regulating Vital and Physical Phejtomena. 341 



are modified by slight differences in the conditions ; and it 

 scarcely seems too much to assume that, ignorant as we are of 

 the nature of these conditions in the living organism, the ac- 

 knowledged differences in the results may be dependent upon 

 their variations ; perhaps, also, other conditions whose nature 

 is yet unsuspected may also have a share in their production. 



Having now endeavoured to shew, by negative arguments, 

 that we are not justified in assuming that the union of elements 

 in organic compounds is effected by a novel kind of affinity, 

 unknown in the inorganic world, we shall next briefly inquire 

 vrhat positive reason we have for believing that chemical affinity 

 is the agent concerned in the formation of organic products. 

 It would be endless to enumerate all the actions which we might, 

 without passing beyond the limits of our absolute knowledge, 

 adduce in support of our argument ; but we may notice a few 

 of the most remarkable. Many of the proximate principles of 

 plants are mutually convertible ; thus gum may be changed to 

 iugar ; and starch, which is a semi-organized form of gum, may 

 be made to undergo a similar transformation, by the action of 

 heat and sulphuric acid. 



This is one of the remarkable instances of the catalytic action described by 

 Berzelius,* which is common to organic and inorganic chemistry, and which 

 is not yet comprehensible in the known laws of chemical affinity ; the pecu- 

 liarity of the action consists in the change produced by one compound upon 

 another, without itself undergoing any alteration. Thus, in the present in- 

 stance, the quantity of sulphuric acid undergoes no diminution, nor does it 

 form any new combination. 



Now, there are two distinct actions in this change, the rup- 

 ture of the vesicles causing the liberation of the contained gum, 

 and the conversion of the gum into sugar ; the former is effect- 

 ed by the heat, the latter by the heat and acid combined. The 

 process is one of the most common in the vegetable economy, 

 being performed whenever an embryo begins to be developed, 

 and makes use of the store of nourishment previously prepared 

 by the parent, as in germination ; but in the plant it is accom- 

 plished in a manner somewhat different. A secretion which 

 has been termed diastase is brought to act on the fecula, and 



• Edinburgh Philosophical Journal, voL xxL 



