CH. XXXVI. THE STUDY OF CHEMISTRY. 369 



of oxygen and hydrogen come off rapidly and vehemently. 

 This led him to invent a useful Httle instrument called a 

 voltameter y which measures the quantity of water decom- 

 posed, and so tells exactly what is the strength of the 

 electric current. *Thus we see,' says Faraday in one 

 of his lectures, ' that the power which decomposes water, 

 or produces the heat and light of the electric spark, is 

 neither more nor less than the chemical force of the zinc — 

 its very force carried along the wires and conveyed to 

 another place.' 



And here again we find ourselves brought face to face 

 with the truth that all the various physical forces are only 

 different forms of one and the same force. We learnt before 

 that motion can be turned into heat and heat into motion, 

 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, motion, electricity, magnetism, and chemical change, 

 are all different phases of one indestructible force 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 them 

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

 into nitrate of copper the iron attracts the nitric acid and 



