ACTION OF FERMFNTS. 723 



pounds, and never like oxygen, nitrogen and hydrogen, as chlorous 

 or electro-negative. Even where chlorine replaces hydrogen, 

 the type is not uniformly preserved, for we find occasionally the 

 same body derived by substitution from two different types, as 

 perchloride of carbon from both Dutch liquid and hydrochloric 

 ether. 



The reference of bodies to a common type has often an 

 advantage over their classification under a common radical 

 or according to any theory of constitution, as it involves less 

 that is speculative. The former asserts only that the bodies 

 contain the same number of atoms and have a common consti- 

 tution, but says nothing as to what that constitution is. Hence 

 a type may be denoted by an empyrical formula of the simplest 

 kind expressing nothing but the elements and their number, in 

 which changes by substitution can be distinctly exhibited. It 

 is useful, in the present uncertain state of our knowledge res- 

 pecting the constitution of organic compounds, to have such a 

 mode of expressing compounds and exhibiting their relations 

 in composition, but it does not supersede rational theories of 

 constitution. 



TRANSFORMATIONS OF ORGANIC SUBSTANCES. 

 ACTION OF FERMENTS. 



Complex organic substances frequently divide themselves 

 into two or more compounds of a simpler constitution, without 

 the intervention of any intelligible chemical agency. The 

 presence of a second organic substance, however, is an essential 

 condition of such transformations, although the latter substance 

 does not contribute to the change by imparting any new element 

 to the decomposing body, nor by abstracting any element from 

 it. The resolution of sugar into carbonic acid and alcohol in 

 fermentation, by the contact of yeast, is a familiar example of 

 such a change (page 760). Decomposition of this kind has 

 been distinguished as catalysis, and the second body which 

 determines the changes in the first, by an action of presence, 

 termed the catalytic agent (page 195). The recent study of 

 such decompositions has revealed the circumstance that the 

 activity of the catalytic agent is connected with its being itself 



