KNOBLAUCH ON RADIANT HEAT. 193 



diathermanous media did not appear to me sufficiently dis- 

 proved, and I therefore endeavoured experimentally to decide the 

 question, 



Whether the power possessed by rays of heat, of passing through 

 certain bodies, has any perceptible relation to the temperature of 

 the sources from which they are derived. 



After the extended investigations made by Melloni on the 

 most dissimilar bodies with such extreme care, I could not expect 

 to discover new substances which (for the sources of heat used 

 by him) might be classified with prepared rock salt as regards 

 the transmission of heat. I therefore preferred changing the 

 sources of heat instead of the diathermanous media. 



1. In the first series of experiments I made use of red-hot 

 platinum, the flame of alcohol, an Argand lamp, and the flame 

 of hydrogen. The former was kept at a red heat without flame 

 (according to Davy^s method*), by being placed on the wick of 

 a spirit-lamp, which it surrounded spirally. The alcohol-flame 

 had a uniformly trimmed wick, which never carbonized, and 

 dipped in the fluid contained in a glass vessel. The Argand 

 lamp, which was at a constant level, with a double current of 

 air, had a cylindrical wick without a chimney. The flame of 

 hydrogen issued from the tube of a gasometer constructed for 

 the purpose, and which allowed the gas to escape under a con- 

 stant pressure. The constancy of these sources of heat during 

 the experiment was tested most carefully. They were, of course, 

 only allowed to act upon the thermoscope to such an extent as 

 would allow of their being submitted to comparison, being pro- 

 tected from the rays of parts accidentally heated with them by 

 polished metallic screens. 



However uncertain determinations of temperature may be 

 with regard to this point, nevertheless all natural philosophers 

 are agreed that the degree of heat of a red-hot spiral of platinum 

 wire is less than that of a flame of alcohol, which is able to raise 

 the wire to a yellow heat; and less than that of an Argand 

 lamp, in which carbon is raised to a white heat. Moreover, all 

 would agree that the hydrogen flame t has the highest tempera- 

 ture among the sources of heat we have mentioned. 



The next question is, whether, in correspondence with the 

 position advanced by Delaroche, the heat of the alcohol flame 

 and the Argand lamp would pass through diathermanous bodies 



• Coinniuniculcd to the Royal Society of Loiiildii, Jan. 2.'), 1817. 

 t Mitsclierlich, Li'/nhuc/i ilcr Chemic, ;h(\ edit., part i. p. 289-290. 



