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XLVII. On, Molecular Physics. By Prof. W. A. Norton. 

 [Continued from p. 282.] 



IN considering the changes of state through which the same 

 substance may pass, we have been led to recognize, as an 

 important physical principle upon which the mechanical pro- 

 perties manifested in each new condition in a great degree depend, 

 that the physical condition of the individual molecules is liable 

 to permanent variations from the effect of heat, and that these 

 variations consist in expansions of the electric atmospheres 

 which surround the atoms of the molecules. If we take a more 

 extended view, and consider the diverse permanent changes of 

 condition which the same substance may experience while in 

 the same state of aggregation, we may discern the operation of 

 a still more comprehensive principle, viz., that the physical 

 state of the atmosphere of a molecule, and therefore the curve 

 which represents its action upon surrounding molecules, is liable 

 to permanent alteration from the action of external forces gene- 

 rally. It is well known that if a mechanical force, of conside- 

 rable intensity, be applied for a short interval of time to a body, 

 the result will be a permanent change in its form. The expe- 

 riments of Hodgkinson have indeed established that a certain 

 set may be imparted to bars of cast iron, by a temporary load 

 which is but a small fraction of its breaking load, and that " there 

 is no weight, however small, that will not injure the elasticity " of 

 such a bar. As we cannot suppose that a given mass of mole- 

 cules, while retaining forces of mutual action of unvarying in- 

 tensity, can take up an infinite number of positions of equili- 

 brium, differing but slightly from each other, we must conclude 

 that the individual molecules experience some change of condi- 

 tion, which occasions a change in the intensities of the forces 

 they exert in a given direction. 



From our present theoretical point of view, such possible 

 changes of condition consist in compressions, or expansions, 

 of the molecular atmospheres, either as a whole or unequally 

 on different sides. In the former case there will be a variation 

 in the size of the atmosphere, and in the intensities of the 

 forces of attraction and repulsion exerted by the molecule at 

 a given distance, but the forces exerted in different directions 

 will have an equal intensity. In the latter case there will be a 

 variation in the form of the atmosphere, and an inequality of 

 action in different directions. The form assumed will be sphe- 

 roidal, or approximately so, supposing it to have been origi- 

 nally spherical; and the mechanical result will be the exer- 

 tion of an increased force of attraction from the sides of the 

 molecule at which its atmosphere is compressed, and a dimi- 



