432 On a New Thermometer for measuring high Temperatures, 



with the greatest accuracy. But it is not thus in the new ther- 

 mometer, on account of the relatively minute quantity of air 

 contained in the capillary portion. In fact, as we have seen, 

 the quantity of air contained in the reservoir at 500° is less than 

 the quantity contained at zero by about eV, on the supposition 

 that the capillary tube possesses an internal diameter of ^ of a 

 millim. The total quantity of air in the capillary tube is even 

 slightly increased ; since this tube already contains air at 0°, 

 the entire quantity at 500° is increased to about -£$ part of the 

 quantity contained in the reservoir. 



It is the temperature of this portion of air in the capillary 

 tube that varies with the surrounding temperature, and conse- 

 quently its volume changes and exercises a certain degree of in- 

 fluence on the position of the mercurial column. 



To calculate the magnitude of this source of error, we will 



suppose the instrument to be graduated to the temperature of 



15°, and the circumambient temperature to vary between the 



limits of 0° and 30°, that is, 15° in either direction. Now, since 



a determinate volume of air varies T l^ of its bulk for each 



degree of temperature, it follows that the extreme variation will 



11 x 15 

 amount to QnAn , or about gV of the volume contained in the 



capillary portion. But this last represents, even in the extreme 

 case, only -gV of the quantity of air contained in the reservoir. 

 Therefore the variation due to the influence of the circumam- 

 bient temperature is equivalent to a change of a roVo~ P ar * a ^ 

 most ; and this represents on the instrument not more than # 3 

 of a degree ; it may therefore be neglected. And even suppo- 

 sing the capillary tube of the instrument to be -J a millim. in 

 diameter, the variation would only amount to jij of the volume of 

 the air in the reservoir, a quantity which scarcely exceeds the 

 limit of experimental error. 



IV. Applications. 



This instrument, when well constructed and its capillary tube 

 properly calibrated, indicates high temperatures within 2° or 3° 

 of the truth ; within these limits I believe it to possess consider- 

 able utility. 



The new thermometer is especially adapted to the determina- 

 tion of temperatures lying between 330° and 500°. It is used 

 exactly as a mercurial thermometer, except that the scale requires 

 to be readjusted at the commencement of the experiment by re- 

 determining either the zero- or the 100°-point. 



I have employed this thermometer in the fractional distillation 

 of coal-tar at temperatures between 330° and 450° with great 

 success. Tubulated retorts may be used in the ordinary way, 



