IMPORTANCE OF THE VEGETABLE ACID. 127 



fully tiie influence which gluten and vegetable albumen exercise ovei 

 the general efficiency of the products of vegetation in the support of ani- 

 mal life, and over the changes which these products must undergo, be- 

 fore they can be converted into the substance of animal bodies. In a 

 former lecture (Lecture IV., p. 66), I have had occasion to draw your 

 attention to the comparatively small proportion in which nitrogen exists 

 in the vegetable kingdom, and to show that it must nevertheless be con- 

 sidered as much a necessary and constituent element in their composi- 

 tion as the carbon itself; the very remarkable properties we have al- 

 ready discovered in the compounds above mentioned strongly confirm 

 this fact, and illustrate in a striking manner the influence of apparently 

 feeble and inadequate causes in producing important natural results. 



5°. With the exception of acetic acid, which in constitution is closely 

 related to sugar* and gum, all the acid substances to which it has been 

 necessary to advert, contain an excess of oxygen above what is neces- 

 sary to form water with the hydrogen they contain. Thus 



Vinegar = C4 H3 O3 contains no excess of oxygen. 



Tartaric Acid = C4 Hg Og . . 3 of oxygen iL excess. 



Malic Acid ) n tt n o 



Citric Acid ^ —^Atlz'^A - - ^ 



Oxalic Acid = Cg O3 . . 3 

 It requires a little consideration to enable us to appreciate the true im- 

 portance of these and other organic acids, in the vegetable economy. At 

 first sight they appear to form a much smaller part of the general pro- 

 ducts of vegetation than is really the case. We must endeavour to 

 conceive the quantity actually produced by a single tree loaded Vith 

 thousands of lemons, oranges, or apples, — or again, how much is formed 

 during the growth of a single comparatively small plant of garden rhu- 

 barb in spring, if we would obtain an adequate idea of the extent to 

 which these acids are constantly formed in nature. On the other hand, 

 we must recollect also that the greater portion of the acid of fruits disap- 

 pears as they ripen, if we would understand the true nature of the in- 

 terest which really attaches to the study of these substances, of the 

 changes to which they are liable, and of the circumstances under which 

 in nature these changes take place. 



6°. I will venture here to draw your attention for a moment to the na- 

 ture and extent of that remarkable power over matter, which the chem- 

 ist, as above explained, appears to possess. Such a consideration will 

 be of value not only in illustrating how far we really can now, or may 

 hereafter, expect to be able to influence or control natural operations, 

 [see Lecture II. , p. 32,] but what is probably of more value still, exhibit- 

 ing the true relation which man bears to the other parts of creation ; and, 

 in some measure, the true position he is intended to occupy among them. 

 1°. We have seen that the chemist can transform certain substances 

 one into the other, in a known order ; but that as yet he cannot reverse 

 that order. Thus far his power over matter is at present limited ; but 

 this limit he may at some future period be able to overpass, and we 



• It is identical in constitution with caramel (p. 114)~the uncrystalliiable sugar of syrups. 

 For 



Vinegar. Carsimel. 



C3X 



(C4 H3 C3 X 3) = Cl2 H9 O9. 



