Refraction and Polarization of Heat. 425 
heat, and in historic order presented the views of Bacon, Boyle, 
Boerhaave, Stahl, and Black, and adverted to the discoveries of 
Black respecting latent and specific heat, and the successive labours 
of Irvine, Crawfurd, Wilke, Magellan, Lavoisier and Laplace, Dulong 
and Petit, in the same field. 
Heat presents itself in two very different conditions ; first when 
combined with matter, pervading bodies slowly, either by commu- 
nication and conduction through and among its particles, or by the 
movements of the particles themselves; secondly, when radiated, 
moving through elastic fluids or empty space with vast velocity. 
The first of these had been studied by the philosophers already 
named, and not long after by Rumford. To the second of these, viz. 
radiant heat, the subject of Professor Forbes’s discovery called upon 
him more especially to allude, and to present a brief historic view. 
The radiation of cold, and its reflection by metallic mirrors, was 
known to Baptista Porta in the sixteenth century ; and observations 
were made on the radiation of heat, by the Florentine academicians, 
towards the middle of the seventeenth century, and by Marriotte in 
1682. About the middle of the 18th century, Lambert published 
his works on pyrometry and photometry, which contained some of 
the first accurate experiments on this subject ; and the facts of the 
difficult transmission and reflection of heat by glass, were pointed 
out by the Swedish chemist Scheele. Pictet of Geneva extended 
his experiments on the radiation and the reflection of the heat de- 
rived from boiling water; and our venerable associate Professor Pre- 
vost of the same place, established the doctrine of the mobile equi- 
librium of heat, in 1802. The triumph of this theory was found in 
the beautiful experiments of Dr. Wells, on dew, in 1813. 
Meanwhile, the experiments of Rumford and Leslie were corro- 
borating and extending these general views, even although the doc- 
trines of radiation were denied by the latter philosopher in all his 
writings. The passage of radiant heat through solid substances, 
such as glass, and through fluids, such as water, had long been ad- 
mitted, in the case where light accompanied heat. But in the case 
of non-luminous heat, it was strenuously denied by Leslie, and others. 
The experiments of De la Roche proved that such was the fact, at 
least in the case of heat derived from terrestrial sources, and at the 
same time luminous. But this subject has received a vast enlarge- 
ment by the recent experiments of Melloni, who has shown that 
substances differ surprisingly in their permeability to heat, and that 
while some, such as alum, stop almost every incident ray, others, 
as rock-salt, transmit almost the whole of the heat, and that from 
whatever source derived, 
The connexion of light with heat was too obvious and important 
to be overlooked. To Sir W. Herschel the world is indebted for 
the first great step in this curious inquiry. He examined the ther- 
mometric qualities of the spectrum formed from the sun’s rays bya 
common prism of glass ; and in 1800 announced the curious fact, 
that the heating power increases, not only from the violet to the red 
end of the spectrum, but even beyond the latter, indicating the exist- 
ence of dark calorific rays. These experiments, though at first denied 
