Cuap. VI, § 8.] 
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 
HEAT.—MELLONI. 
955 
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- 
ies of Heat analogous to Colour.—Experiments in Great Britain on the Polarization 
and Double Refraction of Heat. 
(707.) Te length to which this chapter has already ex- 
piace: tended, must be my apology for bringing concisely toa 
on radiant conclusion what remains to be stated regarding the 
heat, 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. 
(708.) Maceponto Mettront, a native of Parma in Italy, 
pn eel became associated as an experimenter, probably about 
‘hiwofo.the year 1828 or 1829, with Nobili, a skilful and 
Dili. 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 Tuermo-Mvtripxier, athermometer of extreme 
The ther~_ delicacy, though improved by Melloni, was (as has just 
plier, an been stated) the invention of Nobili, It consists of 
instrument two portions, a sentient part and an indicating part. 
of research. (he 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 
(709.) 
(710.) 
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- pre in 
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 emg 
received with marked coldness, if not incredulity, by go» te 
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 
passing heat of any temperature, high or low, through 
polished sereens 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 
the specific action of different bodies in sifting the 
(712.) 
(718.) 
