ON SAP. 8d 



bined in animal and vegetable matter. In the first state 

 the sun's rays decompose the acid, the carbon is deposit- 

 ed, and the oxygen which flies offpurifies the atmosphere ; 

 in the second state, oxygen is absorbed during the night, 

 and combines with the carbon, with which it forms car- 

 bonic acid ; this, during the day, is decomposed, and the 

 oxygen restored to the atmosphere. Thus vegetation 

 serves as a counterpoise to the deleterious effect of the 

 respiration of animals. 



Emily. And should we not add to the contamination 

 of the air by combustion, Mrs. B. ? for oxygen is also ab- 

 sorbed in that process ? 



Caroline. The air of a forest must then be much more 

 wholesome than that of a town, where so many human 

 beings and animals are continually breathing out carbonic 

 acid, and where such numberless combustions are rob- 

 bing the atmosphere of oxygen. 



Mrs. B. No ; the constant motion of the air so rapid- 

 ly restores the equilibrium, that it has been found, by the 

 most accurate chemical experiments, that the air of a 

 crowded city contained precisely the same quantity of 

 oxygen as the finest air of the country. I do not mean 

 to say that the atmosphere is not more impure and un- 

 wholesome in a large town ; but this arises from the smoke, 

 and variety of exhalations, which do not circulate so rap- 

 idly as the oxygen gas. 



The air in a forest is, on the other hand, far from being 

 considered as healthy ; the trees impede the circulation 

 more than the houses in a town, the latter being, in some 

 measure, ventilated by the currents of air which flow 

 through the streets. 



Caroline. But, then, consider the pure breath of the 

 green leaves in a forest." 



Mrs. B. The exhalations arising from the stagnant 

 waters, and the putrefaction of the dead leaves which re- 

 main floating in the confined air, more than counterbal- 

 ance that advantage, and render a dense forest an un- 

 wholesome spot to inhabit. 



452. How does Caroline recapitulate what had before been said of 

 the elaboration of the sap 1 ? 453. What does Caroline say of the com- 

 parative healthfulness of the forest and the town! 454. Why is her 

 opinion incorrect! 455. From what arises the unwholesomeness of 

 the populous city if not from the want of oxygen in the air! 456. 

 And why is it unhealthy in the forest, if vegetation conduces to the purity 

 of the air! 



