620 
ME. J. BEOUGHTON ON A CEETAIN EXCEETION OE 
been arranged to deliver any gas that might be evolved into a eudiometer over mercury. 
But the gas was delivered in a very unsatisfactory manner, owing to the pressure that 
had to be overcome before the gas could pass into the eudiometer. At the end of two 
days, however, a few bubbles had passed, which, when tested by a potash pellet, showed 
the presence of carbonic acid by a marked absorption. 
The plan was then adopted of passing a slow stream of hydrogen (well purified by 
passing through tubes of pumice moistened with a strong solution of argentic nitrate 
and through a pair of potash bulbs) through the intervening space and then through a 
nitrogen bulb containing baryta-water. In this way the constant evolution of carbonic 
acid by the bark was readily detected by the precipitation of baric carbonate. The ex- 
periment was continued for nine days. The last three days the amount was estimated : 
the vols. of carbonic acid, corrected to the ordinary standard of pressure, were as follows : 
7th day 2 TO cub. centims. 
8th „ T54 cub. centim. 
9th „ T64 cub. centim. 
Immediately after the conclusion of the experiment, the bark, which had for so long 
been in contact with hydrogen only, was quickly cut off the plant, and as rapidly as 
possible passed into a eudiometer over mercury. Carbonic acid soon made its appear- 
ance, and after three days its amount was estimated. Its corrected volume was 6’21 cub. 
centims. The surface of the bark included in the tube exposed a surface of 18 square 
inches. 
In applying the same method for the purpose of ascertaining the evolution of carbonic 
acid from leaves while growing on the plant, very considerable difficulty was experienced 
in continuing the experiment long after the exclusion of oxygen. This was owing to 
the plant suffering from the effects of the hydrogen employed. This gas, when gene- 
rated in the usual manner from zinc and sulphuric acid, even after the most careful puri- 
fication I could devise, had such a poisonous effect on the fresh leaves, that after three 
days’ exposure they became yellow and died. I am inclined to attribute this to some im- 
purity in the gas (as a trace of arsenic) which was not perfectly removed by the means 
of purification adopted. I tried several plans of purification and several kinds of 
plants. In all cases I found carbonic acid evolved ; but I was never able to continue 
the experiment a sufficient length of time without the leaves becoming manifestly un- 
healthy, and thus introducing a source of doubt. After many trials I found that care- 
fully washed and purified nitrogen was the gas best suited for my purpose. I found 
also that a branch from a plant of Erica pubescens appeared perfectly healthy and fresh 
after many days’ exposure to the gas. 
I therefore finally adopted the following arrangement, which will be readily understood 
by the annexed sketch (fig. 4). Nitrogen gas contained in the gas-holder was slowly passed 
through two Ltebig’s potash bulbs containing a solution of pyrogallic acid in strong 
potash, then through a similar bulb containing a solution of potash, after this through 
baryta-water in a Will’s nitrogen bulb (which remained clear to the end). The pure 
gas then passed into the three-necked globe containing the branch of Erica , the latter 
