570 SCIENCE PROGRESS. 



ness of pure copper disappears in presence of a trace of 

 alloy, although of much practical importance to copper- 

 smiths and to engineers is scientifically barren, because it 

 has not yet tended towards the attainment of the only aim 

 of all scientific work, namely, the acquirement, at some 

 infinitely distant date, of a perfect knowledge of the con- 

 stitution of matter. 



The history of the physico-chemical investigation of 

 gases, liquids and solids is of the very highest interest, 

 not only as teaching the classic methods which have done 

 such service in the past, but also as indicating the lines 

 upon which work will probably be conducted in the future. 

 Comparatively early in the century the physical and chemical 

 examination was prosecuted with an energy which rapidly 

 furnished us with a fairly complete theoretical knowledge 

 of gases ; and it was not until much later that a vigorous 

 attack, directed against the more complex problems pre- 

 sented by liquids and liquid solutions, led to a vast increase 

 in our store of knowledge of matter. It cannot be doubted 

 that in the future the investigation of the physico-chemical 

 properties of solids will be undertaken by chemists with a 

 unity of purpose which will speedily bring to comparitive 

 maturity our present embryonic knowledge respecting solids. 

 Under these circumstances it may not be unprofitable to con- 

 sider some few of the peculiarities which stamp solids as a 

 class apart from liquids or gases, and to point out in what 

 way these peculiarities affect the questions at issue and how 

 they may easily lead astray. 



Many of the laws governing gases are immediately ap- 

 plicable to liquids, the differences between the two states 

 being largely difierences of degree and not of kind ; the 

 truth of this statement has been repeatedly exemplified by 

 the masterly work of van der Waals on the continuity of 

 the liquid and gaseous states and by many other investi- 

 gations published during recent years. The brilliance of 

 the results which have been obtained by drawing analogies 

 between liquids and gases would at first sight seem to stamp 

 as logical those attempts to learn something of the physical 

 chemistry of solids which have been based on analogy ; but 



