502 



Lord Blytliswood and Mr. H. S. Allen 



place in that direction which is accompanied by a decrease 

 in volume, that is in this case to increase the volume of air 

 absorbed. 



§ 10. For comparison with these results, the amount of air 

 absorbed by the charcoal at atmospheric pressure was deter- 

 mined by direct experiment. The charcoal bulb was immersed 

 in liquid air for six minutes, as it was considered that this 

 time was long enough for the charcoal to absorb its full supply 

 of air from the atmosphere. The bulb was then allowed to 

 regain the temperature of the room, and the air expelled was 

 collected over water in an inverted burette. The mean of 

 three readings gave 69*3 c.c. as the total volume. A blank 

 experiment was then carried out without the charcoal. The 

 mean of three readings was 41*2 c.c. The difference, 28*1 c.c, 

 gives the volume of air absorbed by 2 grams of charcoal when 

 cooled to the temperature of liquid air*. 



§ 11. Observations were made with the apparatus already 

 described to determine the rate at which air was absorbed by 

 the charcoal. Readings of the height of the mercury column 

 were taken at intervals of a quarter of a minute. For one 

 minute after the application of the liquid air. the readings 

 were irregular in consequence of the cooling of the bulb, but 

 at the end of that time the liquid air had ceased to boil 

 violently, and regular readings could be obtained. The 

 mercury in the barometer-tube reached a steady height within 

 from six to ten minutes after the commencement of the 

 experiment. 



The readings obtained in Experiment No. 1 are given in 

 the following table (Table II.) :— 















Table 11. 















t. 1 



(mins.) } 



1. 



li 



li- 



If- 



2. 



2i 



2i 



2|- 



3. 



3i 



3i 



3|- 



4. 



Final. 



h 



400 

 373 

 193 



440 

 333 

 153 



475 



298 

 118 



500 



273 



93 



520 



253 

 73 



535 

 238 



58 



546 



227 

 47 



556 



217 



37 



565 



208 



28 



571 

 202 



22 



575 



198 



18 



580 



193 



13 



583 



190 



10 



593 

 180 



n 



P-Po- 



* Tins result illustrates a point frequently observed in the course of 

 these experiments. If the charcoal is left exposed to the atmosphere it 

 rapidly deteriorates in its capacity for absorbing- air, probably through the 

 absorption of moisture. In an experiment made eight weeks previously 

 over 100 c.c. of air were collected from the same bulb. The power of 

 absorption may be partially restored by heating' the charcoal, and driving* 

 off the condensed moisture, but it appears never to regain its original 

 value. Charcoal bulbs intended for the exhaustion of vacuum tubes 

 should always be kept sealed when not in use. 



