182 



Professor Sir James Dewar 



[Jan. 20, 



40 c.c), a little annular space is arranged at D, into which liquid air 

 is poured immediately before the experiment begins. 



The charcoal, after being placed in the small bulb C, is heated to 

 a low red heat, and simultaneously exhausted by a good air-pump, 

 and after all the gas has been removed, the stop-cock E is closed. In 

 this condition it is placed in the calorimeter B. 



The experiment is conducted by connecting the end of the tube 

 at E by means of an indiarubber tube with the graduated vessel F 

 containing the gas to be examined. When all is ready the stop-cock 

 E is opened, so that the gas may rush into the charcoal, and the heat 

 evolved by its absorption distils off the equivalent quantity of liquid 

 air from the calorimeter B, which is then received and measured in 

 the vessel G. 



When we know the constant of the calorimeter — namely, the 

 number of cubic centimetres of gas evaporated by one calorie (about 

 14 • 5 c.c. in the present case) — from the readings of the two jars F 

 and G we find at once the heat evolved in the condensation per unit 

 volume of gas absorbed. After making a few necessary corrections, 

 the results for different gases are as given in the following table : — 



To render these results comparable, the same specimen of cocoa- 

 nut charcoal was used for them all, and the numbers in the last 

 column are for absorption by one gramme of charcoal. The volumes 

 of the gases absorbed are, both at ordinary and low temperatures, 

 given under standard conditions — namely, 0° C. and 760 mm. of 

 mercury pressure. If it is desired to know the volumes absorbed at 

 — 185° C, when these volumes are measured at — 185°C. and 

 760 mm. pressure, we have only to divide the numbers in column II. 

 by three. 



In each case, a very remarkable increase of absorption takes place 

 at the low temperature. This is shown in column III. It is further 

 remarkable that the increase of absorption diminishes, roughly, as the 

 boiling points of the various gases increase, while column IV. shows 

 a corresponding increase in the quantities of heat evolved. 



