5O ANIMAL CHEMISTRT LECTURE III. 



a pupil: 'I show,' said Gerhardt, writing in 1842, ' how the 

 chemist does everything that is contrary to living nature that 

 he burns, destroys, works by analysis the vital force alone 

 operates by synthesis and reconstructs the edifice destroyed by 

 chemical forces.' But, in reality, there is another side to the 

 shield ; there is a constructive as well as a destructive, a synthetic 

 as well as an analytic organic chemistry ; and to this view of 

 the subject I will now direct your attention. 



(49.) I need scarcely remind you of the mode in which vege- 

 table structures are built up. The minute seed grows into the 

 gigantic tree, the great mass of which is made up of carbon, hy- 

 drogen, and oxygen, that the living organism has stored up 

 from the carbonic acid and water with which it has been sup- 

 plied throughout the period of its existence, and which it has 

 inter- combined into the various forms of vegetable tissue. Now, 

 this storing up of carbon, hydrogen, and oxygen this formation of 

 vegetable compounds is attended throughout by an evolution of 

 oxygen. The proportion of oxygen contained in carbonic acid 

 and water being greatly in excess of the proportion contained in 

 vegetable tissue and secretion, we have throughout the growth of 

 every plant a constant deoxidation of carbonic acid and water 

 the carbon, hydrogen, and necessary oxygen being retained in the 

 substance of the plant, the oxygen in excess of the requirements 

 of the plant being discharged into the atmosphere. Let me re- 

 call to your recollection one of the original experiments of 

 Priestley upon this subject. He showed, for example, that under 

 exposure to sunlight, a quickly-growing leafy plant, immersed in 

 an atmosphere which by the combustion of fuel had been freed 

 from oxygen and charged with carbonic acid, gradually restored 

 that atmosphere to its pristine condition, by an absorption and 

 subsequent decomposition of its carbonic acid into oxygen gas 

 evolved from the leaves, and carbon retained within the vegetable 

 structure. Here we have an imitation of the experiment. A 

 bunch of fresh mint has been thrust into this narrow cylinder of 

 dilute carbonic acid water standing in the small pneumatic 

 trough, and the whole exposed to sunlight. You perceive that 



