INSECTICIDES, FUNGICIDES, ETC. 169 



lime was obtained by mixing it after it was slaked and cold 

 with sulphur (5 per cent, of the total wash), or with solar distillate 

 (6 to 12 per cent.) : the addition of I per cent, of sodium silicate 

 (soluble glass), whilst often quite inoperative, gave, on occasions, 

 better results-than anything else which was tried. Glue or size, 

 when not less than 3 per cent, was used, improved the adhesion 

 to a certain extent. Soap, treacle, the basic sulphates, and 

 other substances examined, w 7 ere all detrimental in their action. 



Lime was found to adhere to the trees far better than whiting 

 (calcium carbonate), although it is itself soon converted into the 

 carbonate : an instance of how a substance formed in situ may 

 behave differently from the same substance when applied after 

 it has been already made (VII, 73). 



(2) LEAD ARSENATE (Reports, VI, 157; VIII, 16) 



Lead arsenate has proved itself an invaluable substitute for 

 Paris green and other arsenical preparations as an insecticide 

 for caterpillars, owing to no appreciable scorching action on the 

 leaves following on its use. There are few preparations, how- 

 ever, as to which so much error and confusion has arisen in the 

 recipes given for its manufacture, partly because the nature of 

 the arsenate formed by different methods was not known, and 

 partly because the reagents used were obtainable on the market 

 in several different forms. These uncertainties were removed 

 by a chemical investigation of the reactions concerned. The 

 results, however, are not at the present day of so much importance 

 to the fruit grower as they were at the time, since most growers 

 are now able to obtain the arsenate in the form of a paste, instead 

 of having to make it for themselves. 



Lead arsenate is made by adding a solution of the acetate or 

 nitrate to a solution of sodium arsenate. If the acetate is used, 

 the arsenate formed is Pb 3 (AsO 4 ) 2 , and some acetic acid is 

 produced at the same time : when the sodium arsenate taken 

 is the " pure," " crystallised " or " hydrated " substance, 

 the proportions required are 1-9 parts of lead acetate to every 

 i part of sodium arsenate : if the latter be in the form of 

 the " dry," " anhydrous " or " crude " arsenate, then for every 

 i part of it taken 3-5 parts of lead acetate are required. Such 

 proportions ensure the presence of a slight excess of lead, which 

 is advisable, as an excess of lead produces less scorching, than 

 an excess of sodium arsenate (VI, 180). Owing to variations in 

 the composition of chemicals concerned, it is preferable not to 

 add the whole of the lead acetate at once to the arsenate, but to 



