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Vi.— Researches on Radiant Heat. Second Series. By Batrour Srewart, M.A. 
~ Communicated by Professor ForBss. 
(Read 18th April 1859.) 
Division of Subject. 
1. The first part of this paper describes the following groups of experi- 
ments :— 
Group I. On the effect which roughening the surface of a body produces upon 
its radiation. 
II. On the nature of that heat which is radiated by rock-salt, at 212° F. 
III. On the radiation of glass and mica, at high temperatures. 
The second, or theoretical portion, of the paper, has reference to the law which 
connects the radiation of a particle with its temperature, and to DuLone and 
PetTIT’s experiments on this subject. 
There is also an addition of a later date than the rest of the paper on General 
Diathermancy. 
Instruments used, and Method of Investigation. 
2. The instruments used, and the method of using them, were much the 
same as described in the first series of these researches, Art. 3. Should any 
difference occur in the method of conducting a particular experiment, it will be 
mentioned when the experiment so performed comes to be described. 
First Group of Experiments described. 
3. This group of experiments has reference to the effect of roughening the 
surface of a body upon its radiation. This was suggested to the writer by Pro- 
fessor Forses. The first substance tried was rock-salt. 
A. Rock-Salt.—lt was found that roughening the surface by means of emery- 
paper, until it became quite dim, had little or no effect in increasing the radiation, 
as will be seen from the following statement, embodying the mean result of three 
sets of experiments.* 
The pieces used were the middle piece (thickness=°36 inch) and thickest piece 
(thickness=-77 inch), described in First Series, Art. 6. These pieces were placed at 
a distance of about 4 inches from the mouth of the polished brass cone, and, in 
* In the experiments with roughened surfaces, only one of the surfaces of the substance was 
roughened, the other being left polished. In radiation experiments, therefore, the roughened sur- 
face was placed next the pile; while in transmission experiments, it was placed furthest from the 
pile. 
VOL. XXII. PART I. Q 
