594 Journal of Agriculture, Victoria. [10 Oct., 1918. 



used, and to cease adding the milk of lime as soon as the paper shows 

 the first signs of an alkaline reaction. If the lime be of poor quality, 

 more than one pound may be required to neutralize 2 lbs. of copper 

 sulphate ; with chemically pure lime, considerably less than one pound 

 would be required. The proportion of lime required thus varies greatly 

 according to its quality. Quicklime changes rapidly on exposure to 

 air; it first becomes slacked, and subsequently carbonated. 100 lbs. 

 of pure quicklime after becoming slacked would weigh 135 lbs. ; 

 when completely carbonated it would weigh 178.6 lbs. Air-slacked lime, 

 provided it be not carbonated, is just as suitable as quicklime, though 

 more of it must be used. Once lime has become carbonated it is no 

 longer fit for the preparation of Bordeaux Mixture ; with it, the chemical 

 reactions which take place are different, and the precipitate varies con- 

 siderably in its nature. Amongst other substances, it contains carbon- 

 ate of copper, a substance which has a severe action on the green tissues 

 of the vine. Hence it is that Bordeaux Mixture prepared with faulty 

 lime (too old), even if this be used in the proportion shown to be neces- 

 sary by test paper, often burns the foliage of the vine. 



In France, all authorities are agreed that 2 per cent, of copper sul- 

 phate crystals is the standard strength for Bordeaux Mixture; it is 

 considered unsafe to place reliance on anything weaker, especially in 

 the case of severe Mildew outbreaks. The recent high price of copper 

 has led to weaker mixtures having been somewhat largely applied during 

 the past couple of years in France. Though satisfactory protection has 

 sometimes been obtained with 1^ per cent., and even with 1 per cent. 

 Bordeaux, the results of a reduction in strength have often led to grave 

 disappointment. Indeed, in the disastrous Mildew visitations of 1910 

 and 1915 in France, an increase of the copper sulphate strength to 

 3 per cent, was often recommended and applied with most satisfactory 

 results. If the copper percentage be reduced below the standard men- 

 tioned above,' the duration of protection is correspondingly reduced. 

 With the standard strength, the vine is provided with a " reserve of 

 copper " such that, even though fairly heavy rain may fall, sufficient 

 remains on the green organs to prevent spore germination and conse- 

 quent infection. 



Chemistry of Bordeaux Mixture. 



This is not nearly so simple as was formerly thought. In many of 

 the older text books the reaction which takes place when lime is added 

 to copper sulphate solution is stated to be as follows: — 



CUSO4 + Ca(0H)2 = CaSO, + Cu(OH)i 



(Copper Sulphate) (Lime Hydroxide) (Lime Sulphate) (Copper Hydroxide) 



According to this, the lime and copper merely change places. In reality 

 the transformations which occur are far more complex. 



When lime is added to copper sulphate solution both substances 

 undergo change; the lime takes part of the sulphuric acid from the 

 copper sulphate to form lime sulphate or gypsum, but so long as the 

 lime is not in excess there is no copper hydroxide formed. The pre- 

 cipitate consists of copper in the shape of basic sulphates — in other 

 words insoluble sulphates of copper which contain more of- the metal 

 than ordinary copper sulphate (bluestone). The greater the amount of 



