436 Prof. Tyudall on the Action of Free Molecules on 



In 1855 Dr. Franz, of Berlin, published a paper " On the 

 Diathermancy of certain Gases and Coloured Liquids "*. He 

 found that air contained in tubes 452 and 900 millimetres 

 long, absorbed 3*54 of the radiation from an Argand lamp; 

 and he concluded that all transparent gases would behave 

 like air. I have given reasons for holding that Dr. Franz in 

 these experiments did not touch the question in hand f. In 

 the arrangement which he describes, the absorption by air 

 was quite insensible. But 60 per cent, of the radiation from 

 his powerful source was lodged in the glass ends of his tubes; 

 these, as secondary sources, radiated directly and indirectly 

 against his pile; and it was their chilling by the cold air that 

 slightly lowered his deflection and produced the supposed 

 absorption. 



It is not improbable that other attempts were made to 

 bring gaseous matter under the dominion of experiment ; bat 

 none to my knowledge are recorded. 



^ 2. Partial Svmmary of previous Work. 



My researches on magne-crystallic action carried with 

 them the incessant use of conceptions and reasonings touching 

 molecular constitution and arrangement. At an early period 

 of these studies it occurred to me that heat, both in its radiant 

 and in its ordinary thermometric form, might be turned to 

 good account as an explorer of molecular condition. The 

 first fruit of this idea was a paper " On Molecular In- 

 fluences " %, in which it was shown that wood possesses three 

 axes of calorific conduction coincident with the axes of 

 elasticity discovered by Savart. Experiments on certain 

 crystals recorded in this paper suggested a possible connexion 

 between diathermancy and conductivity ; and in 1853 I 

 worked at this question. The substances then submitted to 

 experiment were rock-crystal, amethyst, topaz, beryl, rock- 

 salt, smoky quartz, fluor spar, tourmaline, Iceland spar, 

 dichroite, arragonite, heavy spar, flint, and glass of various 

 kinds. These minerals were employed in the shape of cubes 

 carefully cut and polished, the transmission through each of 

 them, in different directions, both of radiant and conducted 

 heat being determined. 



A desire for completeness, not then attained, caused me to 

 postpone, and finally to forego the publication of the results 



* Poggendorli's Anna/en, vol. xciv. p. 337. 



f Philosophical Transactions, 18G1, vol, cli. p. 27, and elsewhere. 



t Ibid. 1863, vol. cxliii. p. 217. 



