RADIANT HEAT, AND ITS CONVERSION THEREBY INTO SOUND. 
317 
image of the spiral, its size being such that it was wholly embraced by the plate of salt. 
Here then was a beam of heat passing through an experimental tube without coming 
into contact either with the surface of the tube itself, or with any coating or lining of 
that surface. With this apparatus all my old experiments on vapours have been 
frequently repeated. There is no substantial difference between the results thus 
obtained, and those obtained with an experimental tube, where nineteen-twentieths of 
the heat which reached the pile, was reflected heat.” 
The tube referred to in this extract was of rough brass, tarnished within. Hence, 
when air entered it, after exhaustion, the dynamic heating of the tube and the 
partial condensation of the vapour when the air was moist, produced an amount of 
radiation from its internal surface which, though small, was a source of some dis¬ 
turbance. In my present experiments, therefore, another tube of the dimensions 
above given is employed, and to reduce to a minimum any radiation from its internal 
surface, it is coated within with silver, deposited electrolytically and highly polished. 
Experiments with this tube show that it does not in any way disturb the true radiation 
from the source. 
In Table I. are recorded five series of measurements executed with a brass tube 
polished within. But as the liquids, though reported pure by the manufacturing 
chemist, vary slightly from time to time, I thought it advisable to adhere to the same 
samples in experiments wherein a tube with reflecting interior is to be compared with 
one permitting of no reflection. The following five series of new measurements were 
therefore executed with the tube first mentioned. 
Table III. 
Brass tube with internal reflection. 
A 
B 
C 
D 
E 
Pressures. 
Spiral 
Spiral 
Lime 
Lime 
Lime 
(dark). 
(bright). 
(free). 
(mirror). 
(lens). 
Bisulphide of Carbon . . 
•48 
5-0 
3-1 
3-4 
31 
2-5 
Chloroform. 
•36 
7-9 
5-3 
5-6 
4-8 
5-0 
Benzol. 
•32 
9-6 
8-6 
7*9 
6-9 
6-3 
Iodide of Methyl 
•46 
12-1 
8-8 
10-2 
8-1 
7-5 
Iodide of Ethyl .... 
•36 
15-0 
11-9 
12-5 
11-3 
10-6 
Amylene. 
•26 
21-9 
15-6 
16-0 
14-4 
12-5 
Sulphuric Ether .... 
•28 
30-9 
23T 
22-7 
19-8 
18-1 
Acetic Ether. 
•29 
36-9 
27-3 
29-6 
24-1 
« • 
Eormic Ether. 
•36 
• • 
. , 
29-6 
25-0 
23-8 
Column A in this table contains the absorptions of the respective vapours, in 
hundredths of the total radiation, when the source was a platinum spiral just under 
incandescence, unaided by either lens or mirror. Under B are the absorptions when 
the source was the same spiral heated to bright redness. The same vibrating atoms 
are preserved, but in A they vibrate on the whole more slowly than in B; and as 
