174 SCIENCE AND FRUIT GROWING 



the copper of the fungicide must be soluble, and penetrate 

 into the cell, or the contents of the cell must come out, and 

 attack the copper compound. Ex hypothesi, they can not do so 

 when the spores are in the resting condition ; and, as fungicidal 

 action occurs even then, it is the copper which must be in the 

 active, i. e. soluble, condition. 



In the case of the first three compounds mentioned above, 

 this condition does exist, for they are all appreciably soluble. 

 The extent to which A dissolves in water is sufficient to allow 

 of copper being recognised in the solution of it by the ordinary 

 tests for that metal; with B and C the solubility is smaller, 

 and the copper is recognisable only by special methods. One of 

 these consists in placing a piece of bright iron (not steel) in 

 the mixture, when a deposit of metallic copper will appear on 

 the iron, and, in time, the whole of the copper will be removed 

 from the mixture, iron taking its place, and changing it from blue 

 to yellow. The rapidity with which this action occurs, in spite 

 of the excessively minute proportions in which the basic sulphate 

 dissolves (less than 0-00005 P er cent, of metallic copper in the 

 case of the compound C) , may appear surprising, but is quite in 

 accordance with the known laws of chemical action, for as soon 

 as such copper as is in solution is removed by the iron, more 

 dissolves to take its place, only to be removed in a similar manner, 

 till all has been removed. In 70 hours one three-inch iron nail, if 

 the surface be kept bright, will remove the whole of the copper 

 from 0-5 or 0-6 grams of the basic sulphate. The rapidity of 

 such actions depends on the extent of surface exposed, and that 

 exposed by the nail would be very small in comparison with that 

 offered by a tree to the same amount of sulphate sprayed on to 

 it, so that a fungicide of scarcely recognisable solubility may 

 exercise a comparatively rapid fungicidal effect (XIV, App. 276). 

 Passing on to the compound D, which is the main constituent 

 of ordinary Bordeaux mixture, it is found to be quite insoluble; 

 no action on iron takes place, and there should, therefore, be no 

 chemical action on the fungus. But, in using it as a spray-fluid, 

 it becomes exposed to the air, and it is found that when a stream 

 of air, or, better still, a stream of carbonic acid, is passed through 

 it, it, after a certain time, begins to act on iron, proving the 

 presence of dissolved copper; this may be equally established 

 by adding a drop of ferrocyanide of potassium, which now gives 

 the characteristic red colour of copper ferrocyanide, which it 

 did not do before the passage of the air. The change is easily 

 explained, for as soon as the excess of lime present in the 



