248 



On the Application of Manures, 



as under the direct influence of the sohar rays, and to be no less 

 common in aquatic than in terrestrial plants. 



I also showed that when a branch loaded with flowers as well 

 as with leaves was introduced into a jar containing a certain pro- 

 portion of carbonic acid, the balance still continued to be in 

 favour of the purifying influence of the vegetable. 



The apparatus I made use of consisted of a large bell-glass 

 jar, containing in one case 600, in another 800, cubic inches of 

 air,'"^ and suspended by pulleys ; its edges dipped into quick- 

 silver, contained in a double iron cylinder of corresponding di- 

 mensions to the jar, which, being closed at bottom, constituted a 

 well of about 6 inches in depth, calculated to receive a fluid, and 

 to admit of the glass vessel moving freely in it. The inner mar- 

 gin of this hollow cylinder was cemented air-tight, according as 

 circumstances required, either to a plate of iron, or to a pot of 

 the same material upon or in which the plant operated on might 

 be placed ; and the jar was then let down upon it until its edges 

 were sunk a little beneath the surface of the mercury. 



Thus all communication with external atmosphere was cut off, 

 and the effect of the plant upon the air enclosed in the jar was 

 readily measured, by simply pressing down the latter, and thus 

 expelling a portion of its contents through a tube communicating 

 with its interior, and introduced at its outer extremity under a 

 pneumatic trough, wherein the air might be collected and ex- 

 amined. By connecting this extremity with a vessel containing 

 a measured quantity of carbonic acid, and raising the jar a little 

 in the well of mercury, it was easy to draw in any proportion of 

 that gas with which it was thought proper that the plant should 

 be supplied. A portion of the air was always tested immediately 

 after the introduction of every fresh portion of carbonic acid, and 

 again after an interval of some hours, and the proportion of this 

 gas and of oxygen present was each time carefully registered. 

 The amount of carbonic acid was determined by a solution of 

 potass, that of oxygen by the rapid combustion of phosphorus 

 with a portion of it introduced into a bent tube. 



Such was the mode of procedure when an entire plant became 

 the subject of experiment ; but some of the most satisfactory 

 trials were with branches of certain shrubs, themselves too large 

 to be admitted under the jar. These branches, without being 

 detached from the parent trunk, were introduced through a hole 

 in the centre of two corresponding semicircular plates of iron, 

 which were cemented, air-tight, to the inner margin of the iron 

 cylinder on the one hand^ and to the stem of the branch on the 



Larger jars, containing from 1200 to 1300 cubic inches, were latterly 

 employed. 



