252 
LIGHT AND RADTANT HEAT. 
been once plate of glass, are proportionally more capable of penetrating 
passTgSrf’ a secot1 d ; which establishes a new proof of the particular state 
more easily, in which these rays subsis' :.s well as the modification they 
acquire. The results here men : : ned relate to the motion of 
caloric ; but its chemical action has also been studied in com- 
Obscnre heat P ar * son with that of light. MM. Gay Lussac and Thenard 
effects the che- have proved, that all the change* of colour produced by light, 
m cal changes. ^ imitated and effected by obscure heat, and by a tempe- 
rature not exceeding 100° of the centesimal thermometer. 
Other phenomena before observed were in proof that, in this 
comparison of the effects of caloric and light as to raising the 
temperature of bodies, or to produce chemical changes, a great 
difference must be admitted between the influences of the 
rays of different colours. In fact, M. Rochon had announced 
that the heat produced by the different rays of the same spec- 
trum are not equal. M. Herschel afterwards found that the 
calorific faculty increases progressively from the violet to the 
red j and he has even fixed the maximum beyond the red, — so 
that, according to his experiments, the most calorific rays are 
entirely, or almost totally, deprived of the faculty of enlighten- 
ing. MM. Wollaston, Ritter, and Beckmann, having likewise 
examined the other extremity of the spectrum, namely, that 
which affords the sensation of violet, have discovered, that this 
extremity also possesses peculiar properties, and that there exists 
beyond the violet, rays quite invisible, which, more than all 
the other rays of the spectrum, possess the faculty of deter- 
mining chemical combinations. But the experiments of Hers- 
chel, though confirmed by several philosophers, have been 
questioned by others, not less skilful, particularly M. Leslie. 
It became, therefore, of importance to remove every doubt 
respecting these uncertainties. It was equally interesting to 
know whether these invisible rays, or nearly so, which lie be- 
yond the limbs of the spectrum, might, nevertheless, possess 
some of the properties of light : for example, whether reflec- 
tion from polished glasses could impress upon them that peculiar 
modification, which M. Malus has denoted by the name of 
polarisation. M. Berthollet engaged MM. Malus and Berard 
to 
Inquiry re- 
specting the 
experiments 
of Herschel. 
