Notices respecting New Books. ‘ee 
the heat by which it had been excited. Or if there is any 
necessary connexion between these events, there remains 
the difficulty of accounting tor the slow communication of 
caloric through elastic fluids. The theory, too, is incom- 
patible with the results of the experiments of Herschel and 
Engiefield; which, if they are admitted as accurate, esta- 
blish the existence of a subtle calorific matter, eapable of 
rapid projectile motion. These experiments, however, 
Mr. Leslie considers as altogether fallacious. 
*1t has sometimes been conceived that radiant caloric is a 
species of light. . Dr. Hutton, assuming that the heating 
powers of the different species of visible light are not pro- 
portional to their power of exciting vision, supposed there 
might be a species of light capable of exciting temperature 
without exciting this sensation, and such he conceived to 
be the nature of radiant caloric. There appears little foun- 
dation for this hypothesis. So far as we can trace, radiant 
caloric has ail the properties of caloric conveyed by slow 
communication, and the mere circumstance of its assuming 
‘a state of projectile motion, if it actually do so, is insufii- 
cient to identify it with light. It exerts none of the che- 
mnical agencies of light. And the very basis of the hypo- 
thesis is subverted; for, as 1s afterwards to be stated, it is 
uncertain if any of the rays of light apart from caloric have 
a heating power: 
«« It 13 an interesting object of investigation, What i is the 
‘relation subsisting between those two modes in which calorie 
is discharged from bodies, that by radiation, and that by 
slow communication ? There appears, in general, reason to 
infer, that those which at a given temperature give off most 
caloric by communication, discharge least by radiation, and 
vice versd,—metals, for example, radiating imperfectly, 
while they yield caloric readily by communication, while 
glass is, with regard to these properties, precisely the re- 
verse. 
«An inquiry of equal importanceis, What proportion “ae 
the caloric discharged by radiation from a body suffering 
reduction of temperature, bear to that given oul by slow 
communication? The influence of each of these modes is 
established by numerous facts, That of slow communica- 
tion is well shown by the different degrees of celerity with 
which a body cools, according to the conducting power of 
the medium with which it is in contact, or according as 
the conducting power is favoured by frequent renewal of 
that mediuin ; as, for example, by the application of a cur- 
rent of air, or agitation in a liquid. The influence of ra- 
diation 
