80 ON SAP. 



the jar containing the sprig of mint. And this cannot be 

 accounted for otherwise than by supposing, that the car- 

 bonic acid has been absorbed by the mint, decomposed 

 by its leaves, the carbon retained, and the oxygen evap- 

 orated. 



M. de Saussure has succeeded in measuring the quan- 

 tity of carbon which plants thus acquire. He transplant- 

 ed fourteen periwinkles into vases, seven of which he 

 watered with distilled water, and the remaining seven 

 with water in its natural state. After some days he ana- 

 lysed these plants, and found that the former had not 

 made any acquisition of carbon, whilst the latter had ac- 

 quired a considerable addition of that substance ; their 

 wood being one-sixth heavier than that of the former. 



Emily. And the periwinkles, which had augmented 

 in weight, had, I suppose, alone given out oxygen by 

 their stomas. 



Mrs. B. No doubt ; but, in making these experiments, 

 attention must be paid to expose the plants, not only to 

 broad daylight, but, if possible, to the full force of the 

 sun's rays ; for the solar light is absolutely necessary to 

 the process of decomposing the carbonic acid. During 

 the night the vegetable laboratory is employed in a very 

 different process ; for, in the dark, plants absorb instead 

 of exhaling oxygen. 



Caroline. You alarm me, Mrs. B. : this is a sort of 

 Penelope's labor, to destroy during the night the work 

 done in the day. And how is the atmosphere to be pu- 

 nned by these means ? 



Mrs. B. It is true that this apparent inconsistency 

 requires some explanation. You must observe, that the 

 solid nutritive particles dissolved in the sap, whether of 

 animal or vegetable origin, are combined with a consid- 

 erable quantity of carbon. The sap therefore contains 

 carbon in two states : in the one gaseous, combined with 

 oxygen, and mixed with the water of the sap ; in the oth- 

 er combined with different solid ingredients, but dissolv- 



435, How did Saussure ascertain the quantity of carbon acquired by 

 plants'? 436. What was the result'? 437. Why is it necessary 

 that this experiment be made in full day light! 438. What different 

 process takes place in the night with plants'? 439 Tf plants, so far 

 as absorbing and exhaling the gases is considered, reverse in the night 

 what they do in the day, how is the atmosphere to be purified! 



