CHAP. VI., 8.] 



HEAT. MELLONI. 



157 



power of different surfaces to account for the appa- 

 rently capricious formation of dew in different situ- 

 ations, establishing that it is moisture deposited 

 from the lowest stratum of air upon surfaces cooled 



below the " dew-point," by their unrequited radiation 

 of heat towards a clear sky. A slight wind, by con- 

 tinually restoring the equilibrium of temperature to 

 the surface, prevents the deposition. 



8. MELLONI. Recent History of Radiant Heat Transmission and Refraction of Heat ; Pro- 

 perties of Heat analogous to Colour. Experiments in Great Britain on the Polarization 

 and Double Refraction of Heat. 



(707.) 

 I Recent ob- 

 servations 

 on radiant 

 heat. 



(708.) 

 Melloni 

 the asso- 

 ciate of No 

 bili. 



(709.) 

 The ther- 

 mo-multi- 

 plier, an 

 instrument 

 of research. 



(710.) 



THE length to which this chapter has already ex- 

 tended, must be my apology for bringing concisely to a 

 conclusion what remains to be stated regarding the 

 progress of the subject of radiant heat. With the ex- 

 ception of the excellent researches of De la Roche on 

 the immediate transmission of radiant heat through 

 glass (mentioned in the preceding Section), but which 

 that ingenious philosopher did not live to extend 

 and complete, little of importance was done between 

 the researches of Leslie and those of Melloni, of 

 which we are now to speak. 



MACEDONIO MELLONI, a native of Parma in Italy, 

 became associated as an experimenter, probably about 

 ,the year 1828 or 1829, with Nobili, a skilful and 

 ingenious physicist of Reggio (Modena). Nobili 

 was well known by his experiments on galvanic Elec- 

 tricity and on Electro-Magnetism. He was also the 

 great improver of Schweigger's Multiplier, rendering 

 it an instrument of precision ; and to him we owe the 

 happy and ingenious application of Thermo-Electri- 

 city to the measurement of minute effects of heat. 



The THERMO-MULTIPLIEK, a thermometer of extreme 

 delicacy, though improved by Melloni, was [as has just 

 been stated) the invention of Nobili. It consists of 

 two portions, a sentient part and an indicating part. 

 The first is composed of a number of short thin bars of 

 antimony and bismuth, arranged like a square faggot, 

 pairs of bars being soldered together in consecutive 

 order at the opposite ends of the faggot, so as to form 

 a single bent metallic conductor. If the junctions 

 exposed at one end of the faggot are subjected to 

 heat, and those at the other end kept cool, the effect 

 will be a thermo-electric current of considerable in- 

 tensity generated by the pile. This current is con- 

 veyed by means of two wires from the opposite ends 

 of the system, which are connected with a delicate 

 galvanometer which forms the indicating part of the 

 apparatus. In practice, one end of the pile, armed 

 with a conical reflector for concentrating the rays of 

 heat, is exposed to a calorific source whose radiant 

 effect is to be measured, whilst the other end is care- 

 fully screened from external influences. The devia- 

 tions of the galvanometer needle indicate the heating 

 effect as on the scale of a thermometer. The pre- 

 cautions required in the construction and use of the 

 instrument, and in the interpretation of its results, 

 are too numerous to be mentioned here. 



Nobili, in conjunction with Melloni, applied the 

 thermo-multiplier (amongst other experiments) to 



the proof of the instantaneous transmission of heat 

 through glass and other solid and liquid bodies. 



From 1831, this enquiry was conducted nearly (711.) 

 exclusively by Melloni, who about that time settled Melloni 

 first in Geneva and then in Paris, having been com- ^^ en 

 pelled, on political grounds, to quit Italy. His first derful 

 and most important original memoir was presented trans- 

 to the Academy of Sciences early in 1833, and was parency of 

 received with mai'ked coldness, if not incredulity, by f^ h^t. 

 that body. A few months later, the writer of these 

 pages had an opportunity of seeing Melloni's ex- 

 periments in Paris, and he made known their im- 

 portance at the immediately succeeding meeting of 

 the British Association at Cambridge. The Royal 

 Society of London in no long time awarded their 

 Rumford medal to Melloni, after which mark of 

 foreign approbation, he first obtained a hearing 

 from the Institute of France. The most consider- 

 able result at which he had then arrived was this : 

 that rock-salt possesses a power unapproached by 

 any other substance of transmitting heat of any 

 temperature and from whatever source, with ex- 

 tremely little loss ; and as a natural consequence of 

 this, that heat wholly devoid of luminosity, such as 

 that from boiling water, or even the heat of the hand, 

 may be refracted by prisms and lenses of rock-salt 

 exactly in the same manner as light is refracted by 

 glass. The reality of these effects (which had excited 

 the persevering scepticism of the Parisian savans) 

 was demonstrated by a great number of most inge- 

 nious experiments, in which every possible source of 

 error and confusion was avoided or allowed for. 



Melloni even believed that the loss observed in (712.) 

 passing heat of any temperature, high or low, through 

 polished screens of rock-salt, was precisely the same ; 

 and, moreover, that it occurred entirely at the two 

 surfaces by partial reflection, so that the solid me- 

 dium was absolutely transparent for every kind of 

 heat. It is certain that the loss is in every case 

 small; but this almost paradoxical conclusion has 

 not been completely confirmed by those who have 

 repeated his experiments. The important and un- 

 expected discovery of the nearly complete trans- 

 parency of rock-salt for heat, enables us to construct 

 complex thermotic apparatus for refracting and con- 

 centrating it, analogous to those of glass which are 

 used in optics. 



The next point clearly made out by Melloni, was (713.) 

 the specific action of different bodies in sifting the 



