292 PROFESSOR TYNDALL ON THE ACTION OF FREE MOLECULES ON 
merits'”. Mellon: extended to radiation his conclusion regarding absorption. “ On 
ne commit,” he writes, “ aucun fait qui demontre directement le pouvoir emissif des 
fluides elastiques purs et transparents. ”t Such was Melloni’s relation to the subject 
now before us. 
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 
millimeters long, absorbed 3 ’54 of the radiation from an Argand lamp, and he con¬ 
cluded 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.§ 
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; but none to my knowledge are recorded. 
§.2. Partial Summary 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 Mole¬ 
cular Influences/j| in which it was shown that wood possesses three axes of calorific 
conduction coincident with the axes of elasticity discovered by Sayart. 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 sub¬ 
stances 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 of this inquiry. It, however, kept alive reflections 
* “ Pour un intervalle cle cinq a six metres, Fair n’exerce aucune absorption sensible pour le rayonne- 
ment des corps cbauds,” ‘La Tbermocbrose,’ p. 136. 
f Annales de Obimie et de Physique, vol. xxii., p. 494. 
t Poggendorff’s Annalen, vol. xciv., p. 337. 
§ Philosophical Transactions, 1861, Vol. 151, p. 27, and elsewhere. 
|| Ibid., 1853, Vol. 143, p. 217. 
