MATERIALS 59 



arsenate, but it does not appear to enhance the action 

 of the latter two to any considerable extent, and it 

 seems better to use nicotine only in cases where the 

 emulsion or arsenate is inadmissible. 



There are various preparations of nicotine or tobacco 

 in use for fumigating greenhouses. 



27. Pyrethrum. — This is obtained by powdering the 

 heads and stems of plants of the genus Pyrethrum. 

 It is useful for the destruction of aphis and cater- 

 pillars, and forms one of the usual constituents of pre- 

 parations used for fumigation. It is applied to plants in 

 the open in the same way as hellebore powder, either 

 dry, with or without the admixture of other powders, 

 or as a 3 to 4 per cent, decoction in water ; or it may be 

 added to a paraffin emulsion. Like hellebore, also, 

 it deteriorates on keeping. 



28. Quassia. — This is made by boiling chips of the 

 wood of Picrcena excelsa in water. Together with 

 soft soap, it forms a wash which is much used in 

 summer for destroying aphis and other insects. 



The water and the chips are boiled together for 

 fifteen minutes — some recipes give a much longer time 

 — and the soap is then added. When the latter is all 

 dissolved, the liquid may be strained off, and is ready 

 for use. The lower proportions of quassia and soap 

 mentioned in the above formula, are those generally 

 adopted in England. 



