1901.] on the Aims of the National Physical Laboratory. 661 



some instruments of Thuringian glass a depression of as much as 

 0° • 65 C. bad been noted. For scientific purposes sucb an instrument 

 is quite untrustworthy. If it be kept at, say, 15°, and then immersed 

 in a bath at 30°, its reading will be appreciably different from that 

 which would be given if it were first raised to, say, 50°, allowed to 

 cool quickly just below 30°, and then put into the bath. This was 

 the defect which the investigators set themselves to cure. 

 Table I. gives some details as to thermometers. 



Table I. 

 Depression of Freezing Point for Various Thermometers. 



Degrees. 



Humboldt, 1835 0-06 



Greiner, 1872 0-38 



Schultzer, 1875 0-44 



Kapps, 1878 0-65 



English glass 0*15 



VerreDur 008 



16'" 0-05 



59'" 002 



Analysis of Glasses. 



Weber bad found in 1883 that glasses which contain a mixture 

 of soda and potash give a very large depression. He made a glass 

 free from soda with a depression of 0°*1. The work was then taken 

 up by the Aichungs Commission, the Eeichsanstalt and the Jena 

 factory. Weber's results were confirmed. An old thermometer of 

 Humboldt's, containing 0*86 per cent, of soda and 20 per cent, of 

 potash, had a depression of o- 06, while a new instrument, in which 

 the percentages were 12*7 per cent, and 10*6 per cent, respectively, 

 had a depression of 0° • 65. 



An English standard, with 1*5 per cent, of soda and 12 '3 per 

 cent, of potash, gave a depression of 0°*15, while a French " verre 

 dur " instrument, in which these proportions were reversed, gave 

 only 0°- 08. 



It remained to manufacture a glass which should have a low 

 depression and at the same time other satisfactory properties. The 

 now well-known glass 16'" is the result. Its composition is shown 

 in the Table. 



Tbe fact that there was an appreciable difference between the 

 scale of tbe 16'" glass and that of the air thermometer led to further 

 investigations, and another glass 59'", a borosilicate containing 12 per 

 cent, of boron, was the consequence. This glass has a still smaller 

 depression. 



Previous to 1888 Germany imported optical glass. At that date 



