26 COMBINATION. 



unquestionable truths, the latter are plausible con- 

 jectures, to which we are led by the attentive study 

 of facts. When a chemist has made a number of 

 experiments, or has observed many phenomena, he 

 endeavors to ascertain the causes of the effects he 

 has been studying. He selects the most probable 

 explanation, and adopting it as a theory or view, to 

 be confirmed or disproved by future experiments, 

 proceeds to try in all possible ways the truth of his 

 conclusion. By thus forming a theory he is enabled 

 to arrive at the truth more easily than if he were 

 merely to continue making experiments at random. 

 It is by reasoning on the results of thousands of ex- 

 periments that chemists have been enabled to reduce 

 the science into a useful form, as they have thus been 

 led to discover certain great leading laws, which 

 govern all chemical changes or operations. 



3. Nearly all the changes which are going on in 

 Nature may be classed under two heads. The one 

 kind of change is that which takes place when two 

 substances come together which have, as it were, an 

 attraction or affinity for each other. As a familiar 

 example of what then happens, we may take the com- 

 mon process of soap-boiling. When an alkaline or 

 caustic lye is boiled with tallow or fat, soap is formed. 

 The alkali which is contained in the lye has an attrac- 

 tion for the fat; the two become thoroughly mixed, 

 and combine or unite together, and form a new sub- 

 stance, quite different from either the fat or the alkali, 

 which new substance is called soap. 



