MATERIALS 49 



coloration is noticed, more lime is required. If this 

 wash is bought in a concentrated form, it should 

 always be rechurned before use, as mentioned under 

 Bordeaux emulsion on p. 29. 



As to the comparative merits of emulsions made 

 with different substances, the insecticidal value of 

 those with copper and iron sulphate will be the same, 

 but that containing copper has the advantage of 

 being a fungicide as well. In the case of aphis, an 

 emulsion made with soap may be preferable to those 

 made with the sulphates, cleansing the leaves more 

 effectually of the honey-dew and excrement deposited 

 on them by the insects. 



Besides soap and the basic sulphates of iron and 

 copper, other substances have the power of emulsify- 

 ing paraffin oil with water : starch, flour, milk and 

 glue, all act in this way, though none of them is superior 

 to soap or the sulphates for this purpose. 



Although paraffin seems of itself to be one of the 

 most satisfactory insecticides for summer use, it may 

 sometimes be desirable to use other substances 

 together with it. With some of these it can be 

 emulsified by simply churning them up with it ; this 

 is so with freshly prepared lead arsenate and calcium 

 arsenate ; in other cases it is necessary to emulsify 

 the paraffin with iron sulphate, as in i6<r/, before 

 adding the other substance ; Paris green, lead arsenate 

 paste and potassium sulphide may thus be incorpor- 

 ated with paraffin (see p. 56). With various lime- 

 washes, including the limc-sulphur-salt washes, paraffin 

 may be emulsified directly by churning, but the 

 emulsion is of a somewhat different nature, and is 

 4 



