THE GASES PLANTS FEED UPON. 



that he did not repeat the experiment with pure carbonic acid artificially prepared, and 

 regrets that he had not yet had leisure to do so. These experiments were repeated and 

 extended by his friend Dr. Percival, who published an account of his results in Hunter's 

 " Surgical Essays." The conclusion to which he arrived, was just the contrary of what 

 might have been expected from Dr. Priestley's experiments; for he states, " that fixed 

 air not only retards decay, but actually continues the vegetation of plants, and aftbrds 

 them a pabulum, which is adequate to the support of life and vigor in them for a consider- 

 able length of time." So satisfied was he of the truth of this, and the important in- 

 fluence that the discovery would have upon the art of gardening, that he goes on to point 

 out how the florist who is anxious to obtain a prize at some great meeting held for the en- 

 couragement of horticulture, might perhaps avail himself of the powers of "fixed air," 

 either to quicken the growth of his plant if still immature, or to preserve it in all its 

 beauty, and retards its decay if fully developed; and he accordingly gives several direc- 

 tions to gardeners who are willing to try the efiect of carbonic acid gas on their flowers. 

 Some years before this time, hov/ever. Dr. Priestley had observed that plants grew well 

 in air which had been vitiated by the breathing of animals, and that they soon restored 

 such air to its original state of purity; it seems that he did not at tha,t time very distinct- 

 ly connect the two phenomena together, for it was not till after Dr. Percival's experi- 

 ments had been published, and after Ingenhousz had promulgated his views upon the 

 subject, that carbonic acid began to be regarded as essential to the growth of plants. It 

 is not easy now, neither in fact is it very important, to decide exactly what degree of merit 

 belongs to each of these three philosophers. The experiments of Ingenhousz were the 

 most numerous and minute, and his views were the most strongly criticised by contem 

 porary naturalists; at the same time he distinctly states that his experiments arose out 

 of those of Dr. Priestley. 



The circumstance Avhich prevented many observers from believing in the importance of 

 carbonic acid to plants, arose from the fact, that as in nature they are only supplied with 

 it very gradually, and in small quantities, and as their organs are only fitted to absorb it 

 when largely diluted with common air, so when placed in an atmosphere containing a con- 

 siderable proportion of this gas, they are alwaj'^s more or less injured; and it was difiicult 

 to understand how, if carbonic acid in small quantities was food and nourishment, that in 

 rather large quantities it would be hurtful and even poisonous. 



Dr. Percival's idea of the probable value of carbonic acid as a powerful agent in the 

 hands of a skillful gardener, is one which naturally leads to several interesting considera- 

 tions, and it has accordingly produced a number of curious experiments, which, though 

 they certainly do not justify his prediction, or, indeed, give much prospect of any really 

 important direct improvement in practical gardening, are nevertheless, very curious and 

 interesting. The primary question of course is, are plants always able to absorb as much 

 carbonic acid from the air as they require, and would benefit be derived by increasing the 

 proportion of it, in the air of a forcing-house for example? When Dr. Priestley found 

 that pure carbonic acid was hurtful to plants, he next tried mixtures of it with common 

 air, and he found that even one-eighth of it rendered the air poisonous. Ingenhousz, 

 after a great number of experiments, and, though quite satisfied that plants really decom- 

 pose carbonic acid, speaks with great doubt of the effects which it produces when artifi- 

 cially added to the air. He says, " plants grow tolerably well in air to which a small 

 quantity of carbonic acid has been added, especially if they are exposed to the sunshine; 

 sometimes even they seemed to grow all the better for it;" but he adds, " that it is doubt- 

 ful if this effect was really due to the presence of the carbonic acid." A second, and very 



