PHYSICS, BIOLOGY, AND PSYCHOLOGY. 147 



the gas-laws to liquids, but by means of them predict 

 with great accuracy a very large number of facts. 



There remain other facts, however, which we cannot 

 predict, for with sufficient further fall of temperature 

 a liquefied gas crystallises. It doing so it may, like 

 water or molten iron, increase in bulk. Now, the simple 

 assumptions on which the kinetic theory of gases and 

 liquids is based are insufficient to explain the phenomena 

 of crystallisation, with the accompanying abrupt change 

 of volume and of other properties. We must, therefore, 

 assume, not merely that the molecules attract one 

 another in the directions joining their centres, after 

 the manner of gravitation, but that they tend to assume 

 a definite position, pole to pole, in relation to one 

 another, and actually assume this position as soon as 

 their mutual movements, due to heat, are insufficient 

 to prevent them from doing so. The liquid thus 

 crystallises at a perfectly definite temperature, unless 

 its enormous intermolecular pressure is sensibly increased 

 by added external pressure. 



This shows us that when we look closely at actual 

 molecules we are forced to the conclusion that the 

 tendency to take specific form or arrangement is always 

 present in molecules, and therefore in what we call 

 matter. We cannot sum up the properties of molecules 

 in the conceptions of mass, extension, and central forces 

 proportional to mass, in accordance with the funda- 

 mental physical conceptions of Newton. The actual 

 properties of molecules can only be expressed in terms 

 of their potential orientations to various other kinds 

 of molecules ; and when we pass beyond the compara- 

 tively simple empirical facts relating to crystallisation, 



