22 INSECTICIDES AND FUNGICIDES. 



Up to a very recent date it was generally supposed that arsenic was 

 present in London Purple in the form of calcium arsenite. During 

 the course of this investigation, however, it was found that the arsenic 

 is present both as calcium arsenite and calcium arsenate. The amount 

 of these two substances varies very much, but where both the arseni- 

 ous and arsenic oxids are calculated to arsenic it is found that the per- 

 centage of this constituent is fairly constant in different samples. For 

 example, the percentage of arsenic in the samples above taken in serial 

 order is as follows: 28.09, 30.39, 29.35, 28.07, 28.63, 29.04, 28.99, 

 30.59, 29.68, 24.55, and 28.00. Sample 19843 is the only one low in 

 arsenic, and this is because it contains the large amount of sand pre- 

 viously spoken of. 



As to the amounts of calcium oxid and dye, these two substances 

 vary in the samples above about as much as might be expected, with the 

 possible exception of 19843, which contains more dye and less calcium 

 oxid than the average sample. As a whole, all of the samples are of 

 about the same strength with the exception of 19843, which is somewhat 

 weaker. 



In the consideration of the soluble portion of London Purple a sur- 

 prise awaits the investigator. According to the old method of consid- 

 ering that London Purple was composed mainly of calcium arsenite 

 and only determining the arsenious oxid in a water extract, 7 of the 

 above samples would have been considered as excellent for spraying 

 purposes, since they contained only very small quantities of soluble 

 arsenious oxid, but on looking at the figures for soluble arsenic oxid, 

 it will be seen that wherever a sample only contains a small quantity 

 of soluble arsenious oxid it contains a very large quantity of soluble 

 arsenic oxid, so that the total amount of arsenic that is dissolved from 

 any sample of London Purple is very great. This perhaps explains 

 the unaccountable manner in which London Purple often scorches the 

 foliage when an analysis has shown that only a small quantity of 

 arsenious oxid is present. But even though London Purple has often 

 unaccountably scorched foliage, there are many cases where it has been 

 used with most excellent results. The only conclusion that can be 

 drawn from this (if we consider that the above 11 samples fairly rep- 

 resent the London Purple as sold on the American market, and there 

 is no reason to believe that they do not) is that plants can stand a vast 

 amount more soluble arsenic than we have before thought possible. 

 It is highly probable that if all of this soluble arsenic were present as 

 free arsenious or arsenic acid all plants would be seriously burned, if 

 not entirely defoliated, by applying such a substance, but where a 

 large part of the soluble arsenic is present as the calcium salts of 

 arsenious and arsenic acid, as shown by the fact that large amounts of 

 lime also go into solution, it appears that this alters the case and that 

 plants can endure much more of these salts than they can of the free 

 acids. Work along this line is much needed. 



