A CENTURY OF CHEMISTRY. 125 



in speaking of the circulation of matter, — a fact to 

 he associated with Liehig's industry — as not less im- 

 portant than Harvey's theory of the circulation of 

 the blood. The idea marks a new era. 



CHEMICAL AFFINITY. 



The Problem of Chemical Changes. — Chemistry 

 has above all to do with changes in the composition 

 of matter, and although in point of time the stiidj 

 of chemical changes was prosecuted, by the alche- 

 mist, for instance, long before there was any sound 

 knoAvledge of material composition, the understand- 

 ing of the former entirely depends on an understand- 

 ing of the latter. 



One of the early results of the careful study of 

 these chemical changes or reactions was to show that 

 though the number of possible experiments is endless, 

 the number of kinds of experiment is limited. It 

 began to be seen that substances could be arranged 

 in various groups, the members of each group acting 

 in a similar way in similar circumstances. Thus 

 a number of substances, like oil of vitriol (sulphuric 

 acid) and spirits of salt (hydrochloric acid) exhibit 

 similar properties, or similar reactions in similar 

 conditions, and may be ranked together as acids; 

 another set of substances, like spirits of hartshorn 

 (ammonia) and slaked lime, are most markedly dif- 

 ferent from the acids, and may be ranked together 

 as alkalis; a third set of substances, like chalk, pro- 

 ducible by the reaction of an acid and an alkali, may 

 be ranked together as salts. Thus there arose a clas- 

 sification of compounds based on similarity of reac- 

 tion in similar conditions. It was merely a prelimi- 

 nary step towards order, and it led to many others 

 of greater importance. 



