PEOFESSOE TYNDALL’S CONTEIBUTIONS TO MOLECTJLAE PHYSICS. 
361 
believe, occurs when we plunge a platinum wire into a hydrogen-flame. The platinum 
is rendered white by the collision of molecules whose periods of oscillation are in- 
competent to excite vision. There is in this common experiment an actual breaking 
up of the long periods into short ones — a true rendering of unvisual periods visual. 
The change of refrangibility differs from that of Professor Stokes, firstly, in its being in 
the opposite direction — that is, from low to high ; and secondly, in the circumstance that 
the platinum is heated by the collision of the molecules of aqueous vapour, and before 
their heat has assumed the radiant form. But it cannot be doubted that the same 
effect would be produced by radiant heat of the same period, provided the motion of the 
ether could be raised to a sufficient intensity. The effect in principle is the same, whe- 
ther we consider the platinum wire to be struck by a particle of aqueous vapour oscil- 
lating at a certain rate, or by a particle of ether oscillating at the same rate. And 
thus, I imagine, by a chain of rigid reasoning, we arrive at the conclusion that a degree 
of incandescence, equal to that of the sun itself, might be produced by the impact of 
waves, of themselves incompetent to excite vision *. 
The change of quality produced in the radiation by the introduction of a platinum 
spiral into a hydrogen-flame is illustrated by a series of experiments, executed for me by 
my assistant, Mr. Barrett, and inserted subsequently to the presentation of this memoir. 
Table XXXIX. a . — Radiation through Liquids. Sources: 1. hydrogen-flame; 
2. hydrogen-flame and platinum spiral. 
Transmission. 
( ; ; 
Name of liquid. Thickness of liquid 0-04 inch. Thickness of liquid 0-07 inch. 
Bisulphide of Carbon . 
Flame only. 
. 77-7 
Flame and spiral. 
87-2 
Flame only. 
70*4 
Flame and i 
86-0 
Chloroform .... 
. 54-0 
72-8 
50-7 
69-0 
Iodide of Methyl . . 
. 31-6 
42-4 
26-2 
36-2 
Iodide of Ethyl . . . 
. 30-3 
36-8 
24-2 
32-6 
Benzol 
. 24-1 
32*6 
17-9 
28-8 
Amylene 
. 14-9 
25-8 
12-4 
24-3 
Sulphuric Ether . . . 
. 13-1 
22-6 
8-1 
22-0 
Acetic Ether .... 
. 101 
18-3 
6-6 
18-5 
Alcohol . . . . . 
. 9-4 
14-7 
5-8 
12-3 
Water ....... 
. 3-2 
7-5 
2-0. 
6-4 
Here the introduction of the platinum spiral changed the periods of the flame into 
others more in discord with the periods of the liquid-molecules, and hence the more 
* Some time after this was written I learned that Dr. Axnsr had previously inferred, from the paucity of 
luminous and extra- violet rays in the hydrogen-flame, that its periods must he extra-red. And he deduced from 
this that the heating of a platinumwire in a hydrogen-flame must consist of a change of period. A very inter- 
esting communication from Dr. Axnsr on this and kindred subjects, will be found in the ‘ Deader ’ for the 26th 
of September 1863. — April 5th, 1864. 
3 c 2 
