486 Professor Sir James Dewar [June 8, 



same property is possessed by meerschaum and silica. The retentive 

 power of alumina is much inferior, however, to that of charcoal, and 

 consequently is of no use in forming high vacua. All charcoals possess 

 the property of gas absorption at low temperatures. Light charcoal, 

 such as is used in the manufucture of gunpowder, or the variety got 

 from animal substances like blood, both act, but experiments made at 

 the Royal Institution show that charcoal prepared by careful carbonisa- 

 tion from cocoa-nut is one of the most convenient varieties to use. 

 While using the same process of carbonisation, the absorptive 

 effect is enhanced by making it take place slowly and with gradually 

 increasing temperature. With the samples made a year ago, it 

 was possible to get an absorption of about 150 c.c. of air per 

 gramme of charcoal at — 185° C, but with care the amount can be 

 raised to as much as 350 to 400 c.c. per gramme. 



The amount absorbed at atmospheric pressure and at the tempera- 

 ture of liquid air can be quickly determined. A gramme of charcoal, 

 previously heated to a red heat is placed in a glass bulb connected by 

 an india-rubber tube and stop-cock to a graduated vessel con- 

 taining air kept over strong sulphuric acid or a high boiling 

 point oil, so that on cooling the charcoal and opening the stop-cock 

 the absorption is measured. This absorptive power can be shown by 

 means of the balance and the electro-magnet. From one arm of a 

 balance was suspended a copper-wire gauze sphere containing charcoal 

 which was carefully counterpoised. A flask partly filled with liquid 

 air was now brought up below it, so that the vapour rising from the 

 liquid air surrounded the charcoal. The absorption soon became 

 visible through the beam of the balance descending as the charcoal 

 became charged, and by adding weights the amount of gas absorbed 

 per unit of time could be determined. If a rod of charcoal was 

 Imng up by a thread between the poles of an electro-magnet and 

 sufficient torsion applied, so that its length was at right angles to the 

 line joining the poles, when the magnetic field was produced, 

 on bringing a vessel of liquid air close below it after a short time 

 on turning on the electric current to " make " the magnet, the rod 

 now set itself along the line joining the poles, proving that it 

 had become magnetic. This action is dependent on the well- 

 known magnetic property of oxygen, one of the constituents of 

 the air, which surrounded the charcoal rod and which had 

 condensed within its pores. On taking away the cool vapour of 

 the liquid air tlie temperature soon rose and the behaviour of the 

 rod under magnetic action showed that it had lost the magnetic 

 property. 



Effect of Increased Pressure of Oas on the Ahsorption of Charcoal 

 at Loiv Temperatures. 



The absorption of hydrogen by charcoal at the temperature of 

 liquid air and under atmospheric pressure is very considerable, and 



