264 



Sir James Dewar 



[Jan. 16, 



settled to a steady boiling point in an enclosed vessel. (It is very- 

 easy for solid carbonic acid to fall to a temperature considerably 

 below its boiling point, by forced evaporation due to mere exposure 

 to the strong air convection currents arising from free exposure of 

 such a cold substance.) Besides this, a set of measurements at 

 gradationally lowered temperatures between 0° and — 78° C. was made, 

 using a cooled alcohol bath as source. Measurements much above 

 100° C. were not satisfactory — e.g. the use of a source of boiling 

 sulphur produced too violent a disturbance and did not give satis- 

 factory values ; but, on the other hand, the difference between old 



iSS -1 ' 2bo ' "IST" 



TEMP"- of BLACK BODY 



Fig. 13. 



DEGREES ABS. 



and new liquid air, say at 89° Abs. and 82° Abs. respectively, was easily 

 registered, either plus or minus as the case might be : and also that 

 between boiling oxygen and boiling nitrogen (90° Abs. and 77° Abs. 

 respectively). No observations were made with liquid hydrogen, 

 either as " black " source (negative) or to cool the thermoscope 

 (hydrogen saturated charcoal), with a suitable Leslie cube of liquid 

 oxygen or nitrogen as black body above, but there is no reason to 

 doubt that this could be arranged satisfactorily. Among other 

 questions the transmissivity for heat of liquid hydrogen could thus 

 be tested. 



