RADIANT HEAT, AND ITS CONVERSION THEREBY INTO SOUND. 
311 
with such tubes, and those obtained with polished tubes in which internal reflection 
came into play. 
Such are the general features and phases of a discussion which, though dealing only 
with a small item of my work, has consumed a considerable amount of time. Other able 
experimenters have entered this field, the latest of whom, MM. Lecher and Pernter, 
have published a long and learned memoir in Wiedemann’s Annalen, which has been 
translated in the Philosophical Magazine for January, 1881. My experiments with 
gases they corroborate, but not those with vapours. Regarding the action of aqueous 
vapour they are especially emphatic, their conclusion being : “ that moist air does not 
perceptibly absorb the heat rays from a source of 100° C.” In fact they found moist 
air a little more transparent than a vacuum. “Ho imaginable source of error,” they 
affirm, “has here been left out of account,” The arrangement for filling with moist 
air was varied, the air allowed to stand for a long time over the water in the gas¬ 
holder, and this moist air then passed through several wash-bottles into the experi¬ 
mental space, but with the same negative result.” In common with Magnus, 
MM. Lecher and Pernter ascribe my results to the condensation of liquid films on 
the rocksalt plates, and on the polished inner surface of my tube.* 
§. 4. Experiments resumed: Verifications. 
With a view to my own instruction, and to the removal of uncertainty from other 
minds, these researches on radiant heat were resumed in November, 1880. A brass 
experimental tube 4 feet long, 2§th inches in diameter, and polished within, was first 
employed.! Interposed between it and the source was a “ front chamber ” through 
which, when exhausted, the rays passed into the experimental tube. A plate of 
transparent rocksalt separated the tube from the chamber; while a second plate 
of salt closed the distant end of the experimental tube. The source of heat was 
at first a Leslie’s cube containing water at 100° C., to one of the faces of which 
the end of the front chamber was carefully soldered. The chamber also passed 
air-tight through a copper cell in which a continuous circulation of cold water was 
kept up. The heat which might otherwise have reached the experimental tube by 
conduction from the source was thus cut off. One face of a thermopile, provided with 
# However I may otherwise differ from MM. Lecher and Pernter, I agree with their opening remark, 
that few other questions of experimental physics present difficulties so great as the one here under con¬ 
sideration. Nor do I see reason to differ from their closing words, that “ the extraordinary difficulty of 
investigations of this sort would he richly repaid by the attainment of quantitative results ; whilst the 
corresponding optical investigations (immeasurably easier) will always remain more of a qualitative 
nature.” It is the difficulty here signalised that has caused so many distinguished investigators to go 
astray in this field of inquiry. I may state here that on the receipt of their paper I wrote to MM. Lecher 
and Pernter, but my communication was returned from Vienna through the dead letter office. 
f The plate answering to this description will be found in the Philosophical Transactions for 1861. 
