10 W. A. Norton—Molecular and Cosmical Physics. 
15. All the mechanical properties of bodies may be ascribed 
to varied values of the quantities ~.. m, and 7? (vol. ii, p. 338), 
and to the variations that may occur in these values under dif- 
ferent circumstances. These quantities must depend primarily 
upon the mass and size of the atoms around which the ethereal 
atmospheres and electric envelopes are condensed. The marke 
difference in the properties of certain substances which have 
nearly the same atomic weight, indicates that atoms of the same 
mass may differ in size. 
It is to be observed that the same substance may assume 
various states of aggregation of its primitive molecules, in which 
it exhibits different properties; under varied thermal or other 
circumstances of solidification, giving rise to modifications of 
the molecular envelopes. One general result may be noted, viz., 
that an aggregation of compound molecules should have less 
tenacity than one of primitive molecules of the same substance; 
since in the latter case no two molecules can be drawn asunder 
sses in 
is, are propagated indefinitely as unneutralized heat-pulses. 
At the same time a change Dace occurs in the physical con- 
