CHAPTER XVII 



INSECTICIDES, FUNGICIDES, ETC, (continued) 

 (i) ALKALINE WASHES (Report VI, u, 53, 143) 



To remove moss, lichen and dead bark from fruit trees, as well 

 as to destroy the eggs and breeding places of certain insects, the 

 stems of the trees are often painted with lime wash. It is better, 

 however, to spray the wash on to the trees, so as to reach the 

 branches as well as the stems, and for spraying purposes a 

 solution of caustic soda is preferable to a lime wash. Either 

 caustic soda alone, or caustic soda mixed with potash, is generally 

 recommended for the purpose, at a maximum strength of 2 per 

 cent., sometimes with the addition of 0'5 per cent, of soft soap or 

 agricultural treacle. 



The term soda is, unfortunately, applied both to caustic soda 

 (sodium hydroxide or hydrate) , and also to the much less caustic 

 alkali, sodium carbonate. No confusion, however, appears to 

 have been made on this account in the case of these washes, and 

 the soda used is always the caustic soda. But it is otherwise 

 in the case of the potash : a similar opportunity for confusion 

 exists, and the confusion has constantly been made, so that it 

 is sometimes impossible to ascertain whether caustic potash or 

 the carbonate is signified. Caustic potash (potassium hydroxide 

 or hydrate) corresponds, of course, with caustic soda : the 

 carbonate is known under various names, such as pearlash, 

 purified potash, potashes and crude potash. It appears to be 

 the carbonate, and not the caustic potash, which was originally 

 suggested for this wash, and which is still generally recom- 

 mended. The reason of this, we surmise, can only be that at 

 the time when this wash was introduced the price of caustic 

 potash rendered its use prohibitive, for it is evident that effective- 

 ness cannot be enhanced by using a mild alkali instead of a 

 strong one, and if there were any special property of the car- 

 bonate rendering it more effective than the hydroxide, this would 

 have been equally so as regards the sodium compound. 



What the origin of using a mixture of soda and potash (in any 



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