MATERIALS 31 



all of which tend to harbour insects, and to" protect 

 their eggs ; but some caustic washes, especially those 

 containing paraffin, will also destroy, by their chemical 

 action, the eggs of certain insects (mussel scale, etc.) as 

 well as some chrysalids of caterpillars. The caustic 

 substances used are caustic soda or caustic lime : the 

 latter, being a much less powerful agent, figures also 

 in certain preparations used in summer. 



Winter washes may be used at any time between 

 the fall of the leaf and the swelling of the buds in the 

 following spring ; but they seem to be generally more 

 effective when applied late in the winter, say, February 

 or March. It is well not to defer the operation too 

 long, however, for bad weather may then occur and 

 render it impossible. One thorough winter-washing 

 every second or third year should be sufficient to keep 

 the trees clean. 



10. Caustic soda. {Soda: Sodium Jiydroxide). — 

 This must not be confused with carbonate of soda or 

 washing soda, both of which are practically useless 

 for cleansing trees. By far the most convenient form 

 in which caustic soda is sold is that known as 

 " powdered " ; it is obtainable in tins holding i lb. 

 and upwards, and is of 98 per cent, purity. If a less 

 pure article is used, the quantities in the following 

 recipes must be increased accordingly. It must be 

 remembered that caustic soda is exceedingly in- 

 jurious to the skin, and may cause considerable 

 injury : the precautions requisite in using it have 

 already been mentioned (p. 18), and it is only 

 necessary to add that the solid absorbs both water 

 and carbon dioxide very eagerly, so that the tins of it 



