98 
PROFESSOR W. RAMSAY AND DR. S. YOUNG 
pressures is sensibly identical with that representing pressures of dissociation and 
vapour-pressures at different temperatures. 
§ 50. It will be noticed that at the highest temperatures the numbers representing 
temperatures of volatilization fall below the vapour-pressure curve : and since the 
temperatures of the jacket of mercury-vapour used for heating the block of 
ammonium chloride was only about 20° higher than the highest temperatures registered 
by the thermometer imbedded in the block, it was deemed advisable to make an 
independent determination of the temperature of vaporization under normal pressure. 
For this purpose a calibrated thermometer into which air had been admitted was 
heated with mercury-vapour under circumstances as similar as possible to those of the 
Fig. 3. 
actual experiment. In the woodcut, A represents a wide tube bent twice at right 
anodes, with a bulb blown at one end. At B a hole was blown, to admit the 
thermometer, which was kept in position by a plug of asbestos, The upper part ot 
the thermometer was protected from hot air-currents by a sheet of asbestos cardboard, 
C ; the mean temperature of the upper part of the stem of the thermometer was 
ascertained in the ordinary way. The result was as follows :— 
Beading of thermometer.351*0° 
Stem heated by mercury-vapour up to graduation . 110° 
Mean temperature of upper portion of stem ... 33° 
Correction for calibration at 351°.— 3'65 
Bise of zero-point owing to heating. 1‘9' 
Corrected temperature 351*0° + 12*3~— 3*65‘ —1*9°= 357 85° 
Barometer. 762*1 millims. at 0 
From Begnault’s table, interpolated. Temperature = 357*6" 
This close approximation between a mercurial thermometer and an air thermometer 
might excite surprise ; but we have frequently noticed that these particular German- 
* From our revised table, 358’3°. 
