383 PRINCIPLES OF rilK.MlSTRY 



and terminates with the complete decomposition of the carbonic 

 anhydride. Phosphorus is used in order to attain the complete absorp- 

 tion of the oxygen. In these examples we see that a known mixture 

 of changeable substances is capable of arriving at a state of stable 

 equilibrium, destroyed, however, by the removal of one of the sub- 

 stances composing the mixture. This represents one of the instances 

 of the influence of mass. And as on rapid cooling the combustible 

 carbonic oxide does not succeed in combining with the oxygen, with 

 which it totally combines if slowly cooled, forming carbonic anhydride, 

 here, as in all chemical phenomena, the action of time is apparent that 

 is, the existence of a definite velocity of reaction. 



Although carbonic anhydride is decomposed on heating, yielding 

 oxygen, it is, nevertheless, like water, an unchangeable substance at 

 ordinary temperatures. The decomposition of carbonic anhydride, as 

 effected by plants, is on this account all the more remarkable ; in this 

 -case the whole of the oxygen of the carbonic anhydride is separated in 

 the free state. The mechanism of this decomposition is that the heat 

 and light absorbed by the plants are expended in the decomposition of 

 the carbonic anhydride. This accounts for the enormous influence of 

 temperature and light on the growth of plants. But it is at present 

 not clearly understood how this takes place, or by what separate inter- 

 mediate reactions the whole process of decomposition of carbonic 

 anhydride in plants into oxygen and the carbohydrates (Chapter VIII.) 

 remaining in them, takes place. It is known that sulphurous anhy- 

 dride (in many ways resembling carbonic anhydride) under the action 

 of light (and also of heat) forms sulphur and sulphuric anhydride, SO 3 , 

 and in the presence of water sulphuric acid. But no similar decompo- 

 sition has been directly obtained with carbonic anhydride, indeed 

 it forms the highest degree of oxidation of carbon ; perhaps for that 

 reason the oxygen separates. On the other hand, it is known that 

 plants always form and contain organic acids, and these must be 

 regarded as derivatives of carbonic acid, as is seen by all their 

 reactions, which we will shortly treat of. For this reason it might be 

 thought that the carbonic acid absorbed by the plants first forms 

 organic acids in them, and that these latter in their final trans- 

 formation form all the other complex organic substances of the plants. 

 Many organic acids are found in plants in considerable quantity ; for 

 instance, tartaric acid, C 4 H 6 O 6 , found in grape-juice and in the acid 

 juice of many plants ; malic acid, C 4 H 6 O 5 , not only found in unripe 

 apples but in still greater quantities in mountain ash berries ; citric 

 acid, C 6 H 8 O 7 , found in the acid juice of lemons, in gooseberries, 

 -cranberries, &c. ; oxalic acid, C. 2 H 2 0.,, found in wood-sorrel and 



