CHAP, viz THEORETICAL DISCOVERIES IN PHYSICS. 57 



alloy must have a new arrangement of the molecules. 

 Eut the interesting point is that alloys can be produced 

 without melting the metals, for mere pressure often pro- 

 duces an alloy at the surfaces of contact; while in other 

 cases, if fine filings of the component metals are thor- 

 oughly mixed together and then subjected to continued 

 pressure, true alloys are produced. 



Another interesting fact is that metals, and probably 

 all solids, evaporate at ordinary temperatures. It has 

 long been known that ice evaporates very rapidly, and 

 now it is found that metals do the same, and the evapora- 

 tion can be detected at temperatures far below their 

 melting points. All these curious phenomena give us 

 new ideas as to the constitution of matter, and lead us to 

 the conclusion that the extreme mobility of the molecules 

 of gases has its analogue in liquids and even in solids. 

 The flow of metals, their diffusion into other metals, and 

 their evaporation, lead to the conclusion that a propor- 

 tion of their molecules must possess considerable mo- 

 bility, and when these reach the surface they are enabled 

 to escape either into other bodies in contact with them or 

 into the atmosphere. This proportion of rapidly mov- 

 ing molecules gives to solids some of the characteristics 

 of liquids and of gases. 



Before leaving this part of our subject we must refer 

 to a most interesting and suggestive discovery which 

 throws still further light on the constitution of matter, 

 and on the forces which give to matter many of the prop- 

 erties without which neither vegetable nor animal life 

 would be possible. It has been found that all gases ex- 

 pand or contract equal amounts for every degree of heat, 

 the amount being -g-Ts" of their volume for each degree 



