658 
AllSENIC IN ARTIFICIAL MANURES. 
My attention was first called to this subject by the diffi¬ 
culty I experienced in procuring any commercial sulphuric 
acid which did not contain a comparatively large proportion 
of arsenic, rendering it quite unfit and dangerous to be used 
for many purposes of experimental illustration. This arises 
from the fact, that the vitriol manufacturer has found that it 
is far more economical for him to make sulphuric acid from 
iron pyrites (a compound of sulphur and iron), which he can 
obtain for about twenty-five shillings a ton, than from native 
sulphur, for which he is obliged to pay about seven pounds 
for the same quantity. This ore of iron contains almost in¬ 
variably more or less arsenic, which passes into the sulphuric 
acid manufactured from pyrites; whereas, the native sulphur 
containing little or no arsenic, the sulphuric acid made from 
it is not so liable to be contaminated with that poisonous 
substance. Pyritic sulphuric acid, on account of its being 
much cheaper, seems in a great measure (at least in Dublin) 
to have taken the place of that manufactured from native 
sulphur, and hence the occurrence latterly of so much 
arsenical sulphuric acid in commerce, and the presence of 
arsenic in so many substances in the preparation of which 
that acid is directly or indirectly employed. 
The vitriol and manure manufacturers have been in the 
habit of making an inferior kind of pyritic sulphuric acid, 
which, owing to its dark colour, is termed brown sulphuric 
acid; this contains a comparatively" large proportion of 
arsenic, and is chiefly used in making superphosphate and 
other artificial manures ; and the manufacturer appears to 
think that the acid which is too impure to be used in the arts 
is good enough for making manures and for other agricultural 
purposes. This, however, from the experiments I shall 
presently refer to, appears to be a great mistake. 
Knowing that sulphuric acid containing arsenic was so 
largely employed in making superphosphate and other 
artificial manures, and that they therefore must contain variable 
quantities of that substance, I have for some time thought 
that it was not improbable that plants grown with such 
manures might imbibe or take up from the soil where those 
substances had been employed, a certain quantity of arsenic, 
and in this way be rendered more or less unwholesome as 
articles of food. 
As a preliminary experiment, to ascertain if plants had the 
power of taking up arsenic when it was presented to their 
roots in the soil, I transplanted into a flower-pot in June, 1857, 
three small plants of peas, and when they had recovered the 
transplanting, I commenced watering them every second or 
