THE GASES PLANTS FEED UPON. 



remarkable series of experiments is described by De Saussure, the result of which show 

 ed that the addition of carbonic acid, where more than a tenth was added, always did harm, 

 whilst a twelfth produced a decidedly beneficial effect, if the plants were exposed at the 

 same time to the inlluence of light; there are, however, unfortunately, several interfering 

 causes in these experiments, which Saussure overlooked, and which diminish their real 

 value. Three 3'ears since some valuable observations on this subject were made by Dr. 

 Daubent, who conducted a careful series of experiments, Avith the object of ascertaining 

 what proportion of carbonic acid in the air, healthy plants are able to endure, without in- 

 convenience. The result of his investigations proves that ten per cent of carbonic acid is 

 not at all injurious to plants, and that it is sometime before even rather larger proportions 

 of that gas begin to produce bad effects. Even as much as twenty per cent produced no 

 injury in ten days. 



In all considerations of this sort, it is obviously the best way to commence by studying 

 the natural conditions of plants, before we begin to place them in new and unnatural cir- 

 cumstances. In nature, plants are supplied with an almost infinitely small quantity of 

 carbonic acid, and the effect of their growth is to deprive the air which surrounds them 

 of that carbonic acid, and to replace it by an equal volume of oxygen. 



The chief difficulty which exists in comprehending the true bearings of this great fact, 

 depends on the enormous magnitude of the entire atmosphere, and the almost infinitely 

 large quantities which we have to consider. It is easy to form some idea of the quantity 

 of carbonic acid which is produced in a given time on a square mile of the earths's sur- 

 face, and which the plants growing on it consequently may be supposed to have to decom- 

 pose in the same period. But it is very difficult to form any just idea of the whole at- 

 mosphere, or to comprehend the relation which exists between the bulk of the carbonic 

 acid and that of the entire atmosphere. Large as the former appears to us, and enor- 

 mous as its quantity is, when compared to the largest tree, yet it is a mere trifle, a 

 thing of very little account, when contrasted with the bulk of the whole atmosphere. 



If, for example, we assume, for the mere purpose of comparison, that on an average 

 each human being on the surface of the earth gives out, in the courseof twenty -four hours, 

 twenty-four ounces of carbonic acid gas, a proportion probably not very far from the 

 truth, the volume of the gas thus expired would be about twelve and a half cubic feet; 

 and the entire quantity given out during a whole year at this rate, would be 4744 cubic 

 feet of carbonic acid gas; having a weight of about 547 lbs., very nearly a quarter of a 

 ton! If then we go a little further, and calculate what would be the bulk and weight of 

 the quantity of carbonic acid that would be given out by the entire population of the 

 globe, during a whole year, assuming its number to be 600 millions, and supposing that 

 every individual required the same quantity of carbonic acid, we should then arrive at the 

 result of very nearly three million of millions of cubic feet, or 316,266 million cubic yards 

 of carbonic acid. This quantity is so large that it is difficult to conceive anj-^ just idea of its 

 real volume, and the consideration is not rendered much simpler, if we reduce it into 

 cubic miles; we then find that it would amount to about 58 cubic miles. On the other 

 hand, if we calculate the weight of such a quantity of carbonic acid, we are are nearly as 

 much bewildered with high numbers, for we find that it amounts to 326,200 million pounds, 

 or 145 million tons! and this, be it remembered, is but a very small portion of the car- 

 bonic acid which plants must absorb every year. 



