ee ee ee Te a eT EE ee ee ee ee 
W. A. Norton—Molecular and Cosmical Physics. 11 
molecules, which convey impulses of the same intensity as those 
expended in arresting the molecular movements. Heat is 
developed in this way when bodies are compressed by pressure, 
or impact. The heat of friction also originates in this manner; 
for between the molecules of the two rubbing surfaces the force 
for which the ratio ~ exceeds 69, will, when ie aa give 
out heat in this manner; since the mutual attraction of their 
envelopes will diminish. We have seen that the same state of 
things occurs with india-rubber when it is stretched. In fact, 
it appears from the table on page 444 (vol. iii), that all 
bodies of matter for which the ratio - is less than 6°9 are in 
this t in the same condition with iidia -rubber. There are 
certain oe aa reasons for believing that all liquids belong to 
this class, on this view the heat of congelation may be as- 
cribed to a ie apse of the molecular envelopes resulting from the 
expansion of the mass that ensues as the compressive force at 
surface of the liquid ceases to operate. Except in the case 
of water, the molecules of the liquid being in what has been 
e secondary condition, this collapse of the envelopes 
will be attended with an augmentation of their attractive 
actions and a consequent attraction of the mass; but this aug- 
mentation will increase the ratio ~~ and so tend to make the. 
