331 
RADIANT HEAT, AND ITS CONVERSION THEREBY INTO SOUND. 
When the requisite quantity of sulphuric ether vapour, viz. 7'2 mercury inches, 
was in the experimental tube, I tried whether the radiation from a Leslie s cube, 
coated with lampblack and filled with boiling water, could pass through the vapour. 
About 14 per cent, of the incident heat was transmitted. Had I been asked at the 
time whether a liquid layer of sulphuric ether 1 millimeter thick was pervious to the 
heat of the cube, I should have replied with some confidence in the negative. Hence, 
for the moment, I thought the experiment opposed to the law that vapour absorption 
and liquid absorption, when equal quantities of matter are compared, are the same. 
On actually testing a layer of the liquid ether 1 millimeter thick, the transmission of 
upwards of 6 per cent, of the incident heat was observed. So that in this case also 
we have harmony of deportment between liquid and vapour. The absorption of the 
vapour exceeds that of the liquid because the heat from the cube radiated freely 
against the interior silvered surface of the experimental tube, and by its reflection 
from that surface had its path through the vapour augmented in length. This 
augmentation naturally carried with it an increase of the absorption. 
The next substance examined was hydride of amyl, the boiling point of which is 
30° Faiir., or 5° lower than that of sulphuric ether. When a large evaporating surface 
is exposed, there is therefore no difficulty in obtaining, from this liquid, vapour of a 
pressure of 6*6 inches of mercury. This, in a tube 38 inches long, would, if squeezed 
to liquefaction, produce a layer 1 millimeter thick. Vapour absorption and liquid 
absorption being measured in succession, this is the behaviour of the hydride of amyl:— 
Absorption by vapour . . . . 51 per cent. 
Absorption by liquid ... .51 per cent. 
the two absorptions being absolutely identical.'' 5 ' 
Combining this section and the last, their joint results may be thus summed up. 
Beginning with a column of sulphuric ether vapour 38 inches long at 7‘2 inches 
pressure, or with a column of hydride of amyl vapour 38 inches long and at 6'6 inches 
pressure, and gradually shortening the column without altering the quantity, the 
vapour would gradually augment in density and pass wholly, when reduced to a 
thickness of 1 millimeter, into the liquid state of aggregation. Suppose a beam of 
heat of constant value, after passing through the vapour, to impinge upon a thermopile 
and to produce a definite galvanometric deflection ; this deflection would remain 
absolutely fixed during all the changes of density and aggregation which we have 
* When the rocksalt cell was empty, reflection of course occurred at its two interior surfaces. A 
perfectly diathermanous liquid, with the refractive index of rocksalt, would annul this reflection. And 
though the liquids actually employed had a smaller refractive index than rocksalt, and though they were 
far from being perfectly diathermanous, their introduction into the cell must nevertheless have diminished 
the reflection and thus added to the transmitted heat. This addition, having been determined by calcu¬ 
lation, was sensibly neutralised by the introduction of washers of thin note paper which slightly augmented 
the thickness of the liquid stratum traversed by the calorific rays. 
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