Prof. TyndalFs Contributions to Molecular Physics. 439 



the sether in which this matter is immersed. Regarding the 

 motions of the aether itself, as illustrated by the phenomena of 

 reflexion, refraction, interference, and diffraction, the optical 

 investigations of the last half century have left nothing to be 

 desired j but as regards the atoms and molecules, whence issue 

 the undulations of light and heat, and their relations to the 

 medium which they move, and by which they are set in motion, 

 these investigations teach us nothing. To come closer to the 

 origin of the sethereal waves — to get, if possible, some experi- 

 mental hold of the oscillating atoms themselves — has been the 

 main object of the researches in which I have been engaged for 

 the last five years. In these researches radiant heat has been 

 used as an instrument for exploring molecular condition, and 

 this is the object which I have kept constantly in view throughout 

 the investigation which I have now the honour to submit to the 

 Royal Society. 



The first part of these researches is devoted to the more com- 

 plete examination of a subject which was briefly touched upon 

 at the conclusion of my fourth memoir — namely, the action of 

 liquids, as compared with that of their vapours, upon radiant 

 heat. The differences which exist between different gaseous 

 molecules, as regards their power of emitting and absorbing 

 radiant heat, have been already amply illustrated. When a gas 

 is condensed to a liquid, the molecules approach and grapple 

 with each other by forces which are insensible as long as the 

 gaseous state is maintained. But though thus condensed and 

 enthralled, the sether still surrounds the molecules. If, then, 

 the powers of radiation and absorption depend upon them indi- 

 vidually, we may expect that the deportment towards radiant 

 heat which experiment establishes in the case of the free mole- 

 cule, will maintain itself after the molecule has relinquished its 

 freedom and formed part of a liquid. If, on the other hand, the 

 state of aggregation be of paramount importance, we may expect 

 to find on the part of liquids a deportment altogether different 

 from that of their vapours. 



Melloni, it is well known, examined the diathermancy of 

 various liquids, but he employed for this purpose the flame of 

 an oil-lamp, covered by a glass chimney. His liquids, moreover, 

 were contained in glass cells ; hence the radiation from the source 

 was profoundly modified before it entered the liquid at all, for 

 the glass was impervious to a considerable part of the radiation. 

 It was my wish to interfere as little as possible with the primi- 

 tive emission, and also to compare the action of liquids with that 

 of their vapours, when examined in a tube stopped with plates 

 of rock-salt. I therefore devised an apparatus in which a layer 

 of liquid of any thickness could be enclosed between two polished 



