60 MR B. STEWART ON RADIANT HEAT—SECOND SERIES. 
order to increase the effect, no diaphragm was used. They were heated in the 
boiling-water apparatus already described. With this arrangement 
The thick piece gave, when polished, a deviation of, ; : ; : 21-1 
when roughened, . ; : : : 21:8 
The middle piece gave, when polished, a deviation of, ‘ : TMP es 13°6 
when roughened, : : P 5 : : 13:5 
4. The next point was to ascertain if roughening rock-salt had any effect upon 
the quality of the heat radiated. 
The following table will show that it does not alter the quality of the heat 
sensibly ; its quality being tested by its capacity of penetrating a screen of rock- 
salt :— 
TABLE I. 
Per-centage of whole Heat which 
Source of Heat. penetrates a Rock-salt Screen 
thickness *29 inch. 

Rock-salt, *77 inch thick, polished, . . . 49 
roughened, . . . 51 
Rock-salt, ‘36 inch thick, polished,. . . . 42 
roughened, . . . 43 


The trifling difference between polished and roughened salt in this table may 
fairly be attributed to error of experiment. We may therefore conclude, that 
roughening by emery-paper neither alters the quantity nor the quality of the heat 
radiated by rock-salt. 
5. Again, the transmissive power of rock-salt for lamp-black heat of the tem- 
perature 212°, is not sensibly altered by roughening the surface. This will be seen 
from the following statement :— 
The per-centage of Lamp-black 
heat transmitted was 
With Screen of Rock-salt, thickness -36 inch, polished, rig 
Ht roughened, Th 
This result naturally follows from the previous one, for it has been shown 
(First Series, Art. 19) that the absorption of a plate equals its radiation ; and since 
roughening the surface does not influence the radiation, it ought not to influence 
the absorption. 
6. B. Glass.—It is already known that roughening the surface of a plate of 
glass does not sensibly increase its radiation. It was only necessary, therefore, 
to ascertain whether roughening the surface of a radiating plate of glass alters the 
capacity of its heat for penetrating a screen of glass. 
Accordingly, a plate of crown-glass, ‘05 inch thick, 3°75 inches square, being 
placed before the cone as a screen, and a similar plate roughened, heated in the 
