72 PROGRESS OF SCIENCE IN THE CENTURY. 



A moment's reflection will show that two some- 

 what different sets of changes go on around us. In 

 the frosty night water changes into ice; the sun 

 rises, and the ice changes into water; in the bright 

 sunshine the water may even pass into the air as 

 vapour. Here we have one of the most familiar 

 instances of a change of state, but the water remains 

 in a real sense water all the time. There is no 

 change in the nature of the stuff, and it is with 

 changes in the nature of the stuff that chemistry 

 has primarily to do, with the change, for instance, 

 which occurs when, by an electric current, water is 

 decomposed into its two constituents, hydrogen and 

 oxygen. The chemist has as his fundamental prob- 

 lem, not merely the recognition and isolation of 

 elements, but their affinity in relation to one an- 

 other, their capacity of exerting chemical action or 

 inducing chemical change. 



Detection of an Element. The question natur- 

 ally rises in the mind, how does the chemist know 

 when a given substance is an element or not ; and the 

 only scientific answer is that all substances should 

 be assumed to be compounds until all known methods 

 of decomposing them have been tried without suc- 

 cess. " If the products we obtain always weigh more 

 than the substance itself and never less, no matter to 

 what changes it has been subjected, then, provided 

 each change is complete and accompanied by no loss 

 of substance through our imperfect methods, we are 

 constrained to regard that substance as an ele- 

 ment," * 



Thus the chemical conception of an element is 

 simply that of an undecomposed not necessarily 



* Ostwald, Outlines of General Chemistry, trans. J. 

 Walker. 1890, Chap. II., " The Elements," p. 9. 



