From the Royal Society catalogue of papers we find that Melloni produced eighty-six memoirs byhimself, as well as three in connexion with other physicists. These embrace a wide range of subjects, but the reputation of Melloni as a physicist rests more especially upon his discoveries in radiant heat. Men of science were, in the early part of this century, very much in the dark with regard to the nature of the invisible heat rays. Leslie and others had indeed advanced the subject by means of the differential thermometer, but such an instrument was at the best a very poor substitute for the human eye. It was necessary to invent an instrument more nearly capable of doing for the dark rays what the eye does for those of light before any great increase in our knowledge of this subject could be expected to take place. This step was taken (shortly after Seebeck’s discovery of thermo-electricity) in the construction of the thermo-multiplier or combination of thermopile and galvanometer which formed the subject of a joint memoir by Nobili and Melloni in 1831. In this memoir, after describing their instrument, these physicists confirmed the experiments of Leslie and others. They tried screens of glass, sulphate of lime, mica, and ice, also of water, oil, alcohol, and nitric acid enclosed in glass, and found an instantaneous effect produced in the index of their instrument except for ice and water, the source of heat being an iron ball below redness. After finding that most substances when used as screens stopped a much larger proportion of dark heat than they did of light, Melloni set himself to discover some body that might be transparent for dark heat. In this search he was rewarded with complete success. Rock-salt was found to possess this property; and he immediately proceeded to construct prisms and lenses of rock-salt with which he proved the refraction of dark heat, that is to say, of the heat proceeding from bodies below incandescence.

Melloni was likewise very successful in studying the action upon dark heat of screens of various substances. His experiments in this and other directions are described by Baden Powell in his report to the British Association on Radiant Heat (1840). The rays of the lamp were thrown upon screens of various materials in such a manner that the effect transmitted from all the screens was of a certain uniform amount. This constant radiation was then intercepted by a plate of alum, and it was found that very different quantities of heat were transmitted through the alum in the different cases. Melloni concludes that the calorific rays issuing from the various diaphanous screens are, therefore, of different qualities, and possess what may be termed the diathermancy peculiar to each of the substances through which they have passed. One of his screens was made of green glass, and he found that a piece of alum transmitted only 1 per cent, of the heat which had passed through this screen. Green glass and alum form, therefore, an antagonistic combination.

These experiments suggest naturally a new analogy between dark heat and light which could not fail to strike Melloni, and accordingly we soon find him describing an experiment with the solar rays transmitted through green glass and then intercepted by other media. They pass copiously through rock-salt, he tells us, but feebly through alum, and hence Melloni concludes that there are amongst the solar rays some which resemble those of terrestrial heat, and in general that the differences observed between solar and terrestrial heat in the transmission of rays are to be attributed merely to the mixture in different proportions of these several species of rays. Λn instrument like the thermo-multiplier could not of course remain a monopoly, and shortly after its completion we find Professor James Forbes making use of it to prove the polarization of heat as well as to extend our knowledge of refraction. The brilliant researches of this experimentalist were, like those of Melloni, crowned with the Rumford medal of the Royal Society. On September 2, 1839, Arago communicated to the academy of sciences a letter by Melloni, who had found that rock-salt acquires by being smoked the power of transmitting most easily heat of low temperature, or dark heat. Forbes had discovered a similar property in mica split by heat, and he now showed that rock-salt roughened and mica scratched possess similar properties to blackened rock-salt. Melloni on his part took up the subject of polarization, and decided in favour of the equal polarizability of heat from different sources, a conclusion that did not then appear to Forbes to be in conformity with his experiments. It is very instructive to notice the loyalty which held both these experimentalists to the results of their observations. While Melloni differs from Forbes with regard to polarization, he will not allow the truth of a generalization proposed by Ampère, who had endeavoured to explain on the theory of undulations the identity of light and heat,—the difference of effect being dependent solely on the different wave-lengths, those producing heat being larger than those giving rise to light. Melloni admits that many phenomena may be explained by this hypothesis, but he mentions some experiments in which he thinks that this theory will not hold. The brilliant generalize!· from without has of course a different point of view from the laborious experimentalist within. They are all worthy of scientific honour—for it is by the seeming conflict, but in reality the united efforts of workers such as these that the essential element of stability in the structure of scientific knowledge is finally secured.