160 SCIENTIFIC THOUGHT. 



back in the history of the subject and draw attention 

 to the gradual change which the nineteenth century 

 has brought about in our ideas regarding the different 

 states in which matter is supposed to exist, be it 

 in motion or in rest : the solid, the liquid, and the 

 gaseous states. 



Not very long ago the impressions of common-sense, 

 according to which a fundamental difference separates 

 solid from liquid and liquid from aeriform bodies, per- 

 meated scientific treatises also. Eigid demarcations 

 were maintained between hydrostatics and pneumatics, 

 and likewise between the doctrines of bodies at rest 

 and such as are in a state of perceptible motion. 

 One of the most marked changes which the centmy 

 has witnessed, has been the breaking down of these 

 older landmarks of science. The state of rest — once 

 supposed actually to exist — has had to give way to a 

 state of concealed yet measurable motion, as in the 

 case of the kinetic theory of gases, which explains dead 

 pressure by the bombardment of innmnerable particles 

 darting about. The idea of dynamical equilibrium — 

 i.e., the maintenance of a state of uniform motion — has 

 in many cases taken the place of static equilibrium or 

 rest, as in the doctrine of the flow of heat, the theory 

 of exchanges of radiation, and the conception that the 

 rigidity of solids depends upon a peculiar form of whirl- 

 ing motion — the vortex. Similarly the intermediate or 

 transition states which lie between the solid and fluid, 

 the properties of viscosity and of colloidal substances, 

 and of vapours as marking the transition between 

 liquids and gases, have attracted more attention in pro- 



