286 



Drs. De La Rue and H. W. Muller. [Nov. 20, 



The experiments were made in a bell- jar, containing the terminals, 

 which could be gradually exhausted after having been filled with air 

 or other gas. One of the terminals was fixed to the bottom plate, the 

 other could be adjusted to any distance from it by a rod sliding 

 through a stuffing-box in the glass cover. The foot of the stand was 

 insulated by a disk of ebonite, on which it stands. One such bell-jar 

 is 9J inches (23*4 centims.) high and 5J- inches (14*9 centims.) in 

 diameter ; its cubical content, obtained by covering the open ends with 

 glass plates and filling with water from a graduated measure, was 

 found to be 3,787 cub. centims. 



A remarkable phenomenon was observed on making connexion 

 between the terminals and the battery by means of the discharging 

 key (already described in Part I, page 4), namely, that within certain 

 limits of pressure in the bell- jar a sudden expansion of the gas took 

 place, and that as soon as the connexion was broken the gas then 

 as suddenly returned nearly, but not quite, to its original volume in 

 consequence of a slight increase of temperature. The effect was 

 exactly like that which would have been produced if an empty bladder 

 had been suspended between the terminals, and suddenly inflated and 

 as suddenly emptied. 



The followed experiment in rarefied air, at a pressure of 56 m.m., 

 at a temperature of 17°*5 C, will give an idea of the amount of in- 

 stantaneous expansion which occurs when the terminals are connected 

 with the poles of the battery of 11,000 cells, current 0-01102 W ; 

 the resistance of the bell- jar was reproduced by substituting 600,000 

 ohms wire resistance. 



m.m. M. 



Distance of the terminals — the top one a 

 point, the lower a disk — 6 inches ; pressure 56 73,684 



On making contact the arc passed and the 



column of mercury was depressed 15*8 20,789 



Pressure on connexion 71*8 = 94,473 



The increased was to the normal pressure in the ratio of 1*282 to 1 

 as the gas was kept at a constant volume, and supposing the expansion 

 to be due to an increase of temperature, the pressure would vary 



T' 71*8 



as the absolute temperature,* therefore — — = 1*282, whence 



T 56 



T'=l-2S2x291-2=373°-3 C. ; (373*3-273*7) = 99°'6 C, the tempera- 

 ture of the bell-jar, and (99"6 — 17'5) = 82°*1, the rise of temperature 

 while the discharge was taking place. But the temperature of the 

 bell-jar as determined by a thermometer enclosed in it with its bulb 

 uppermost only rose o, 64 C. per second, taking into account the rate 



* Absolute zero =273*7 C, 273*7 + 17*5 =291*2. 



