PROFESSOR TYNDALL’S CONTRIBUTIONS TO MOLECULAR PHYSICS. 359 
vapour, as compared with those of carbonic acid, are slow, may therefore be inferred 
from these experiments. 
The two following Tables illustrate the action of carbonic acid gas and olefiant gas 
respectively, on the radiation from a flame of hydrogen : — 
Table XXXVII. — Eadiation through Carbonic Acid Gas. Source, hydrogen-flame. 
Tension in inches. 
Deflection. 
Absorption. 
1 
5-5 
7-4 
2 
9-5 
12-8 
4 
11-0 
14-9 
30 
19-0 
25-7 
Total heat . . . 
. 48-5 
100-0 
Table XXXVIII.- 
•Eadiation throug 
h Olefiant Gas. 
Source, hydrogen-flame. 
Tension in inches. 
Deflection. 
Absorption. 
From Table XXXYII. 
1 
12-0 
16-2 
7-4 
2 
18-0 
24-3 
12-8 
4 
24-0 
32-4 
14-9 
30 
38-5 
58-8 
25-7 
Total heat 
. . 48-5 
100-0 
100-0 
A comparison of the last two columns, one of which is transferred from Table 
XXXVII., proves the absorption of the rays from a hydrogen-flame by olefiant gas to 
be about twice that of carbonic acid; while, when the source was a carbonic oxide 
flame, the absorption by carbonic acid at small tensions was more than twice that effected 
by olefiant gas. 
§n. 
Water at moderate thickness is a very transparent substance; that is to say, the 
periods of its molecules are in discord with those of the visible spectrum. It is also 
highly transparent to the extra-violet rays ; so that we may safely infer from the deport- 
ment of this substance its incompetence to enter into rapid molecular vibration. When, 
however, we once quit the visible spectrum for the rays beyond the red, the opacity of 
the substance begins to show itself: for such rays, indeed, its absorbent power is 
unequalled. The synchronism of the periods of the water-molecules with those of the 
extra-red waves is thus demonstrated. I have already proved that undried atmospheric 
air manifests an extraordinary opacity for the radiation from a hydrogen-flame, and from 
this deportment I inferred the synchronism of the cold vapour of the air and the hot 
vapour of the flame. The vibrating-period of a molecule is, no doubt, determined by 
the elastic forces which separate it from other molecules, and it is worth inquiring how 
these forces are affected when a change so great as that of the passage of a vapour to a 
liquid occurs. The fact established in the earlier sections of this paper, that the order 
of absorption for liquids and their vapours is the same, renders it extremely probable 
mdccclsiv. 3 c 
