REPORT ON RADIANT HEAT. 11 
water of increasing thickness, the maximum successively occurs 
in the red, and thence upwards to the green. A similar effect 
is produced by colourless glasses; but with colowred glasses, 
whilst the Jwmtnous spectrum is variously absorbed and altered, 
the place of the maximum of heat remains unaltered, and the 
decrease from it quite regular. 
Another experiment consists in interposing a diaphanous 
body, which absorbs all the calorific, but only a part of the 
luminous rays. On using in this way a peculiar species of 
green glass coloured by oxide of copper, the greenish light 
transmitted “‘ exhibits no calorific action capable of being ren- 
dered perceptible by the most delicate thermoscopes, even when 
it is so concentrated by lenses as to rival the direct rays of the 
sun in brilliancy.” 
On these points Prof. Forbes has made some remarks in the 
London and Edinburgh Journal of Science, March, 1836. 
Such experiments as these, he justly observes, and indeed 
many more simple, clearly show that heat is not light, but 
nothing more. It is a question, then, what is the point really 
aimed at in these speculations. The author agrees with Mel- 
loni in the result, “ that one and the same undulation does not 
invariably impress the senses of sight and feeling at once. The 
great difficulty is this—to account for the equal refrangibility 
of two waves having different properties.” 
New Phenomena of Transmission: Melloni and Forbes. 
It appears by the Comptes Rendus, that on September 2nd, 
1839, M. Arago communicated to the Academy of Sciences a 
letter by M. Melloni, containing some new and highly interest- 
ing experiments on the transmission of radiant heat. He found 
that rock salt acquires, by being smoked, the power of transmit- 
ting most easily heat of low temperature, or of that kind which 
is stopped in the greatest proportion by glass, alum, and (ac- 
cording to his view) all other substances. 
Upon this point, Prof. Forbes was led to some further con- 
siderations, and thence to fresh series of researches ‘* On the 
effect of the mechanical Textures of Screens on the immediate 
Transmission of Radiant Heat,’ an account of which he com- 
municated to the Royal Society of Edinburgh, Dec. 16, 1839. 
Upon the above-mentioned result of Melloni, Prof. Forbes 
remarks, that according to the conclusions indicated in his own 
Researches (third series), Melloni’s view of the interception of 
heat of low temperature by all substances alike, is equivalent to 
saying that substances in general allow only the more refrangi- 
ble rays to pass, or that while rock salt presents the analogy of 
white glass, by transmitting all rays in equal proportions, every 
