170 SCIENCE AND FRUIT GROWING 



add in the first place only three-quarters of the amounts men- 

 tioned above, and then add more in small quantities at a time, 

 only so long as such additions cause a further cloudiness. 



When the nitrate of lead is used, instead of the acetate, a 

 different lead arsenate is formed, namely, Pb 2 H 2 (AsO 4 ) 2 , though 

 it is not very constant in composition. The proportions of the 

 reagents required in that case are ij parts of the nitrate to every 

 i part of crystallised sodium arsenate, or 2\ parts to every I part 

 of the crude arsenate. 



It was found that, contrary to statements made in various 

 recipes, it was immaterial whether the sodium arsenate were 

 added to the lead salt, or the lead salt to the arsenate, but that 

 it was advisable that the arsenate solution should be as weak 

 as possible, and, therefore, the lead solution (whether acetate or 

 nitrate) as strong as possible ; since the weaker the arsenate, 

 the more finely divided was the precipitate of lead arsenate 

 formed. The precipitate formed by using lead acetate is 

 more finely divided than that obtained when lead nitrate is 

 used. 



The lead arsenates formed, whichever reagent is used, are 

 appreciably soluble in water, but not sufficiently so to cause 

 any recognisable scorching of the foliage of apple trees, and in 

 this respect lead arsenate compares very favourably with Paris 

 green, even when the latter is mixed with its own weight, or 

 even double its weight, of lime (VI, 192) ; with still larger pro- 

 portions of lime ten times its weight was tried the scorching 

 action of Paris green was prevented. 



'Lead arsenate may, contrary to statements made on the subject, 

 be mixed with Bordeaux mixture without decomposition, so as 

 to obtain fungicidal as well as insecticidal effects ; but it must 

 not, of course, be mixed with potassium sulphide (VI, 184). 



Calcium arsenate, which had been suggested as a cheap sub- 

 stitute for lead arsenate, was examined. It was found to have 

 the same constitution as the lead arsenate obtained from lead 

 acetate, but to be more soluble than it. This solubility, however, 

 was reduced by adding excess of lime, a suitable spray-fluid 

 being obtained by the addition of i part of lime, in the form 

 of milk of lime, to every i part of crystallised sodium arsenate 

 dissolved in 400-500 parts of water. Its insecticidal value was 

 about equal to that of the lead arsenate (VIII, 17, 92). l 



1 Such obervations as have been made with nicotine and other insecticides 

 will receive sufficient notice under the headings of the insects for the 

 destruction of which they have been applied (pp. 196-229). 



