GENERAL PRINCIPLES 377 



lamps or candles, combustion cannot go on without continual 

 access of fresh supplies of air, for the sake of the oxygen gas 

 which is contained in it. The burning of a candle is due to a 

 process of what is known chemically as oxidation, the oxygen of the 

 air uniting with the wax of the candle, which as a result is trans- 

 formed into simpler chemical substances, chiefly water (H2O) and 

 carbonic acid gas (CO2). If a cold dry tumbler is held over a 

 burning candle a sort of mist condenses upon the inner side of the 

 glass, and this is some of the water in question. And again, if a 

 candle-end is burnt for a time in a tumbler into which a small 

 quantity of clear lime-water has been poured, a little gentle shak- 

 ing will be followed by a milky appearance in the lime-water, due 

 to the formation of carbonic acid gas as one result of the burning. 

 This gas unites with the lime-water so as to produce minute par- 

 ticles of carbonate of lime, which give rise to the milkiness. The 

 slow combustion constantly going on in the animal body similarly 

 causes its complex living substance to break down into a number 

 of much simpler compounds, among which are to be found both 

 water and carbonic acid gas. Such compounds, being of no use, 

 are called waste products, and have to be passed out of the body. 

 And one of the reasons why an animal is obliged to breathe is that 

 it may get rid of carbonic acid gas, together with a large amount 

 of water. You can easily satisfy yourself that this is true as re- 

 gards a human being by directing some of your outgoing breath 

 against a cold looking-glass, when a film of moisture {i.e. water) 

 will be seen. And the presence of carbonic acid gas in such 

 breath can be proved by breathing into some clear lime-water, 

 which will at once become milky. 



Breathing or respiration has a double purpose, for it not only 

 gets rid of waste products, but is also the means by which the 

 oxygen necessary for promoting the breaking-down processes of 

 the body is taken into the system. This oxygen is absorbed into 

 the blood, or, in some of the simpler animals, what corresponds to 

 it, and is taken to all parts of the body, in order that they may 

 " waste " and give up their share of the energy necessary for 

 working the different organs. 



Some of the smallest and simplest animals breathe by the 

 general surface of the body, but in higher forms, especially those 

 which live on land, this is not possible, and all sorts of compli- 

 cated breathing arrangements exist. We get, in fact, special 



