32 FRUIT TREES AND THEIR ENEMIES 



must be kept tightly closed, or it will deteriorate and 

 cake together. A solution of it prepared for spraying 

 should be exposed to the air as little as possible. 

 The strength to be used is — 



{\0(i) Caustic soda (98 per 



cent, purity) . . 2 to 2^7 lbs. 

 Water . . .10 galls. 



The water should be stirred while the soda is shaken 

 in, or the latter may cake together at the bottom. 



The smaller proportion — 2 lbs. — is sufficient to 

 effectively remove moss and lichen from the trees 

 under ordinary conditions, but a still smaller propor- 

 tion will be only partially successful. By increasing 

 the amount to 2\ lbs. the action on scale is con- 

 siderably increased, but the risk to the workmen 

 using it is proportionately greater. It appears that 

 in the case of scale, the action does not consist of a 

 direct destruction of the eggs, but of a softening, and 

 subsequent glueing of the covering scale to the bark, 

 which results in imprisoning the insects when the 

 eggs hatch. A 2 per cent, solution is not strong 

 enough to do this with certainty, though it is generally 

 effective. 



Caustic potash is just as effective as caustic soda, 

 when equivalent chemical quantities are used, which 

 means using about three parts of the potash instead 

 of two parts of the soda ; but, as potash, is much 

 clearer than soda, there is no object in making the 

 substitution. Many of the recipes still in circulation 

 recommend substituting for half of the soda an equal 



