vni THERMO-ELECTRIC CRYOSCOPY 167 



are subjected to precisely similar conditions in this respect. 

 The freezing-bath is also kept stirred. The galvanometer 

 may now be put in circuit with the thermocouple by 

 fixing the clip on the support in the petroleum key ; and 

 the petroleum is occasionally stirred. Immediately on 

 making the contact the spot of light travels from zero. 

 At first its motion is rapid, but becomes slower and slower 

 till at last it moves with an almost imperceptible creep. 

 It comes to rest about 60 sees, after contact is made. It 

 will be found convenient to allow 75 sees, to elapse before 

 making a reading. During this time the stirring of the 

 test-tubes is actively kept up ; for it is surprising how. 

 quickly the ice rising in the solution allows the lower 

 layers round the junction to become supercooled. In the 

 other test-tube the same does not occur, as the ice soon 

 forms a lining lying against the wall of the test-tube, and 

 the junction is supported in water surrounded by ice. 



When the first reading is made, the clip is disconnected 

 and the galvanometer mirror swings free. Reversed con- 

 nection is made when the spot of light is at the limit of 

 its swing on the side on which the first deflection was 

 recorded. In this way the suspension of the galvanometer 

 is kept from any sudden strain which might be produced 

 by suddenly checking its movement. After 75 sees., 

 during which the same active stirring is kept up, a second 

 reading is made. This first observation after putting in 

 the solution should be regarded merely as a preliminary 

 one ; but still, if too much ice has not been present, it 

 will give the freezing-point within a couple of hundredths 

 of a degree. 



The test-tubes are now raised from the freezing-chamber, 

 and the one containing the solution momentarily touched 

 by the finger to give it a little heat. When the upper 

 cork is readjusted and stirring recommenced, it will be 

 noticed that the spot of light retires towards zero. If all 

 the ice is not melted, it will quickly recover its former 



