Elastic Force, Density, and Temperature in Gases. 57 



taining silver, gold, and platinum) is less than ^ J th of an inch*, 

 was considerably agitated by the jet, and the index-needle set 

 into vibration, so that tenths of degrees could not be read off 

 with quickness and certainty. 



In January 1854 I used a method of experiment similar to 

 that with Marcetfs boiler, described in the paper of 1853, namely, 

 the exterior thermometer having been compared with Breguet's 

 within the reciver, this latter fell on the pump being worked ; 

 and when it had risen gradually to some particular degree, the 

 jet was allowed to pass for about one second, and the effect upon 

 the -index-needle noted. When the barometer-gauge of the air- 

 pump stood about half the height of the barometer, we had 



S=z ^ ; =1 nearly; and if Breguet's thermometer stood 4°, 



3°, 2^°, 1 t 4 q°, or 1° degree below the exterior thermometer, 

 it rose instantly on the jet passing through the helix; but when 

 the difference was only ~o° & fell. These are in accordance 

 with the former results, which gave J° as the temperature of 

 such a jet ; but the actual readings of the index, contrary to 

 what I had hoped for, were irregular and uncertain. 



Experiments were then tried of rarefactions about five and 

 three ; or if the barometer stood at 30 inches, the jet was passed 

 through the helix when the barometer-gauge stood at about 25 



7) "—'2) 



inches and about 23 inches, for then 8=- — ~- became 

 S= 30-5 =5jandS= 30-i = 3f 



The first motion of the needle was always in the direction due 

 to cold ; but the very great agitation of the helix and index-needle 

 from the violence of the jet, prevented a reading being obtained 

 before the temperature approached that of the external air. 

 Accordingly this method of experiment only confirmed the 

 results of the paper of 1853, without adding anything to them. 

 . The next method of experiment which I adopted was with a 

 Newman's air-pump, and the Breguet's thermometer placed 

 under a glass receiver upon the plate. Newman's air-pump 

 having a single barrel of large dimensions, a considerable ex- 

 haustion of a moderate-sized receiver is obtained by the first 

 stroke of the piston ; and this can be easily performed, and the 

 communication cut off by the stopcock in one second of time. 

 The object was to find the effect of such rapid exhaustion on the 

 thermometer, and then by using receivers of different sizes 

 giving different degrees of exhaustion for the same single stroke 

 of the pump, to find the law for the corresponding changes of 

 * gVth of a millimetre. 



