254 PRINCIPLES OF VENTILATION. 



anced by the good they effect. " New Haven," justly called the 

 City of Elms, is almost embowered in the shade of lofty trees, 

 and is remarkable for the salubrity of its atmosphere, and the 

 health of its inhabitants. There, almost every house has its 

 garden ; and the daily consumers of its deleterious exhalations 

 stand in the open streets, at once the ornaments of the city and 

 the scavengers of the air. The cutting down of a healthy tree, 

 in the midst of a large town, without some very strong reason, 

 should be regarded as an offence to the community, and an 

 injury to the public weal. It is much to be wished that other 

 towns, that are rapidly increasing in extent and population, 

 would follow the example of New Haven, and bad ventilation 

 and impure air would, in a very great degree, be deprived of 

 their injurious effects. 



2. Under favorable circumstances, plants are able to appropriate 

 a larger amount of carbonic acid than that commonly existing 

 in the atmosphere. The vegetation around the springs, in the 

 valley of Gottingen, which abound in carbonic acid, is very rich 

 and luxuriant, appearing several weeks earlier in spring, and 

 continuing much later in autumn, than at other spots in the 

 same district. But it is probable that, taking the average of the 

 whole globe, and at all seasons, the quantity of carbonic acid 

 existing in the air is that most adapted to maintain the health 

 of the plants at present inhabitants on its surface, as well as to 

 interfere as little as possible with the animal creation. In hot- 

 houses, however, the case is different, especially in winter ; for, 

 although carbonic acid be not produced by the respiration of 

 animals, it is produced in abundance by other causes, and these 

 same causes also depriving the atmosphere of oxygen and its 

 aqueous vapor, the carbonic remains in excess, and its effect 

 upon the plants is easily perceived. The presence of oxygen, 

 in proper quantity, in the atmosphere of a green-house, or hot- 

 house of any kind, is even more necessary to be artificially 

 maintained, than carbonic acid, because the oxygen affords the 

 means by which the superfluous carbon is removed. We know 

 that plants in a hot-house suffer more frequently from an excess 

 of carbon than an excess of oxygen, arising from the causes 



