I 



PHYSICAL LABORATORY OF GREAT BRITAIN. 349 



exhibition — "the manufacture of thermometers had come to a dead 

 stop in Germany, thermometers being then invested with a defect, 

 their liability to periodic changes, which seriously endangered German 

 manufacture. ' Comprehensive investigations were then carried out 

 by the Normal Aichungs commission, the Reichsanstalt, and the Jena 

 glass works, and much labor brought the desired reward."' The defect 

 referred to was the temporary depression of the ice point which takes 

 place in all thermometers after heating. Let the ice point of a ther- 

 mometer be observed; then raise the thermometer to say 100° and 

 again observe the ice point as soon as possible afterwards; it will be 

 depressed below its previous position; in some instruments of Thu- 

 ringian glass a depression of as much as 0.65° C. had been noted. For 

 scientific purposes such an instrument is quite untrustworthy. If it 

 be kept at say 15° and then immersed in a bath at 30° it will be appre- 

 ciably different from tha" 1 " 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. 



Depression of freezing /mini far various thermometers. 



Degree. 



Humboldt, 1835 0. 06 



Greiner, 1872 38 



Schultzer, 1875 44 



Rapps, 1878 65 



English glass 15 



Yer Deer 08 



16"' 05 



59"' 02 



Analysis of glasses. 



Si0 2 Na 2 G CaO A1 2 3 ZnO B,0 3 

 W— 67.5 14 7 2.5 7 2 



59'"— 72 11 5 12 



Weber had found in 1883 that glasses which contain a mixture of 

 soda and potash give a very large depression. He made in 1883 a 

 glass free from soda with a depression of 0.1°. The work was then 

 taken up h\ the Aichungs commission, the Reichsanstalt, 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 0.06°, while a new instrument, in which the per- 

 centages 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, 

 12.3 per cent of potash, gave a depression of 0.15°, while a French 

 "Ver Deer"" instrument in which these proportions were reversed gave 

 only 0.8°. 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 



