FUNCTION.] CARBONIC ACID. 271 



It has been observed by a modern writer, "that, if the 

 roots of a plant are placed in a close vessel, in distilled water, 

 from which carbonic acid has been carefully expelled, the 

 plant may increase a little in size, in consequence of the 

 decomposition of the water, and the combination of its ele- 

 ments with the vegetable system ; but it is only when carbonic 

 acid is added, that the plant acquires its natural vigour and 

 rate of growth. But if a plant is placed in solid carbon, and 

 you water it with distilled water, it might as well be planted 

 in powdered glass, until the carbon begins to combine with 

 the oxygen of the air, and to form carbonic acid. Sir Humphry 

 Davy placed a plant of Mint in water mixed with carbon in a 

 state of impalpable powder, and he found that not a particle 

 could enter the roots. If we look to the effects of manures, 

 we shall find that in most cases, except when their object is 

 to alter the state of the soil mechanically, or to act as stimu- 

 lants, as is probably the case with sulphate of iron, their 

 energy is in proportion to their capability of forming carbonic 

 acid. Yeast, for instance, which is one of the most active 

 manures we have, is so from possessing, beyond all other sub- 

 stances, the power of exciting fermentation, and thus of 

 causing the formation of carbonic acid among the vegetable 

 matter which lies buried in the soil.* 



* Dr. Seller calculates the annual conversion of the carbon of organic matter 

 into inorganic carbonic acid at not less than 600 millions of tons ; and infers, on 

 the most favourable aspect of the amount of soil over the earth's surface, that 

 such an annual loss could not be withstood beyond 6000 years ; and, on a less 

 exaggerated assumption of its amount, probably very near the truth, that the 

 waste would absorb the whole of the existing organic matter of the soil in about 

 740 years. Dr. Seller contends that the truth of these conclusions remains 

 unaltered, even if it be conceded that much of the carbon of plants is drawn, not 

 from the organic matter of the soil, but from the inorganic carbonic acid of the 

 atmosphere, unless some inorganic source of their hydrogen and oxygen be at 

 the same time admitted. He therefore regards Liebig's view of the inorganic 

 nature of the food of plants as supported not merely by many special facts for 

 example, by the increase of the organic matter of the soil, often observed during 

 the growth of plants, but also by the general view of the earth's surface just 

 taken, because there is nothing in its aspect to warrant the idea that its means of 

 maintaining the organic kingdoms are declining with the rapidity indicated in 

 the statements just made. Dr. Seller also examines Liebig's view of ammonia ; 

 first, as the sole source of the nitrogen of plants, and thereby of animals ; second, 

 as having its exclusive origin from the interior of the earth, and never from the 

 nitrogen of the atmosphere. In regard to these statements he makes it appear, 



