness of the wash. A moderate excess of lime produces a more 

 adhesive mixture than either a great excess or minima quantities. To what 

 are these changes due? Let us first consider the case of neutral and ap- 

 proximately neutral mixtures. 



The decrease of adhesiveness of Bordeaux mixture as neutrality is ap- 

 proached is not due to the fact that the precipitate formed is less gela- 

 tinous than that found in alkaline washes. In fact the rate of settlement of 

 Bordeaux mixtures increases as the ratio cupric sulphate-calcic oxide 

 approaches 1:1 even when the strength of the mixture in copper sulphate is 

 sufficiently high to give rise to the formation of 10 CuO SO 3 in all cases as 

 will be seen from the following table. 



Table VII. Settlement, in relative numbers, of 1 per cent Bordeaux mixture 

 prepared with different ratios cupric sulphate to calcic oxide. 



1% 1:1 Settlement after: 



1% 1:0.5 Settlement after: 



1% 1:0.25 Settlement after: 



1% 1:0.2 Settlement after: 



% hour, 100; 1 hour, 205; 2 hours, 455. 



% hour, 72; 1 hour, 150; 2 hours, 355. 



% hour, 55; 1 hour, 122; 2 hours, 294. 



i/ 2 hour, 5; 1 hour, 10; 2 hours, 20. 



Hence the physical state of the precipitate is not directly the cause of 

 the lesser adhesiveness of "acid" and "neutral" Bordeaux mixtures. The 

 lesser adhesiveness of these washes which is, as we have seen, quite marked, 

 is, however, readily explainable. It will be remembered that the composing 

 precipitate in these mixtures is wholly gelatinous or almost wholly gela- 

 tinous, in other words the precipitate is quite homogeneous and in drying 

 will behave like a body, and a stress or strain developed at any point will 

 be accompanied by a reaction in other parts. When, therefore, "acid" or 

 "neutral" Bordeaux mixtures are sprayed on glass, the precipitate dries 

 more rapidly on the surface exposed to the air and in drying shrinks, which 

 results either in fissuring or a curling of the edges, depending on the thick- 

 ness of the film and the degree of hygroscopicity of the atmosphere. When 

 the washes are sprayed on foliage, the tendency to fissure and curl is greatly 

 augmented owing to the transpiration of the leaves retarding the drying out 

 of the lower surface of the films. I have seen spots of Woburn Bordeaux 

 mixture, for instance, curl up to such an extent that the slightest mechanical 



