REFRACTIVE INDEX OF GASES ON TEMPERATURE. 
437 
vertical tubes F at each end of the jacket admitted the introduction of a 
thermometer fitted through a rubber cork. Steam or water entered at G and was 
pumped out at the corresponding hole at the other end of tlie jacket. C is an 
optically plane and parallel plate of glass, 17 millims. diameter, TS millims. thick. 
The four plates were all cut from the same plate of worked glass by Reinpelder unl> 
IJertel. D is a piece of hollow cork to reduce eddies of cold air. 
I experienced very great difficulty in making the joints between the glass plates 
and the brass tubes absolutely air-tight under the varying conditions of temperature 
and pressure. I succeeded finally Ijy using a rubber washer d millim. thick Ijetween 
the glass and tlie brass, and then painting bicycle enamel round the junction. This 
material dries rajjidly and hardens, but still with sufficient elasticity to avoid straining 
of the glass. It is not porous, nor does it melt or even soften at 100° C. It is, 
moreover, soluble in ether, so that the glass plate can be recovered unimpaired. 
Small brass tubes (see fig. 3) passed through the side of the jacket and were 
screwed and soldered, one to each of the long brass tubes. These served to connect 
the tubes with the manometer for recording the pressure. 
These small brass tubes passed through short brass tubes of slightly larger 
diameter, soldered to the jacket, thus allowing play during alteration of temperature. 
The joint was made by a short jjiece of thick rubber tube, wired, and painted over 
with black enamel. 
The steady temperatures required were obtained as follows:—-Tap water was 
drawn through the jacket by means of a water pump. This gave temperatures abouf; 
10° C. Higher temperatures, such as 20° C., or 30° C., were obtained by drawing 
the w^ter through lead spirals of different sizes, immersed in a saucepan of water 
kept boiling. Temperatures from 50° C. to 100° C. were obtained by boiling water 
under reduced pressure in an old mercury bottle, and drawing the steam tlirough the 
jacket by the water pump. 
