210 On the Vegetative Efficacy of Sulphur. 
important degree, the su/phuric acid may be considered either 
in itself, or as it sets other active agents at work, the main 
spring of operation in plaister. It is commonly used by che- 
mists to separate the carbonic, and all other acids, from their 
combinations, wherever they are found. The earth, accord- 
ing to the theories before stated, is constantly filled with the 
carbonic acid, by furnishing carbon to the air it mhales. Itis 
found in calcareous substances, with which, in great varieties, 
the earth abounds ; it exists in, or is produced by, the roots 
of decaying or decayed vegetables, trees, and all animal or 
vegetable manures. I therefore think it a corollary fairly to 
be drawn ‘rom this theory, and the actual analysis of the gyps, 
that it is this sw/phuric or vitriolic acid which constitutes its 
operative principle, c,”—And see volume first, page 158. 
I have since ascertained by a variety of practical tests, to 
my satisfaction, that this opinion was well founded. Aiter 
separating the vitriolic acid, the other parts of the plaister 
are inoperative ; and have no effect whatever on vegetation. 
As to my conjectures about the mode of its operation, they 
yet remain mere conjectures ; though practical effects are be- 
yond all doubt. I believe also that, *‘ the mode in which that 
substance acts upon vegetation, has remained, and will always 
be, a mystery.” Why it acts on some plants, and not on others, 
is as mysterious and inexplicable, as is its mode oi acting 
on those whereon it produces invariable and wonderful effects. 
We know what will assist its operation ; and can supply 
artificially what it does not find, or has exhausted, in the 
earth. And this is enough for us to know ; for all practical 
uses. 
R. P, 
