226 ELEMENTARY GENERAL SCIENCE CHAP. 



carbon di-oxide is much greater, indicating that this is pro- 

 duced as a product of the animal economy, resulting, in fact, 

 from the union of the atmospheric oxygen with the carbon 

 present in animal tissues. Owing to this all animals require 

 oxygen, and soon expire if deprived of it. Fish and many 

 other marine animals are thus entirely dependent, for their 

 supply of oxygen, upon the air which is dissolved in the water. 



The experiments (225-226) indicate the absorption of mois- 

 ture by the calcium chloride and sulphuric acid, and it is this 

 moisture which is present in the air that becomes deposited upon 

 cold objects in the form of dew, as in Expt. 227. 



The quantity of moisture which is present in the air in the 

 form of water vapour varies considerably from time to time, 

 being as a rule greater in hot weather than in cold. 



In the neighbourhood of towns, also, there are always present 

 small quantities of compounds which are formed in works, &c. 

 These include compounds of ammonium, oxides of sulphur, &c. 

 Large quantities of dust particles and minute bacteria are also 

 found in air, the number of these little organisms varying from 

 one or less to many thousands per cubic centimetre. 



Air Compared with Oxygen. We have now seen that 

 when a substance burns in oxygen a new compound is formed, 

 which is an oxide. But we must go further, and ask Is this 

 also the case when it burns in air ? Does it take away the 

 oxygen and form an oxide, leaving the nitrogen unacted upon, 



as we should expect ? This can be readily tested. 

 . 



EXPT. 228. Put a small piece of dry phosphorus in a test- 

 tube, cork the tube tightly, and warm. Note that the phos- 

 phorus ignites and forms white fumes, as when it burns in 

 oxygen. But the flame is not so bright. When cool again, 

 take out the cork under water. Note what happens. Put 

 back the cork and shake up, test the remaining gas, and test 

 the water with litmus. Measure the volume of the water, i.e. 

 the gas abstracted, also the volume the tube holds. 



As was the case in rusting, one-fifth of the air has been used, 

 this being what we found was the quantity of oxygen present ; the 

 white fumes formed, too, combine with water to make an acid, 

 as when the phosphorus burned in oxygen. In fact, all tests 

 prove the products to be identical. Hence, in air, also, the 

 burning consists of the union of the burning substance with the 

 oxygen of the air to form a new compound an oxide. 



