177 
and the Mode of its Communication. 
the concentration of the solar rays, is not simply as the conden- 
sation of those rays, but in a higher proportion ; and, that it 
depends much on their direction , being greater, as the angle is 
greater at which they meet at the focus of the lens. 
That fact is certainly very remarkable. It has often been the 
subject of my meditations ; and it has contributed not a little to 
the opinion I have been induced to adopt, respecting the nature of 
light and of heat. I never could reconcile it with the supposi- 
tion that heat is caused by the accumulation of any thing emitted 
by the sun; or by any other body which sends off calorific 
radiations. 
Reserving for a future communication, an account of the 
sequel of my enquiries respecting the subject which I have un- 
dertaken to investigate, I shall conclude this long Paper with 
some observations, concerning the practical uses that may be 
derived from a knowledge of the facts which have been esta- 
blished by the results of the foregoing experiments. 
In all cases where it is designed to preserve the heat of any 
substance which is confined in a metallic vessel, it will greatly 
contribute to that end, if the external surface of the vessel 
be very clean and bright: but, if the object be to cool any 
thing quickly, in a metallic vessel, the external surface of the 
vessel should be painted, or covered with some of those sub- 
stances which have been found to emit calorific rays in great 
abundance. 
Polished tea-urns may be kept boiling hot with a much less 
expence of spirit of wine (burnt in a lamp under them) than 
such as are varnished ; and the cleaner and brighter the dishes, 
and covers for dishes, are made, which are used for bringing 
mdccciv. A a 
