MATERIALS WHICH PLANTS DERIVE FROM THE AIR. 21 



sand can be observed, and which, according to the quantity 

 of soda employed, either dissolves entirely in boiling water, 

 or forms an insoluble transparent substance like glass.* 

 These experiments afford us an example of the changes 

 which the union of elements or compounds is capable of pro- 

 ducing. In the first case, the new body produced by simply 

 mixing the substances together partakes of the properties of 

 its constituents. We can distinctly observe the sand and 

 soda unchanged. But in the second case, when they com- 

 bine^ a body may be formed possessing properties and appear- 

 ance totally unlike those of the substances employed to 

 produce it. In the first case, the substances are said, as 

 with the sand and soda, to be merely mixed; in the second, 

 to be in chemical combination. 



5. There is, also, another mode in which changes are pro- 

 duced in the form and appearance of the bodies around us, 

 and to which I shall have frequent occasion to allude. Its 

 operation is just the reverse of those which I have above de- 

 scribed; for, instead of causing variety, by bringing bodies 

 together in new forms, it separates or breaks up the com- 

 pounds formed by nature. It acts as the lime-burner who 

 places in his kiln the compound limestone, and, by means of 

 heat, tears asunder its constituents, producing a substance 

 possessing properties very different from those of the lime- 

 stone rock of our hills. This process of change, which is 

 continually going on around us, is termed decomposition. 

 1 will again return to its consideration. 



6. Let us, however, continue our inquhy into the nature 

 of that great mixture of gases with which we were engaged, 

 (a) If you were to take some bm*ning wood or coal, and 

 attempt to kindle a fii'e, and at the same time close up the 

 top of the chimney and the mouth of the fireplace, you 

 would find that the fire would not bum. You must ^ give 

 it air," or the lighted fuel would be extinguished, {h) If 

 you were to place a healthy plant, growing in a pot, within 

 a large, carefully closed bottle, and supply it only with dis- 



* A soluble glass has been employed on the Continent as an appli- 

 cation to paper and wood, in theatres and public buildings, to diminish 

 the liability to take lire. It is best made by fusing together a mixture 

 of 70 parts carbonate of potash (salt of tartar), 54 parts carbonate of 

 soda, and 152 parts of ground flints or pure white sand. The glass pro- 

 duced- will readily dissolve in water, and may be applied like a varnish. 



