94  REPORT—1858. 
of heat radiated from polished plates of different substances at a given temperature, 
are compared with the quantity radiated from a similar surface of lampblack at the 
same temperature. The result of this group of experiments is, that glass, alum, and 
selenite radiate about 98 per cent. of what lampblack does; thick mica, 92; thin 
mica, 81; and rock-salt only 15 per cent. The second group of experiments was 
designed to compare together the quantities of heat radiated at the same temperature 
from polished plates of the same substance, but of different thicknesses. The result 
of this group was, that while the difference between the radiating power of thick and 
thin glass is so small as not to be capable of being directly observed, there is a per- 
ceptible difference between the radiation from thick and thin mica, and a still more 
marked difference between the radiation from plates of rock salt of unequal thick- 
ness. The third group of experiments was made with the view of comparing the 
radiations from various polished plates with that from lampblack, as regards the 
quality of the heat,—its quality being tested by its capability of transmission through 
a screen of the same material as the radiating plate. From this group of experi- 
ments it appears that heat emitted by glass, mica, or rock-salt is less transmissible 
through ascreen of the same material as the heated plate than heat from lampblack,— 
this difference being very marked in the case of rock-salt, which only transmits about 
one-third of the rays from heated rock-salt. The common opinion that rock-salt is 
equally diathermanous for all descriptions of heat is therefore untenable. The fourth 
group of experiments shows that heat from thick plates of glass, mica, or rock-salt is 
more easily transmitted by screens of the same nature as the heated plate than heat 
from thin plates of these materials. It was shown that all these experiments may 
be explained by Prévost’s theory of exchanges, somewhat extended. This extension 
consists of the following laws :—1. Each particle of a substance has an independent 
radiation of its own equal in all directions and without regard to the distance of the 
particle from the surface of the body. 2. The radiation of a particle equals its 
absorption, and that for every description of heat. 3. The flow of heat from within 
upon the interior surface of a polished plate of indefinite thickness is proportional to 
the index of refraction of the body, and that for every description of heat. The bear- 
ing of these experiments on Dulong and Petit’s law of radiation was then attempted 
to be traced. It was shown that unless bodies from simply being heated change 
their transmissibility for the same description of heat (which there is no reason to 
suppose), the radiation of thin plates or particles at a high temperature will bear a 
less proportion to the total radiation of that temperature than at a low,—the conse- 
quence will be, that the radiation of single particles will increase with the tempera- 
ture in a less degree than Dulong and Petit’s law would indicate. It may even be 
that the radiation of a particle or very thin plate may be proportional to the abso- 
lute temperature of that particle. Taking a piece of glass or mica, therefore, ata 
low temperature, as it is very opake with regard to the heat radiated by itself, we 
may suppose that the total radiation consists of that of the outer layer of particles 
only, that from the inner layers being all stopped by the outer. At high tempera- 
tures, however, we may suppose that there is not only the radiation of the outer 
layer, but also part of that of the inner layer which has been able to pass, swelling 
up the total radiation to what it appears in Dulong and Petit’s experiments. This 
way of looking at radiation may possibly bring the radiative power of particles to 
obey the same laws with the conducting power of particles, which Prof. Forbes has 
shown decreases with an increase of temperature. The author of this communica- 
tion is indebted to Prof. Forbes for the use of the instruments and substances em- 
ployed, and also for many valuable suggestions with regard to the experiments it 
contains. 
ELEctTRIcITy, MAGNETISM. 
On the Intensity of the Terrestrial Magnetic Force. By J. DRumMonp. 
In comparing the observations of the dip with those of the intensity, the author 
found some anomalous results, of which the following is anexample. In the diurnal 
