REPORT ON RADIANT HEAT, 3 
It was in the course of these researches that the author made 
the important discovery of the singular property possessed by 
Rock Sa tr, viz. that it is almost entirely permeable to heat even 
from non-luminous sources. He found its transmissive power six 
or eight times greater than that of an equal thickness of alum, 
which had nearly the same transparency and refractive power. 
He also discovered that (unlike other diathermanous media) it 
is equally diathermanous to ail species of heat, 7. e. to heat from 
sources of all degrees of luminosity or obscurity; or that it 
transmits in every case an equal proportion of the heat incident. 
Thus he found a plate of 7 mm. (*28 inch) in thickness trans- 
mits about 92 out of 100 rays, whether from flame, red-hot iron, 
water at 212°, or at 120° Fahrenheit. A plate 1 inch thick gave 
a similar constant ratio. 
M. Melloni’s ** Memoir on the Free Transmission of Radiant 
Heat through Solid and Liquid Bodies,” was presented to the 
Academy of Sciences at Paris, Feb. 4, 1833, and published in 
the Ann. de Chimie, No. liii. p. 1 ; a translation of it is given in 
Taylor’s Scientific Memoirs, Part I. 
The author commences with a slight sketch of the researches 
of previous experimenters, but omits to notice any distinctions 
between the characters of the heat from different sources, or the 
different Ainds of heat from one and the same source, when lu- 
minous, especially as indicated by my experiments published in 
the Phil. Trans. for 1825. 
He then proceeds to some “ general considerations on free 
transmission of caloric through bodies, and the manner of mea- 
suring it by means of the thermo-multiplier.”” This, in fact, con- 
stitutes a supplementary and more enlarged portion of his for- 
mer researches. He goes into extensive details on the precau- 
tions necessary to be used in such investigations ; especially for 
guarding against the interference of secondary radiation: as 
this changes with the change of place of the screen, he thus al- 
lows for its effects. He also gives some general observations 
on the use of the galvanometer, and the correct estimation of the 
forces acting upon it. 
The next subject of inquiry is the effect due to “the polish, 
thickness, and nature of the screens.’’ The source of heat being 
a lamp, screens were employed of glass rendered of different de- 
grees of opacity by grinding, &c.; and the effects by transmis- 
sion through them were found to be in proportion to the trans- 
parency, or that the heat follows the same proportion as the 
light. 
The effect of liquids between glass plates was then tried; and 
more rays were found to be absorbed in proportion to the increase 
B2 
