254 Sir James Dewar [Jan. 16, 



shown in the diagram as a single crystal, but small crystals were 

 commonly used in a chiffon gauze bag. This substance becomes 

 electrified at low temperatures with the slightest friction, and 

 effectively removes floating crystals of ice or other impurities, thus 

 keeping the liquid air perfectly clear. It becomes moist in use after 

 some time, and care must be taken not to dehydrate it when drying, 

 as it would then become inoperative. 



The cell and tube are shown only in part (the tube being bent 

 under and passing up behind the cell), and the trap door, etc., are 

 omitted. With the most effective internal cooling, the trap door 

 can be omitted, since the disc stopper, if well polished, almost com- 

 pletely cuts off external radiation. The drawback to this form is 

 the stray radiation from the upper portion of the inner wall, whose 

 temperature cannot be kept steadily down to that of the liquid bath. 

 A lining of black bibulous paper or thin blackened sheet copper is of 

 assistance, but does not eliminate the difficulty in such a vessel, which 

 has to be opened intermittently during use. 



Some of these drawbacks are eliminated in C, which was after- 

 wards developed almost into a standard form. The inner vessel holds 

 a separate small quantity of liquid, but being of plain metal is 

 unisolated, and takes throughout the uniform temperature of the 

 liquid in the outer vessel, which extends farther above the top of the 

 inner vessel than the diagram indicates. The top is closed by a 

 light metal cup fitting with the minimum of play ; into this is 

 placed some of the same liquid air as is used in the vessels. A 

 uranium bag can be attached below, as with the disc stoppers, but in 

 the figure the cell is shown "gas cooled," i.e. above the liquid in the 

 inner vessel. A light cross support serves as a handle to remove 

 this " bucket " stopper for an exposure. It is of course supported 

 meanwhile in a suitable glass vacuum vessel of liquid air. The 

 objection to this form, apart from the need of rather frequent 

 replenishment of the bucket, is the cloud which sometimes forms 

 when the atmosphere is moist, or if the bucket is lifted too rapidly. 

 But with careful use very good results are obtained. 



Curves of Displacement of Gas with Time. 



The evolution of air or gas from the cell, as revealed by time and 

 displacement curves, showed only a small departure from a linear 

 rate for 2 or 3 minutes. After this the gas was expelled more slowly, 

 and in from 10 to 15 minutes the manometer had almost stopped. 

 Two manometers were used. The first was the ordinary U-tube 

 form, to register the rate of growth of pressure ; while in the second 

 the rate of gas evolution at sensibly constant pressure was indicated 

 on a horizontal scaled tube, with a bulb reservoir at the end furthest 

 from the cell. In this form the linear character of the earlier dis- 

 placement range continued longer than the ordinary U-tube, and 



