THE SUMMER HEAT OF CITIES. 445 



the most dangerous malaria with the utmost impunity, provided a 

 grove intervene between his home and the marsh. This function of 

 trees was known to the Romans, who enacted laws requiring the 

 planting of trees in places made uninhabitable by the diffusion of 

 malaria, and placed groves serving such purposes under the protec- 

 tion of some divinity to insure their protection. It is a rule of the 

 British army in India to select an encampment having a grove be- 

 tween the camp and any low, wet soil. 



Finally, trees purify the atmosphere. The process of vegetable 

 nutrition consists in the appropriation by the plant or tree of carbon. 

 This element it receives from the air in the form principally of car- 

 bonic acid, and in the process of digestion the oxygen is liberated 

 and again restored to the air, while the carbon becomes fixed as an 

 element of the woody fiber. Man and animals, on the contrary, re- 

 quire oxygen for their nutrition, and the supply is in the air they 

 breathe. Carbon is a waste product of the animal system, and, unit- 

 ing with the oxygen, is expired as carbonic acid, a powerful animal 

 poison. A slight increase of the normal quantity of carbonic acid 

 in the air renders it poisonous to man, and continued respiration of 

 such air, or a considerable increase of the carbonic acid, will prove 

 fatal. The animal and vegetable world, therefore, complement each 

 other, and the one furnishes the conditions and forces by which the 

 other maintains life and health. " Plants," says Schacht, " imbibe 

 from the air carbonic acid and other gaseous or volatile products ex- 

 haled by animals, developed by the natural phenomena of decomposi- 

 tion. On the other hand, the vegetable pours into the atmosphere 

 oxygen, which is taken up by animals and appropriated by them. 

 The tree, by means of its leaves and its young herbaceous twigs, pre-- 

 sents a considerable surface for absorption and evaporation; it ab- 

 stracts the carbon of carbonic acid, and solidifies it in wood fecula, 

 and a multitude of other compounds. The result is that a forest 

 withdraws from the air, by its great absorbent surface, much more 

 gas than meadows or cultivated fields, and exhales proportionally a 

 considerably greater quantity of oxygen. The influence of the for- 

 ests on the chemical composition of the atmosphere is, in a word, of 

 the highest importance." * 



In large cities, where animal and vegetable decomposition goes 

 on rapidly during the summer, the atmosphere is, as already stated, 

 at times saturated with deleterious gases. At the period of the day 

 when malaria and mephitic gases are emitted in the greatest quantity 

 and activity, this function of absorption by vegetation is most active 

 and powerful. Carbonic acid, ammoniacal compounds, and other 



* Les Arbres, quoted by Marsh. 



