446 Professor Sir James Bewar [June 8, 



atmospheric pressure, was immersed in a bath B of the corresponding 

 liquefied gas contained in a vacuum-jacketed vessel ; and a tube C 

 was led from it to a Leslie thermometer containing a sulphuric acid 

 cylinder used as a registration instrument for volume. The gauge 

 consisted of a U-tube, with bulbs on each side, to prevent any sudden 

 sucking back of the drop of sulphuric acid E, renders visible the 

 slightest alteration of the volume of gas occluded in the charcoal. 

 The upper ends of the U-tube were connected by a by-pass containing 

 a stop-cock D. On shutting this stop-cock after the complete 

 saturation of the charcoal with air has been effected, equilibrium of 

 pressure between C and F could be obtained, while the sulphuric 

 acid drop E occupied a convenient position in the gauge. On 

 the approach of a candle-flame to the charcoal bulb, the gauge 

 immediately showed a considerable expansion, notwithstanding that 

 the radiant heat had to traverse a considerable quantity of liquefied 

 gas, and several thicknesses of glass tubing. In the same manner a 

 beam of light thrown on the charcoal bulb when charged with 

 hydrogen instead of air immediately affected the gauge in a striking 

 way. 



The apparatus can be rendered still more sensitive by the following 

 device (Fig. 21). The charcoal bulb A, saturated with air at - 185° C, 

 is surrounded with another larger bulb giving an annular space that 

 can be filled with liquid air by opening the stop-cock C, the orifice D 

 being open while immersed in a large vacuum -jacketed reservoir, V, 

 of liquid air. The tube E leading from A passes to a sulphuric acid 

 manometer, with by-pass and stop-cock, F, as before, thence to the 

 open end H. The apparatus works thus : Before the radiant 

 beam is turned on A, the annular space between the bulbs is 

 cleared of liquid by blowing a little air through the tube C, and 

 shutting the stop-cock, thus forcing the liquid air out by the 

 aperture D. The charcoal bulb is now surrounded by an atmosphere 

 of liquid air vapour, or hydrogen, as the case may be, maintained at 

 the boiling point by the liquid in V. In these circumstances the 

 maximum effect of any heat given to the charcoal bulb is obtained. 

 The effects are magnified two or three times compared with those 

 given by the charcoal bulb left in direct contact with the liquid. 



The special value of this instrument for thermometric measure- 

 ments is that it becomes more sensitive as the temperature falls, thus 

 placing in our hands a most efficient thermometer for the lowest 

 temperatures which can possibly be reached. Thus, when the char- 

 coal is saturated with hydrogen gas at the boiling point of hydrogen, 

 the instrument has its maximum delicacy under ordinary circum- 

 stances ; but provided helium was absorbed in charcoal at and below 

 the temperature of solid hydrogen, then we can predict that the 

 instrument would be^still more sensitive to radiant energy. 



If, further, the charcoal occupies an annular space surrounding 

 another smaller bulb, into the interior of which any small heated 



