250 



Oa the ylpplication of Manurew 



stances above stated (which appear more favourable to the due 

 performance of the functions of life than those to which Mr. 

 Ellis's plants were subjected), the amount of oxyn^en evolved was 

 much smaller than it would have been in the open air ; for I have 

 succeeded, by introducing several plants into the same jar of air 

 in prettv quick succession, in raisins^ the amount of oxygen con- 

 tained from 2 1 to 39 per cent. ; and probably had not even then 

 attained the limit to which the increase of this constituent might 

 have been brought. 



How great then must be the effect of an entire tree in the open 

 air, under favourable circumstances ! And we must recollect 

 that, ccBteris paribus, the circumstances will be favourable to the 

 exertion of the vital energies of the plant, within certain limits at 

 least, in proportion as animal respiration and animal putrefaction 

 furnish to it a supply of carbonic acid. 



These experiments were published in the ' Philosophical 

 Transactions for 1836/ and have been noticed in Dr. Lindley's 

 popular ' Introduction to Botany;' neither am I aware that the 

 deductions Avhich were drawn from them have anywhere been 

 disputed : I know not, therefore, to what class of physiologists 

 Professor Liebiar can allude when he asserts* that the source from 



o 



whence plants obtain their carbon is still doubted ; nor to whom 

 he may address the arguments by which he thinks it necessary to 

 support a doctrine which, in this country at least, is considered 

 so fully established. 



We are, however, indebted to him for explaining more clearly 

 than had hitherto been done how it happens that, if the atmo- 

 sphere be the source of the carbon present in the constitution of a 

 plant, the growth of the latter is so materially influenced by the 

 quality of the mould with which its roots are in contact, and also 

 why it is that without the presence of this substance it speedily 

 languishes and dies. 



In order to account for this, Professor Liebig begins by show- 

 ing that vegetable mould contains in general two principles^ both 

 of which have often been confounded under the common designa- 

 tion of humus. 



The first of these, which is soluble in alkalies and in certain of 

 the earths, has been termed humic acid, although, correctly 

 speaking, the latter is the substance produced from that descrip- 

 tion of humus which is in a state to be acted upon by alkalies in 

 consequence of their operation upon it. 



The second principle, which should be distinguished by the 

 term coal of humus\ is indeed derived from the former by the 

 gradual process of decay, but is already advanced to that con- 



* Chemistry applieil to Agriculture, p. 20. 



