2

COMBINATION.

latter are plausible conjectures, to which we are led bythe attentive study of facts. When a chemist has madea number of experiments or has observed many pheno-mena, he endeavours to ascertain the cause of the effectshe has been studying. He selects the most probableexplanation, and adopting it as a theory or view, to beconfirmed or disproved by future experiments, proceedsto try in all possible ways the truth of his conclusion.By thus forming a theory he is enabled to arrive at thetruth more easily than if he were merely to continuemaking experiments at random. It is by reasoning onthe result of thousands of experiments that chemistshave been enabled to reduce the science into a usefulform, as they have thus been led to discover certaingreat leading laws, which govern all chemical changes oroperations.

3. Nearly all the changes which are going on in naturemay be classed under two heads. The one kind ofchange is that which takes place when two substancescome together which have, as it were, an attraction oraffinity for each other. As a familiar example of whatthen happens, we may take the common process ofsoap-boiling. When an alkaline or caustic ley isboiled with tallow or fat, soap is formed. The alkaliwhich is contained in the ley has an attraction for thefat; the two become thoroughly mixed, and combineor unite together, and form a new substance, quite dif-ferent from either the fat or the alkali, which newsubstance is called soap.