360 PROFESSOR TYNDALL’S CONTRIBUTIONS TO MOLECULAR PHYSICS. 
that the period of vibration is not materially affected by the change from vapour to 
liquid ; for, if changed, it would probably be changed in different degrees for the 
different liquids, and the order of absorption would be thereby disturbed *. The follow- 
ing Table, in which the deportment of our series of liquids towards the radiation from 
a hydrogen-flame is recorded, will throw additional light upon this question : — 
Table XXXIX. — Radiation though Liquids. Source, hydrogen-flame. Thickness 
of liquid layer 0 - 07 of an inch. 
Name of liquid. 
Absorption. 
Transmission. 
Bisulphide of Carbon . 
27-7 
72-3 
Chloroform 
. 49-3 
50-7 
Iodide of Ethyl .... 
. 75-6 
24-4 
Benzol 
. 82*3 
17-7 
Amylene 
. 87-9 
12-1 
Sulphuric Ether . . . 
. 92-6 
7-4 
Formic Ether .... 
. 93-5 
6-5 
Acetic Ether .... 
. 93-9 
6-1 
Water 
. 100-0 
0-0 
Through a layer of water 9-21 millimetres thick, Melloni found a transmission of 
11 per cent, of the heat of a Locatelli lamp. Here we employ a source of higher tem- 
perature, and a layer of water only one-fifth of the thickness used by Melloni, and 
still we find the whole of the heat intercepted'!’. A layer of water, 0'07 of an inch in 
thickness, is sensibly opaque to the radiation from a hydrogen-flame. Hence we may 
infer the coincidence in period between cold water and aqueous vapour heated to a 
temperature of 3259° C.; and inasmuch as the period of the water-molecules has been 
proved to be extra-red, the period of the vapour-molecules in the hydrogen-flame must 
be extra-red also. 
Another point of considerable interest may here be adverted to. Professor Stokes has 
demonstrated that a change of period is possible to those rays which belong to the violet 
and extra-violet end of the spectrum, the change showing itself by a degradation of the 
refrangibility. That is to say, vibrations of a rapid period are absorbed, and the 
absorbing substance has become the source of vibrations of a longer period. Efforts, I 
believe, have been made to obtain an analogous result at the red end of the spectrum, but 
hitherto without result ; and it has been considered improbable that a change of period 
can occur which should raise the refrangibility of the light or heat. Such a change, I 
* The general agreement, in point of colonr between a liquid and its vapour, favours the idea that the period, 
at all events in the great majority of cases, remains constant when the state of aggregation is changed. 
t From the opacity ol water to the radiation from aqueous vapour, we may infer the opacity of aqueous 
vapour to the radiation from water, and hence conclude that the very act of nocturnal refrigeration which 
causes the condensation of water on the earth’s surface gives to terrestrial radiation that particular character 
which renders it most liable to he intercepted by the aqueous vapour of the air. 
