in connexion chiefly with the Metamorphosis of Matter. 107 



The force which is the agent in the motion with which heat 

 is now identified is believed, with reason, to be connected with 

 the diffusion of highly attenuated matter throughout space. 

 Adopting the terms of Sir John Herschel, on the relative motions 

 of "sethereal molecules" and the "gross molecules" of sen- 

 sitive matter seem to depend all the phenomena of heat. Two 

 distinct sets of theories have been advanced : — those like that of 

 Mossotti, which make the gross molecules centres of force; and 

 that of Rankine, which makes the aethereal molecules the nuclei, 

 and allots to the gross molecules the functions of an encircling at- 

 mosphere. The former theories conceive the sether to have a kind 

 of motion analogous to the planetary orbital motion round the 

 gross molecules, the latter theory ingeniously combines the two 

 motions of waves and whirling. But on both hypotheses these 

 orbits or these oscillations and vortices must have some typical 

 form. Reasoning from analogy, these forms will be conic sec- 

 tions ; and upon the kind of conic section, we conceive, depends 

 the condition of the mass of matter, — the solid, liquid, and ga- 

 seous states corresponding respectively to elliptic, parabolic, and 

 hyperbolic orbits. An addition of heat to a body accelerates the 

 velocity of the revolving molecules ; and on this increase rela- 

 tively to the distance of the revolving molecules from the centre 

 of force in the nucleus depends the change in the form of the 

 orbits, and consequently the metamorphosis in the condition of 

 the substance. As long as the space due to the velocity is less 

 than the distance, the mass will be solid ; while the space due to 

 the velocity is equal to the distance, the mass will continue liquid; 

 and when the space due to the velocity is greater than the dis- 

 tance, the mass will become gaseous. The velocity and the 

 distance may increase together : if they do so at an equal rate, 

 the size of the orbit will be increased, while its excentricity will 

 remain unchanged, and the body, while continuing in the same 

 physical condition, will expand equally in every direction, which 

 is the case with liquids ; if the velocity and distance increase, 

 but not in the same ratio, the excentricity of the orbits will be 

 changed as well as their size, and the body will generally expand 

 unequally in definite directions and ultimately change its state 

 from solid and liquid, for example. 



The chief objection which may be urged to the above theory 

 is in connexion with the liquid condition of matter. If our analogy 

 hold, it may be said, the liquid state will be an unstable one, and 

 a body will suddenly pass from solid to gas. The parabola being 

 merely the limit between the ellipse and the hyperbola, no body 

 can in reality continue to move in that form of orbit ; but we 

 know that many comets do move about the sun in ellipses and 

 hyperbolas at very nearly the limiting state; and it may be 



