180 



SCIENCE AND FRUIT GROWING 



filtering the mixture, or by allowing it to settle. Testing it by 

 inserting a knife-blade into it, and seeing if a deposit of copper 

 forms on the steel, is apt to be very misleading, for it was found 

 that steel would often remain quite bright for a long time, even 

 in a 10 per cent, solution of the sulphate. An iron wire nail is 

 preferable to a piece of steel. 



A more interesting point, however, was the change which 

 Bordeaux mixture undergoes after it has been made. The pro- 

 portion of lime to copper sulphate taken is more than sufficient 

 to form the compound F (p. 177), and that is a very compact 

 substance ; but the lime which reacts with the sulphate in the first 

 instance is limited to the amount which is actually dissolved in 



1 



FIG. 1 8. SETTLEMENT OF PRECIPITATES CONSISTING OF 

 ORDINARY BORDEAUX MIXTURE (AB) AND OF THE BASIC 

 SULPHATE ioCuO,SO 3 (A A). 



the water ; consequently, the first product is one of the less basic 

 sulphates, for the formation of which the amount of lime in 

 solution is sufficient, and these are bulky substances : but the 

 lime, having thus been removed from the water, some of the 

 excess present will dissolve in it, and this will react with the basic 

 sulphate already formed to produce the more basic compound, 

 which is much more compact. In other words, there will be a 

 considerable shrinkage in the precipitate some little time after 

 the reagents have been mixed. The line AA in Fig. 18 shows 

 the volume occupied by ioCuO,SO 3 made by adding the requisite 

 amount of lime-water to copper sulphate, whilst the line AB 

 shows that of the precipitate obtained when the lime added 

 is five times greater than that dissolved by the water, the 



