The Necessity of Pure Air for Health. 251 



of London only 20*85 P^"^ cent. From this it can be at 

 once seen how slight the variation is, and how well the 

 difficulty is overcome by natural forces. The constant 

 movement of the air just referred to also allows the 

 oxygen to come in conta6l with and destroy by oxidation 

 a large amount of the organic matter given off from the 

 lungs and skin of man and other animals. The amount 

 of carbonic acid gas present is also maintained at the 

 uniform level by the same laws of diffusion which allow 

 it to come in conta6l with the vegetation of the earth, 

 to which it is one of the important foods. And here 

 I may mention the wonderful adjustment that exists 

 between the animal and vegetable world, which alone 

 allows animal life to continue on the globe. The 

 chemical changes in the animal and the produ6ls of 

 animal organic matters ultimately resolve themselves 

 into carbonic acid, water, earthy salts and nitrogenous 

 compounds of a more or less complicated nature. As I 

 shall show later on that these nitrogenous compounds 

 and carbonic acid gas are poisons to animal life, it is 

 necessary that they should be destroyed in some way, 

 i.e, changed, for matter cannot be destroyed. Now it 

 has been demonstrated that the vegetable world plays 

 this important part in nature's economy. For the 

 chemical changes in the vegetable kingdom resolve 

 themselves into the formation of oxygen, given to the 

 air by the leaves, hydrogen, non-poisonous nitrogen 

 compounds, starches and sugars (carbo-hydrates) and 

 earthy salts. The following tabular view by DUMAS 

 points out the contrast and antagonism between the 

 processes of animal and vegetable life, and shows the 

 mode in which they are made mutually subservient to 



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