166 SCIENCE AND FRUIT GROWING 



but only as a substitute for part of the caustic soda in the wash, 

 and in every case where such a substitution was made there was 

 a reduction in effectiveness. The proportions of carbonate 

 used, however, were only half of those mentioned above 

 namely, 1-9, instead of 3-8 parts, for every I of soda J though 

 they were double those usually recommended (VI, 53). 



A comparison was also made between caustic soda and caustic 

 potash as regards their efficiency in cleansing trees from moss : 

 in three cases soda was the more effective, and in three cases, 

 potash; and there was no gain in using a mixture of the two, 

 instead of one only; nor in substituting potassium carbonate 

 for a part of the caustic soda (VI, 146). 



Where an alkaline wash is sprayed on to trees there is a rapid 

 disappearance of caustic alkali, this becomes converted by the 

 carbonic acid in the air into carbonate, and then, partially, into 

 the acid carbonate. But the wood of the tree plays a part in 

 these actions. When a glass rod was taken, dipped into a 2 per 

 cent, solution of caustic soda, and exposed to the air, it was 

 found that, after 24 hours' exposure, all the caustic soda in the 

 wash applied to it had been converted into normal carbonate, 

 and about three-fifths of the latter had been converted a stage 

 further into the acid carbonate. The amount of the two car- 

 bonates together corresponded very satisfactorily with the 

 amount of caustic soda in the wash applied. In the case of fresh 

 cut twigs, however, no trace either of caustic soda or of normal 

 carbonate, was found after 24 hours' exposure, and the acid 

 carbonate extracted during 16 hours' soaking in water corre- 

 sponded to but three-quarters of the caustic soda in the wash 

 applied, so that, evidently, some of this soda must have 

 penetrated very deeply into the wood, or must have reacted 

 with the substances present in it. On further soaking, more 

 acid carbonate was extracted, till, after 4 days, the amount 

 of it apparently corresponded to about 50 per cent, more than 

 that in the wash applied, so that some of this acid carbonate 

 extracted, or, rather, some other organic salt which behaves in 

 a similar manner towards the tests used as does the acid carbonate, 

 must have been derived from the juices of the twig. 



A similar result was obtained when the caustic soda solution 

 applied contained paraffin or soap. 



If, instead of a fresh twig, one was taken which had become 



1 It having been ascertained only more recently (Proc. Roy. Soc., A, 93, 

 548) that to produce the same alkalinity it was necessary to use an amount 

 of carbonate containing double the proportion of alkali metal of that 

 in the caustic alkali. 



