368 PEOEESSOE TYNDALL’S CONTEIBTJTIONS TO MOLECI7LAE PHYSICS. 
with those of MM. Wiedemann and Franz on conduction, we obtain the following 
Table : — 
Table XLII. — Comparison of Conduction and Radiation. 
Name of metal. 
Conduction. 
Eadiation. 
Silver . . , 
. . . 100 
11 
Gold . . , 
. . . 53 
27 
Brass . . 
. . . 24 
42- 
Tin ... . 
, . . 15 
90 
Platinum . 
. . 8 
100 
We here find that, as the power of conduction diminishes, the power of radiation 
augments — a result, I think, completely in harmony with that to which a consideration 
of the molecular mechanism leads us. There is but one serious exception known to me 
to the law here indicated ; this is copper, which MM. De la Provostaye and Desains 
place higher than gold as a radiator, though it is also higher as a conductor. When, 
however, the immense change in radiative power which the slightest film of oxide can 
produce, and the liability of heated copper to contract such a film, are taken into account, 
the apparent exception will not have too much weight ascribed to it. I have had a 
cube of brass coated electrolytically with copper, silver, and gold ; and, of all its faces, 
that coated with copper has the least emissive power. This is probably due to some 
slight impurity contracted by the silver. What we know of the deportment of minerals 
also illustrates the law. Rock-salt I find to be a far better conductor than glass, while 
MM. De la Provostaye and Desains find the relative emissive powers of the two 
substances to be as 17 to 6 : the radiant power of the salt is little more than one-third 
that of the glass. So also with regard to alum : as a conductor it is immensely behind 
rock-salt ; as a radiator it is immensely in advance of it. 
Royal Institution , March 1864. 
