ON SAP. 79 



Mr. Sennebier covered a plant which was growing in 

 a pot of earth with a glass bell full of water ; and, in the 

 course of a few hours, found a quantity of air within the 

 bell. Whence came this air ? Did it proceed from the 

 plant or the water in which it grew ? He repeated the 

 experiment with water which liad been boiled, for the 

 purpose of depriving it of its air, and in this instance no 

 air was produced in the bell. 



Caroline. Of what nature was this air ? 

 Mrs. B. Dr. Priestley ascertained that it consisted of 

 oxygen gas, and conceived that it was produced by the 

 decomposition of the water, which, you know, is com- 

 posed of oxygen and hydrogen ; but then he could not 

 understand why boiled or distilled water, which contains 

 as much oxygen as rain or spring water in their natural 

 state, should not produce this air in the glass bell. 



At length Mr. Sennebier, in the prosecution of his ex- 

 periments, discovered the mysterious origin of this air to 

 be in the carbonic acid, which water, in a natural state, 

 always contains. I trust that you have not so far for- 

 gotten your lessons of chemistry, as not to recollect that 

 carbonic acid is composed of oxygen and carbon : the 

 plant absorbs this gaseous acid. It is decomposed in 

 the leaves by the sun's rays: the carbon, which it is es- 

 sential to the plant to retain, is deposited ; and within it 

 the oxygen, which it does not require, flies off by the 

 stomas. 



Caroline. Then the little vegetable mouths breathe out 

 pure oxygen, and retain the carbon : this is just the re- 

 verse of the operation performed in the lungs. 



Mrs. B. You may prove this by a very neat experi- 

 ment. Place two glass jars over the same water-bath, 

 with a means of communication through the water ; fill 

 one of them with carbonic acid, and put a sprig of mint 

 in the other. After some time, a vacuum will be produced 

 in the upper part of the jar of carbonic acid ; and a quan- 

 tity of oxygen gas, corresponding exactly to the quantity 

 of carbonic acid which has disappeared, will be found in 



430. What experiment was made by Sennebier 1 ? 431. How did 

 Dr. Priestley suppose this air was produced 1 ? 432. What was there 

 about it which he could not understand 1 ? 433. How was Mr. Senne- 

 bier finally enabled to ascertain the source of the air contained within the 

 belli 434. What is shown by the experiment of the two glass jars 

 over the same water bath 7 



