INFLUENCE OP INCREASED PROPORTION OF CARBONIC ACID. 191 



this evil is much more than counterbalanced by the good they 

 effect ; so that the cutting down of a tree in the midst of a large 

 town, without some very strong reason, should be regarded as 

 an offence not easily to be atoned for. It is much to be wished, 

 that the law of the land required such an open space to be set 

 apart, whenever the population of an extending town or district 

 increases beyond a certain amount. 



296. There is good reason to believe, that Confervas and 

 other aquatic plants exercise a similarly important influence, in 

 keeping the water they inhabit in a state fit for the support of 

 animal life ; since it appears probable, that they absorb the pro- 

 ducts of the decomposition of that foul matter, by which all 

 ponds and streams are constantly being polluted ; and that at 

 the same time they yield a supply of oxygen to the water. It 

 is a fact well known, that Fishes are never so healthy in reser- 

 voirs destitute of aquatic plants, as in those in which they 

 abound. The lower Cryptogamia appear to flourish better than 

 higher plants would do, when supplied with a large quantity of 

 carbonic acid, whilst the amount of light they receive is but 

 moderate. In the lake Solfatara in Italy, are several floating 

 islands, consisting chiefly of Confervas and other cellular plants ; 

 which are copiously supplied with nutriment by the carbonic 

 acid, that is constantly escaping from the bottom of the lake, 

 with a violence which makes the water appear as if boiling. 



297. Under favourable circumstances, too, the highest plants 

 are able to continue appropriating a larger proportion of carbon 

 than that commonly existing in the air. 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 of the same district. But it is probable that, taking the 

 average of the whole globe and of all seasons, the quantity of 

 carbonic acid commonly existing in the air, is that which is 

 most adapted to maintain the life of the race of Plants at present 

 inhabiting its surface, as well as to interfere as little as possible 

 with the well-being of the Animal creation. 



298. It is not improbable, however, that, in former epochs 



