Prof. Norton on Molecular Physics. 277 



which the laws of effective molecular action, as shown by the 

 proper curve, depend. 



Contemplating from our present point of view the varying 

 mechanical states and conditions which the same substance may 

 assume under different circumstances, we are led to recognize, as 

 an essential physical feature, upon which such changes either 

 wholly or partially depend, the fact that the mechanical condi- 

 tion of the individual molecules is not fixed and unchangeable, 

 but liable to material variations. We perceive their atmo- 

 spheres expanding under the influence of heat, and contracting 

 from the effect of external pressure, and that certain phenomena 

 and permanent changes of property result from these atmospheric 

 changes (e. g. changes of property in passing from the solid 

 to the liquid form, or vice versa; permanent displacement of 

 particles produced by the temporary action of forces of a cer- 

 tain intensity upon bodies). 



States of Aggregation of Matter. — These are three essentially 

 different states of equilibrium. In the solid form, the particles 

 immediately contiguous to each other are in a condition of equi- 

 librium under the action of their own molecular forces ; if more 

 distant particles exercise any effective action, it is attractive, and 

 neutralized by a similar action on the other side of the particle. 

 To be more definite, each molecule of the mass is surrounded 

 by others at various orders of distance from it ; and each pair 

 of molecules at the first order of distance from each other are in 

 a condition of equilibrium by themselves, which is equivalent to 

 saying that their electric atmospheres are separated by the distance 

 0«, fig. 1 (p. 203). For the second order of distance the action 

 should then be attractive ; but it may very well be that when a 

 permanent equilibrium of the mass has been reached, the atmo- 

 spheres of two particles at this order of distance will be so ex- 

 panded by their attractive action, on the line of their centres, 



n 

 that, for the diminished value of - thus resulting, the distance 



between the atmospheres on this line will be the increased dis- 

 tance Oa for the curve corresponding to this diminished value of 



— . Upon this supposition, each particle would be separately in 



equilibrium with every particle contiguous to it, both at the first 

 and second order of distance. We shall have occasion to note 

 hereafter that this state of things is probably more or less perfectly 

 realized under different circumstances of solidification. As to 

 the action of more distant molecules, it is first to be observed 

 that, if two molecules are in equilibrium under their mutual 

 actions, the attractive and repulsive impulses exerted by each 

 upon the central atom of the other must be equal, and therefore 



