SECT. ii. DYNAMIC ABSORPTION AND RADIATION. 51 



of an amount of vapour in the tube possessing a tension 

 of less than the thousand millionth of an atmosphere is 

 perfectly measurable. The temperature imparted to 

 this infinitesimal quantity of matter did not exceed 0*75 

 of a centesimal degree. The molecules which consti- 

 tuted this intensely attenuated vapour, though incon- 

 ceivable, had as true an existence as the suns which 

 constitute the star-dust of the nebulae. e A platinum wire 

 raised to whiteness in a vacuum by an electric current, 

 becomes comparatively cold in a second after the current 

 has been interrupted ; yet that wire, while ignited, was 

 the repository" of an immense amount of mechanical 

 force. What has become of this? It has been con- 

 veyed away by a substance so attenuated that its very 

 existence must for ever remain an hypothesis. But 

 here is matter that we can weigh, measure, taste, and 

 smell ; that we can reduce to a tenuity which, though 

 expressible by numbers, defeats the imagination to 

 conceive of it. Still we see it competent to arrest and 

 originate quantities of force which on comparison with 

 its own mass are almost infinite, a small fraction of this 

 force causing the double needle of the galvanometer to 

 swing through considerable arcs. When we find pon- 

 derable matter producing these effects, we have less 

 difficulty in investing the luminiferous ether with those 

 mechanical properties which have, long excited the 

 interest and wonder of all who have reflected upon the 

 circumstances involved in the undulatory theory of 

 light.' 



The dynamical principle was next applied to deter- 

 mine the radiation of a gas through itself; or through 

 any other gas having the same period of vibration. For 

 that purpose Mr. Tyndall made use of the hollow cylin- 

 der 49*4 inches long already mentioned, closed at both 

 ends by plates of rock-salt, and divided internally into 

 two chambers by a movable plate of the same substance. 



E 2 



