THE QUARTERLY 
JOURNAL OF SCIENCE. 
OCTOBER, 1864. 
ORIGINAL ARTICLES. 
ON RADIANT LIGHT AND HEAT. 
By Batrovur Stewart, M.A., F.R.S. 
Waite the progress of knowledge more and more reveals the intimate 
relationship which subsists between different truths, it may nevertheless 
be sometimes expedient to set apart for separate consideration some 
field of science possessing a boundary line which is perfectly natural 
and definite. The subject of our choice has this advantage. We all 
know that heated bodies give out a species of influence capable of 
traversing space with enormous velocity, and in virtue of which the 
eye is enabled to perceive the sun and stars. Science further informs 
us, that such bodies emit also non-luminous rays, some of which have 
a chemical virtue, and all, including also the luminous ones, have the 
power of heating those substances upon which they fall and whereby 
they are absorbed. 
The remarks we are now about to make are capable of extension 
to the whole of this complex radiation, but we have preferred to 
embrace them under the term Radiant Light and Heat,—a title which, 
although not complete, yet recalls those properties of rays with which 
we are most familiar. 
Our limited space will not, however, permit of our discussing 
more than one of the many interesting problems presented by this 
subject for our consideration. 
An idea very generally adopted until lately, was that which regards 
a luminous body as discharging through space innumerable particles 
of exceedingly small magnitude with the almost incredible velocity of 
nearly 200,000 miles per second. But objections of a very formidable 
nature have gradually gathered around this view, until it has been 
generally abandoned, and light is now rather considered as an undulation 
which is propagated in all directions from a luminous centre, through 
some very attenuated medium pervading space. Accepting this view 
of the case, let us now briefly inquire into the nature of these waves, 
VOL, I. 28 
