PROEESSOR TYNDALL’S CONTRIBUTIONS TO MOLECULAR PHYSICS. 
331 
but in practice the pile P and the cube C' are carefully protected from the capricious 
action of the surrounding air. 
The experiments are thus performed. The empty rock-salt cell C being placed on its 
stage, a double silvered screen (not shown in the figure) is first introduced between the 
end of the tube B and the cell C ; the radiation from the spiral being thus totally cut 
off, and the pile subjected to the action of the cube C' alone. By means of the screen Q, 
the total heat to be adopted throughout the series of experiments is obtained : say that it 
is s uffi cient to produce a galvanometric deflection of 50 degrees. The double screen used 
to intercept the radiation from the spiral is then gradually withdrawn until this radiation 
completely neutralizes that from the cube C', and the needle of the galvanometer points 
steadily to zero. The position of the double screen, once fixed, remains subsequently 
unchanged, the slight and slow alteration of the source being neutralized by the rheo- 
cord. Thus the rays in the first instance pass from the spiral through the empty rock- 
salt cell. A small funnel, supported by a suitable stand, dips into the aperture which 
leads into the cell, and through this the liquid is poured. The introduction of the liquid 
destroys the previous equilibrium, the galvanometer needle moves, and finally assumes a 
steady deflection ; and from this deflection we can immediately calculate the quantity oi 
heat absorbed by the liquid, and express it in hundredths of the entire radiation. 
For example, the empty cell being placed upon its stand, and the needle being at 0°, 
the introduction of iodide of methyl into the cell produced a deflection of 30°-8. The 
total radiation on this occasion was 44°-2. Taking the force necessary to move the 
needle from 0° to 1° as our unit, the deflection 30°’ 8 corresponds to 32 such units, 
while the deflection 44 0, 2 corresponds to 58 - 3 such units. Hence the statement 
58-3:100 = 32:54-9, 
which gives an absorption of 54" 9 per cent, for a layer of liquid iodide of methyl 0-07 oi 
an inch in thickness. 
§ 2 . 
The following Table contains the results obtained in this manner with the respective 
liquids there mentioned. It embraces both the deflection produced by the introduction 
of the liquid, and the quantity per cent, intercepted of the entire radiation. It has 
been intimated to me by some of my Continental friends that the publication of such 
details as would enable a reader to judge of the precision attainable by my apparatus 
would be desirable. In this paper I shall, to some extent, endeavour to satisfy this 
desire, making use, however, of my ordinary experiments. 
Table I.— Radiation of heat through Liquids. Source of heat, red-hot platinum spiral. 
Thickness of liquid layer 0-07 of an inch. 
Name of liquid. Deflection. Absorption per 100. 
Iodide of Methyl .... 30-8 54-9 
Iodide of Ethyl 33-0 60-4 
