584 



Frofessor Sir James Dewar 



[Jan. 19, 



volume of the free space in different thermometers. Thus, if P 

 and p are pressure readings of the manometer at T" and 27b° Abs. 



V V (7) - V\ , 

 respectively, then T' = 273 - - — ^ ^ where c is a constant 



100 ISO 200 



PRESSURE, m.m. Hg. 



Fig. 5. 



varying inversely as the ratio of the volume of the capillary to that 

 of the bulb of the thermometer. Helium under a pressure of 273 mm. 

 may be regarded as reliable as low as 3" Abs. If the pressure be 

 reduced to 27*3 mm. at 273°, the limit of temperature is about 1°' 7 



Insulation by Powders. 



In all low temperature work heat isolation is of the utmost 

 importance. The double-walled glass vacuum flasks now so gene- 

 rally used for this purpose were first exhibited on this Lecture 

 Table nineteen years ago. The efficiency of these vessels is much 



