INTKnIMVTION 19 



Lavoisier's experiment led to other no less important results. The 

 weight of the air taken decreased by as much as the weight of the 

 mercury increased in oxidising ; that is, the portion of the air was not 

 destroyed, but only combined with mercury. This portion of the air 

 may be again separated from the mercury oxide, and has, as we saw 

 (example 3), properties different from those of air. That portion of 

 the air which remained in the apparatus and did not combine with the 

 mercury does not oxidise metals, and cannot support either combus- 

 tion or respiration, so that a lighted taper is immediately extinguished 

 if it be dipped into the gas which remains in the bell-jar. * It is ex- 

 tinguished in the remaining gas as if it had been plunged into water/ 

 writes Lavoisier in his memoirs. This gas is called ' nitrogen. 5 Thus 

 air is not a simple substance, but consists of two gases, oxygen and 

 nitrogen, and therefore the opinion that air is an elementary substance 

 is erroneous. The oxygen of the air is absorbed in combustion and the 

 oxidation of metals, and the earths produced by the oxidation of 

 metals are substances composed of oxygen and a metal. By mixing 

 the oxygen with the nitrogen the same air as was originally taken is 

 re-formed. The existence of compound substances was incontestably 

 proved by these experiments. It has also been shown by direct experi- 

 ment that on reducing an oxide with carbon, the oxygen contained 

 in the oxide is transferred to the carbon, and gives the same gas as is 

 obtained by the combustion of carbon in air. Therefore this gas is 

 a compound of carbon and oxygen, just as the earthy oxides are com- 

 posed of metals and oxygen. 



The many examples of the formation and decomposition of sub- 

 stances which are met with convince us that the majority of substances 

 with which we have to deal are compounds made up of several other 

 substances. By heating chalk (or else copper carbonate, as in the 

 second example) we obtain lime and the same carbonic acid gas which is 

 produced by the combustion of carbon. On bringing lime into contact 

 with this gas and water, at the ordinary temperature, we again obtain the 

 compound carbonate of lime, or chalk. Therefore chalk is a compound. 

 So also are those substances from which it may be built up. Car- 

 bonic anhydride is formed by the combination of carbon and oxygen ; 

 and lime is produced by the oxidation of a certain metal called ' cal- 

 cium.' By breaking up substances in this manner into their component 

 parts, we arrive at last at such as are indivisible into two or more sub- 

 stances by any means whatever, and which cannot be formed from other 

 substances. All we can do is to make such substances combine together 

 or act on other substances. Substances which cannot be formed from or 

 decomposed into others are termed simple substances (elements). Thus 



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