Dr. P. Chappuis on Gas- Thermometry. 437 



that of Fizeau. But up to the present no experiments have 

 been made by this method above 100°. 



It follows that coefficients deduced from experiments made 

 with this apparatus are only rigorously applicable over the 

 range 0°-100°*; and it may justly be objected that extra- 

 polation to 450° might introduce into the results quite sensible 

 errors. The dilatation of Berlin porcelain, which is the 

 material out of which the reservoirs employed for the higher 

 temperature work were made, was determined by me over the 

 interval 0°-100°. I found for the cubical dilatation 



v t =v {l + 8-0703 xl0- 6 *.+ 8-98xl0- 9 * 2 ). 



The importance of the second term is easily seen. It is 

 sufficient to note here that if, instead of this expression, we 

 take 



v t =v {l + 8-96835 xlO- 6 *) 



(which is the mean coefficient between 0° and 100°), we should 

 have for the sulphur boiling-point 



T s = 444-3 



instead of the value 445'2, which results from our calculation. 



Mr. Bedford has recently published in this Journal f the 

 results of ,a determination of the dilatation of a tube of 

 Bayeux porcelain, obtained by a method previously employed 

 by Callendar and Griffiths. 



The porcelain tube, which served for Mr. Bedford's mea- 

 surements, had external and internal diameters of 17 mm. 

 and 10 mm. respectively. Near the ends were two marks, 

 91*4 cm. apart, made by a diamond. The variations in the 

 distance apart of these two marks were measured by a com- 

 parator provided with two microscopes under which could 

 be brought successively the porcelain tube and a glass tube 

 maintained at constant temperature. 



The porcelain tube could be heated to any temperature 

 between 0° and 840°, which was measured by the change of 



* The fundamental study of the dilatation of the three screws of the 

 platinum-iridium tripod, which constitute the standard to which all 

 measurements with this apparatus are referred, was made with great care 

 by M. Benoit, the Director of the Bureau International, and is described 

 in full in the Trau. et Mem. t. vi., 1888. This laborious piece of work, 

 which is attended with peculiar difficulties, would have to be extended 

 to higher temperatures, if it were desired to extend upwards the range 

 over which the Fizeau apparatus can be employed. 



t Phil. Mag. vol. xlix. p. 490. 



Phil, Mag. S. 5. Vol, 50, No, 305. Oct. 1900. 2 H 



