210 On the Vegetative Efficacy of Sulphur. 



important degree, the siilp/iut'ic acid may ht considered either 

 in itself, or as it sets other active agents at work, the main 

 spring of operation in piaister. It is commonly used by che- 

 mists to separate the carbonic, and all other acids, from their 

 combinations, wh rever 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 inhales. It is 

 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 thmk it a corollary fairly to 

 be drawn from this theory, and the actual analysis of the gyps^ 

 that it is this sulphuric or vitriolic acid which constitutes its 

 operative principle, £^c," — And see volume first, page 158. 

 I have since ascertained by a variety ox practical tests, to 

 mv satisfaction, that this opinion was well founded. Alter 

 separating the vitriolic acid, the other parts of the piaister 

 are inoperative ; and have no effect v/natever 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 of acting 

 on those whereon it produces invariable and wonderful effects. 

 We know what will assist its operation ; and can supply 

 artificially v/hat it does not find, or has exhausted, in the 

 earth. And this is enough for us to know ; for all practical 

 uses. 



R. P* 



