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IX. Measurement of the Expansibility of a Hard Jena Glass. 

 By J. T. Bottomley, M.A., JD.Sc, F.R.S., and W. T. 

 Evans *. 



IN the course of investigations which we are carrying out 

 on the subject of Thermal Radiation it became necessary 

 to construct an Air-Thermometer which should be capable 

 of being used at high temperatures, above the softening- 

 point of ordinary English glass. The form of air-thermometer 

 employed is thnt which was described in the Philosophical 

 Magazine for 1888 1 by one of the present writers, Ln 

 this thermometer, which is a constant-volume air-thermo- 

 meter, the volume-gauge and pressure-gauge are separated, 

 and only connected by means of indiarubber pressure-tubing. 

 The instrument has proved eminently satisfactory, and 

 convenient to work with. 



The volume-gauge is the only part which it is necessary 

 to make of hard glass, and, after some trials with specimens 

 of the Jena glass of Messrs. Schott & Genossen, who specify 

 three kinds of hard glass, we determined to use the glass 

 termed by them " Verbrennungsrohrenglas." This is a 

 very hard glass, little acted on by water or alkalis (though 

 this property was not of much consequence for our purpose). 

 It is slightly green in colour, and slightly opalescent. ]t 

 is just possible to work it with an ordinary blowpipe, 

 but for any complicated construction an oxygen flame is 

 required. 



In order to use this glass for the volume-gauge of the air- 

 thermometer, it was necessary to determine the expansibility 

 of the glass. This we have done; and the present note is 

 published in the hope that the results may be of use to 

 others. We were surprised to find so small an amount of 

 expansibility. This property makes the glass of special 

 value for thermometric purposes, at high temperatures. 



To determine the expansibility of the glass, the weight- 

 thermometer method was used, and the cubical expansion 

 was found directly. This seemed more satisfactory than the 

 determination of the linear expansion, and the inference, 

 therefrom, of the cubical expansion. 



The bulb of the air-thermometer is a tube of Jena glass 

 12 cm. long and 1*5 cm. in diameter inside. A similar tube 



* Communicated by Lord Kelvin. 



t J. T. Bottomley/' On a Practical Constant- Volume Air-Thermometer," 

 Proc. Roy. Soc. Edfo. Dec. 19th, 1887, and Phil. Mag. August 1888. 



