20 LESSONS ON CHEMISTRY. 



3. We will commence with the air or atmosphere, as that 

 great ocean of vapour which surrounds the earth is called^ 

 and in which both plants and animals live. This gi'eat mass 

 of gases,* without the presence of which neither plant nor 

 animal could exist, possesses very remarkable physical proper- 

 ties ; but to these it is not necessary at present to direct your 

 attention. We cannot see the air, but we feel it in the 

 breeze which strikes upon us as we walk briskly along, re- 

 sisting our progress; and we hear it sighing in autumn 

 among the falling leaves, and howling in winter in the fierce 

 wind which rushes through our valleys. You can easily 

 convince yourselves that, though invisible, it possesses sub- 

 stance, by trying to press together the sides of an inflated 

 bladder, and, when the bladder is compressed, if you pierce 

 it with a pin, the air will rush out with force.f For our 

 pui-pose, it will be necessary that we should consider the na- 

 ture of the substances which compose the atmosphere, for it 

 is not an elementary body, as the ancient philosophers taught, 

 and as many uneducated people yet imagine, but formed by 

 the mixture of several airs, which possess, when separate, the 

 most energetic properties. 



4. It may be useful in this place to explain the meaning 

 of the term mixture, which I have just employed as distin- 

 guished from chemical combination, as these tenns frequently 

 occur in works which treat of the chemistry of agriculture. 

 When we stir together a quantity of sand and common soda, 

 such as is used in bleaching, we produce a mixture of these 

 substances which partakes of the characters of both. We 

 can at once perceive that it is a compound of sand and soda. 

 If we place the mixtm-e in boiling water, the soda will dis- 

 solve, and can be poured off with the water, leaving the sand 

 unchanged. But if, after mixing the sand and soda, we place 

 them in an iron or clay vessel, and expose them to a very 

 strong heat, we produce a compound in which no trace of the 



* Gases are thin, elastic, and invisible substances, of which the gas 

 employed in illuminating towns is an example. Many of them have 

 neither colour, taste, nor smell ; but, though not to be detected by our 

 senses, they possess most energetic and remarkable properties. 



f Illustration Attempt to fill a wide-mouthed bottle by placing 



it with its mouth downwards in a basin of water, and you will find that 

 the water will not pass into it: something (the air) resists its entrance ; 

 but if you incline the bottle to one side, the air, being lighter than water, 

 will escape in bubbles, and the water will enter and occupy its place. 



