300 Brenchley . — On the Action of Certain Compounds of 
that had made the most growth. A like result was obtained from 
similar analyses of solutions in which barley plants had been grown in the 
presence of varying amounts of boric acid, as in every case at least 
50 per cent, of the original nitrate remained in the solution. The mineral 
constituents were supplied in such excess in the food solution that it was 
impossible that their supply should have been exhausted, the nitrogen 
being the only food constituent liable to complete removal. Consequently 
it may safely be assumed that, other things being equal, in solutions of 
inorganic compounds in progressive concentration, the depression and 
absence of stimulation of plant growth are the direct result of the dele- 
terious action of the compounds upon the plant, and that they are not 
bye-results caused indirectly by interference with food supply outside the 
plant. An interesting point is the consistent amount of nitrate that is not 
accounted for by the analyses of the solutions and the plant. In the course 
of the experiment there is unavoidable loss due to mechanical manipula- 
tion, and possibly to bacterial action, as a little solution is always adherent 
to the roots when they are removed for washing, and bacteria may lock up 
in themselves a certain amount of organic nitrogen that is not accounted 
for by the methods used in detecting nitrate nitrogen in the solutions. 
Bacterial counts were also made to see whether denitrification played much 
part in determining the results, but no regularity was found in the numbers 
of bacteria present in the various solutions. 
The general outcome of the experiments with arsenic on peas and 
barley is that on the whole arsenious acid and sodium arsenite are more 
poisonous in their action than are arsenic acid and sodium arsenate. This 
distinction is more sharply marked with the acids themselves than with 
their salts, especially with regard to peas. Calculation of the actual 
quantities of arsenic in the various concentrations used show that the 
difference of toxicity is real, and that the same quantity of arsenic presented 
in the different forms of arsenic and arsenious acid varies in its action, being 
more toxic in the latter case. The reason for this is not obvious. A very 
tentative hypothesis may perhaps be put forward — that the acid radical 
formed from the lower oxide is absorbed by the plant in the usual way, 
and when within the plant tissues undergoes a process of oxidation, the 
necessary oxygen being withdrawn from the protoplasm, to the detriment 
of the latter substance. If this be true, it would indicate a double toxic 
action, partly due to the specific toxicity of the arsenic, which would 
manifest itself with greater strengths of arsenic acid, and partly due to 
a physiological action induced by the breaking down of part of the proto- 
plasm to provide oxygen for the purpose of oxidation. 
B. Boron . Boric acid differs very considerably from zinc sulphate and 
arsenic compounds in its action upon plants. Although it is toxic in high 
concentrations, very considerable quantities are necessary to kill peas and 
