THE MOLECULAR STKUCTURE OF MATTER. 717 



The practical importance of the views I have enunciated are I think 

 sufficiently obvious. Every one will admit that an external force can 

 not be applied to a system in motion without aflfectiug that motion ; 

 consequently matter, in whatever state, can not be touched without 

 changes taking place which will be more or less permanent. The ap- 

 plication of heat will cause a change of volume, and at last, a change 

 of condition; the application of external stresses will also produce a 

 change of volume; and it is natural to infer that there must be some re- 

 lation between the two, and accordingly Professor Carnelley has drawn 

 attention to the fact that the most tenacious metals have high melting- 

 points, though here again there is a great want of exactness, partly on 

 account of the difficulty of measuring high temperatures, and partly 

 by reason of the scarcity of pure materials. Again, long-continued 

 stresses, or stresses freciuently applied, may be expected to produce 

 permanent changes of form, and so we arrive at wliat is termed the 

 fatigue of substances. Stretched beyond their elastic limits, (which 

 limits I do not suppose to exist except when stresses are applied 

 quickly,) substances are permanently deformed, and the same effects 

 follow the long application of heat. 



The constant recurrence of stresses, even those within the elastic limit, 

 causes changes in the arraugement of the particles of substances which 

 slowly alter the properties of the latter, and in this way pieces of ma- 

 chinery, which theoretically were abundantly strong for the work they 

 had to perform, liave failed after a more or less extended i)eriod of use. 

 The effect is intensitied if the stresses are applied suddenly, if they 

 reach nearly to the elastic limit, and if they are imposed in two or more 

 directions at once, for then the molecular disturbance becomes verj' 

 intense, tlie internal equilibrium is upset, and a tendency to rui)ture 

 follows. Such cases occur in artillery, in armour-plates, in the parts of 

 machinery subject to impact; and, as might be expected, the destructive 

 effects do not always ap[)ear at once, but often after long periods of 

 time. When considerable masses of metal have to be manipulated by 

 forging, or by pressure in a heated condition, the subsequent cooling of 

 the mass imi)oses restrictions on the free movement of some, if not all 

 of the particles ; internal stresses are dev'eloped which slowly assert 

 themselves, and often cause unexpected failures. In the manufacture of 

 dies for coining purposes, of chilled rollers, of shot and sliell hardened 

 in an une(]ual numner, spontaneous fi'actures take place without any 

 aj)parent cause, and often after long delay, the reason being that the 

 constrained molecular motion of the inner particles gradually extemls 

 the motion of the outer ones until a solution of continuity is caused. 

 Similar stresses occur in such masses as crank shafts, screw shafts, gun 

 hoops, etc. - - - 



The influence of time on steel seems to be well established ; the high- 

 est qualities of tool steel are kept in stock for a considerable period ; 



