THE LAWS OF SCIENCE 43 



No more difficult question could be asked, and I cannot 

 pretend to answer it completely, even for that small 

 branch of science which is my special study. The reason 

 for the difficulty is interesting, and we must examine it. 



Let us return to our first " law," namely that steel 

 will rust if exposed to damp air. I said that the use of 

 the word steel implied an invariable association of 

 properties which is asserted by the more elementary law : 

 There is such a thing as steel. But if we look at the 

 matter closely we shall see ^that this is not really a law. 

 For there are many kinds of steel ; the substances, all 

 of which the man in the street would call equally " steel," 

 are divided by the fitter into mild steel, tool steel, high- 

 speed steel, and so on. And the scientific metallurgist 

 would go further than the fitter in sub-division ; he would 

 recognize many varieties of tool steel, with slightly 

 different chemical compositions and subjected to slightly 

 different heat-treatments, which might be all very much 

 the same thing for the purposes of the fitter. Tjjrtjf we 



f st.ef>l,,_wft are in effect 



saying that the_association of the properties of steel is 

 notinvariabTe, that therecan be many su&stances which, 

 thougTTlfiey "agree in some of their properties, differ in 

 others. Thus everything anybody would call steel con- 

 tains, according to the chemist, two elements, iron and 

 carbon ; but most steel contains some other element 

 as well as these two, and these other elements differ 

 from one steel to another ; one contains manganese, 

 another tungsten, and so on. It is not a law that every 

 substance which contains iron and carbon (and has cer- 

 tain physical properties of steel) contains manganese ; 

 for there are substances which agree in all these respects, 

 but differ in containing nickel in place of manganese, 

 and in certain other physical properties which are not 

 common to all steels. 

 There may seem to be an easy way out of the difficulty 



