182 SCIENCE AND FRUIT GROWING 



ought to be the case) the copper solution should be added to 

 the lime- not the lime to the copper and this copper solution 

 should be as strong as possible, nearly the whole of the water 

 being previously mixed with the lime. 



The changes occurring in Bordeaux mixture are by no means 

 complete within the 18 hours allowed in the experiments 

 here illustrated, and eventually all the deposits, whatever the 

 method of mixing may have been, become similar, and consist 

 more or less entirely of the, compound F, (ioCuO,3oCaO), which 

 is of a violet-blue colour, and is very dense. This compound is, 

 apparently, a cuprite of calcium, the copper in it being in a 

 different condition (electronegative) from that of the copper 

 in ordinary copper salts, and it does not respond to the ordinary 

 tests for that metal, just as is the case with iron present in ferro- 

 cyanides. One characteristic of copper in this condition, besides 

 its peculiar colour, is that it combines with various organic 

 substances. Amongst these may be mentioned dextrose and 

 cellulose. In treacle, this compound dissolves completely, 

 forming a violet-blue solution, and such a solution attacks and 

 dissolves cellulose. Bordeaux mixture mixed with treacle has 

 been used as a fungicide, and it is probable that the action of it 

 on the fungus is different from that of ordinary copper salts, not 

 being merely a case of poisoning by copper, but of the destruction 

 of the cell walls of the fungus which are composed of cellulose 

 by the action of the cuprite. Indeed, it is probable that copper 

 when in this form has no direct poisoning effect (see p. 194). 



But Bordeaux mixture and treacle is not a satisfactory spray- 

 rluid, as it decomposes spontaneously, forming cuprous oxide, 

 and it is also decomposed by carbonic acid, with the result that 

 an unknown, and often dangerous, amount of copper in the 

 ordinary form is liberated ; for alkalinity is one condition of the 

 existence of copper in the form in which it is present in the cuprite, 

 and carbonic acid destroys this alkalinity. The compound D, 

 (ioCuO,SO 3 ,3CaO), is also alkaline, and shows properties akin 

 to those of the compound F, for, though it does not dissolve in 

 dextrose to any recognisable extent, it gradually decomposes it, 

 forming cuprous oxide (XI, 44). 



Various other alkaline solutions of copper, with ammonia as 

 the solvent, have been used for spraying purposes (XI, 16), 

 but none of them have proved satisfactory, and experiments at 

 Woburn (XI, 19, 132) have shown that the presence of ammonia 

 greatly increases the scorching effect on the leaves, without 

 increasing the fungi cidal action of the copper present. 



