CH. xxxvu. CHEMICAL METHODS. 397 



And here again we find ourselves brought face to fare 

 with the truth that all the various physical forces are only 

 different forms of one and the same force. We learnt be- 

 fore that mechanical energy can be turned into heat, and 

 heat into mechanical energy, while heat, magnetism, and 

 electricity all in the same way give rise to each other ; and 

 now we learn that chemical change gives rise to electricity, 

 and electricity in its turn to chemical change. So that the 

 whole set of physical forces, heat, electricity, magnetism, 

 and chemical change, are all different phases of the same 

 indestructible energy which we lose sight of in one shape, 

 only to find it in another. 



Methods of Studying Chemistry. We have now 

 learnt how most of the chief methods of producing chemical 

 change have been worked out. The science of chemistry 

 consists in using these methods to test and decompose all 

 the substances in our earth and atmosphere, and so learning 

 their nature. 



We have seen that there are four ways of thus analysing 

 compound bodies. First, by testing them with other sub- 

 stances which attract some of their elements, and draw these 

 out of the compound, as when by plunging a piece of iron 

 into nitrate of copper the iron attracts the nitric acid and 

 draws it out, leaving the copper to fall down as a metal. 

 This was the method chiefly worked out by Bergmann in 

 1761, and which has since then been brought to much 

 greater perfection by other chemists. 



Secondly, by heating substances gently and examining 

 the vapours which rise from them, and afterwards analysing 

 what remains by burning. This" method was fairly under- 

 stood by Geber, and was first applied to organic substances 

 by Boerhaave. 



Thirdly, by passing an electric current through a com- 



