118 An Instrument for the Comparison of Thermometers. 



and outside tube, is condensed in the condenser D, and runs 

 down to the bottom again. 



Fig. 2. 



Fiar. 3. 



(SB 



The thermometers are fastened together, their bulbs resting 

 in a small glass vessel K (fig. 2) which is filled with mercury. 

 This vessel is supported by a glass rod which rests on the 

 table on which the instrument stands. To remove the 

 thermometers the instrument is drawn to the edge of the 

 table till this rod can be lowered and with it the thermo- 

 meters. The scales are read by means of two telescopes, it 

 being quite easy to see through the vapour if suitable liquids 

 are used. The three liquids recommended by Ramsay and 

 Young, viz. carbon bisulphide (20° to 46°), ethyl alcohol (46° 

 to 79°), and chlorobenzene (79° to 120°) do very well. 

 Water cannot be used as it forms drops on the inside of the 

 glass so that the thermometers cannot be read. For tempera- 

 tures higher than 120° the indiarubber cork closing the tube 

 will not do, but the arrangement shown in fig. 3 may be used. 

 The disadvantage of this form is that it can hardly be made 



