Prof. Norton on Molecular Physics. 279 



The views which have now been presented enable us to form 

 a definite conception of the probable arrangement of the mole- 

 cules of a liquid. If the state of equilibrium be such as we have 

 represented, we must conclude that a perfectly symmetrical ar- 

 rangement of particles, similar to that which subsists in crystals, 

 prevails throughout the whole mass. 



We conceive the fundamental distinction between a solid and 

 a liquid, from the mechanical point of view, to be that the exter- 

 nal impulses which fall upon the molecule of a solid, are propa- 

 gated, either wholly or chiefly, in their original line of direction ; 

 while those which fall upon the molecule of a liquid are radiated 

 in every direction from it. The physical cause of this differ- 

 ence in the mode of propagation of a force appears to be the 

 simple fact that in the process of liquefaction the molecular 

 atmospheres are forced by the heat of fluidity to a decidedly 

 greater distance from the atoms which they surround; thus 

 leaving below them a much larger volume of universal gether, to 

 receive the impulses propagated down to it. If this difference 

 between the mode of propagation of impulses by the molecules 

 of a solid and liquid be admitted, it is not difficult to see that 

 we have a sufficient cause for the different mechanical properties 

 attendant upon these two states of aggregation, without having 

 recourse to the prevalent idea of a permanent polarity of simple 

 atoms. So far as any polarization of molecules comes into ope- 

 ration, we shall have occasion to remark, in discussing briefly 

 the topic of crystallization, that it is simply an induced, and for 

 the most part a temporary condition of the molecular atmo- 

 spheres, developed in the act of solidification. 



In the aeriform state the particles are so widely separated that 

 each is repelled by all those which surround it, within the limit 

 of effective action, and the equilibrium is determined by external 

 pressure. The properties of gases and vapours, and the laws 

 of their expansion and contraction, are deducible from equation 

 (3), p. 200. The value of x that obtains when a vapour formed 

 at any temperature has its maximum tension, is the distance 

 Od, fig. 1, answering to the maximum molecular repulsion dn; 



and this varies for different temperatures, because the ratio - 



m 



decreases as the temperature rises. (See different values of 



maximum repulsion answering to different values of the ratio 



- given in Table I., p. 200.) 



The process of transition from the solid to the liquid state 

 occurs at the surface of the mass. As the heat is absorbed, the 

 molecules near the surface recede from each other ; and when 

 this expansion has reached a certain point, the attractive forces 



