10 Feb., 1919.] Copper Fungicides for Tine Diseases. 109 



nearer one keeps to the lower limit of 3.2 lbs. of soda, the higher the 

 proportion of tetracupric sulphate. Alkaline copper soda consists mainly 

 of copper hydrocarbonate. In all three there will be also sodium sul- 

 phate. As this salt is inactive, its presence may be ignored. The respec- 

 tive values of the different types of copper soda are intimately bound up 

 with the fungicide and other qualities of the two leading constituents, 

 viz., tetracupric sulphate and copper hydrocarbonate. The weight of 

 evidence is entirely in favour of the former, which, under the action of 

 the ammonia, but more particularly of the carbonic acid present in the 

 air, gradually gives up small quantities of soluble copper; it is to these 

 that the protection , of the vine from fungus contamination is due. 

 According to most authorities, tetracupric sulphate combines in. a high 

 degree the desiderata of considerable stability and durability of action. 



One of the chief faults of copper hydrocarbonate is its liability to 

 burn the tissues of the vine ; damage to foliage is, in fact, nearly always 

 due to copper carbonate in one of its forms. It is, in fact, for this reason 

 that copper soda is more severe on the tissues of the vine than Bordeaux 

 mixture. 



Malachite Deterioration. 



The transformation of copper hydrocarbonate into malachite is one 

 of the chief defects of alkaline copper soda. 



. . . a change of more or less sudden nature eventually occurs. The blue, bulky 

 precipitate of SCuOj, CO5, changing into malachite 2CuO, CUj, which is green 

 and dense, settles very quickly and occupies but a small volume, being therefore 

 very unsuitable as a spray material. At the same time, nearly all the copper 

 which was dissolved in the liquid becomes converted into malachite also, so tiiat 

 this liquid retains only such quantities as can be accounted for by the solubility 

 of the malachite itself, and this amounts to only 0.001 to 0.006 per cent., depend- 

 ing on the degree to which the liquid is saturated with carbon dioxide.* 



This change to malachite, which is really a dehydration, is promoted 

 in several ways. It is favoured by high temperature, and usually com- 

 mences at about 86 degrees F., a temperature which often prevails when 

 spraying is in progress in Northern Victoria. It is also facilitated by 

 the addition of a small quantity of previously formed malachite, which 

 acts as a starter, hence the recommendation often made to keep all 

 vessels used in the making of copper soda scrupulously clean. 



This change is also most rapid in an alkaline medium, an argument 

 which has frequently been advanced in text-books against alkaline or 

 basic copper soda. 



The Different Forms of Soda. 



In the foregoing remarks on the chemical aspect of the question, the 

 quantities of soda mentioned refer to chemically pure anhydrous soda 

 carbonate, in other words, I^aaCOg of 100 per cent, purity, and not to 

 washing soda, though this last is the form commonly used in the orchard 

 and potato field, f 



Washing soda is, however, a most undesirable form for the purpose, 

 owing to the variability of its content of pure soda carbonate. When 

 pure and freshly prepared, it answers to the formula — 



Na CO , lOH O, 



2 3 2 ' 



in other words, it contains 37.04 per cent, of pure anhydrous soda car- 

 bonate and 62.96 per cent, of water of crystallization. The large crystals 



• Eleventh report of the Wobiirn Experimental Fruit Farm (1910), p. 89. 

 + The well-known 6-8-50 formula means 61bs. l)hiestone; 81bs washing soda ; and 50 gallons of water. 



