A. L. Bay and J. K. Clement — Gas Thermometer. 421 



parison that the derivation of the Kelvin thermodynamic scale 

 from the expansion of nitrogen is probably not encumbered with 

 any error of sufficient magnitude to require consideration when 

 compared with the errors and corrections inherent in the 

 experimental measurements. From the standpoint of the 

 underlying theory of the instrument and the interpretation of 

 its results in terms of the thermodynamic scale, a new and 

 extended experimental study of the Joule-Thomson effect is 

 very greatly to be desired, but there is no reason for apprehen- 

 sion in the application of. existing data, imperfect and limited 

 as they are. Experimental confirmation of this is contained in 

 the recent work of Jaquerod and Perrot in which the expansion 

 of five different gases (nitrogen, air, CO, C0 2 , O a ) between 0° 

 and 1067° was studied in the same bulb under identical experi- 

 mental conditions. The maximum difference which occurred 

 in any of their measurements was only *4 of a degree, which 

 is easily within the limits of error of their apparatus. 



The Bidb. — The question of a suitable bulb to contain the 

 expanding gas has been and is to-day one of the most serious 

 which gas thermometry confronts. The first experiments 

 (Prinsep) w^ere made with a bulb of gold, which was soon 

 abandoned because of its low melting point. Following this, 

 platinum was employed (Pouillet), but here a difficulty was 

 encountered which eventually caused its abandonment in favor 

 of porcelain on account of its supposed porosity (Deville and 

 Troost, JBecquerel). Iodine vapor had been used in the experi- 

 ments of Deville and Troost as the expanding gas and very 

 high values of several temperature constants in the region of 

 1000° had been obtained and quite generally accepted, while 

 the much lower values obtained with a platinum bulb with air 

 were discredited for the time. These high values were sub- 

 sequently shown by Yictor Meyer to be due to the dissociation 

 of the iodine, but the controversy resulted in the unfortunate 

 (as it turned out) substitution of porcelain for metal bulbs, a 

 step which was not retrieved for thirty-six years. The porce- 

 lain bulb without glaze is porous ; with a glaze it is a chemically 

 undefined mineral mixture which not only softens below 1200° 

 with more or less change of volume, but also gives out gas 

 (either original or previously absorbed), so that the porcelain 

 gas thermometer, as it is commonly called, never returned to 

 its original zero after heating to high temperatures.* The 

 uncertainty in the zero which arises through the use of the 

 porcelain bulb causes an error of the order of 5° at 1000°, which 

 is practically impossible of satisfactory correction. 



The return to metal bulbs is due to Prof. Holborn of the 

 Reichsanstalt, who has successfully used a platinum bulb (con- 



* Holborn and Day, 1899, loc. cit. 



Am. Jour. Sci. — Fourth Series, Vol. XXVI, No. 155. — November, 1908. 

 30 



