326 Expansion of Mercury behveen 0° C. and —39° C. 



by an india-rubber stopper through which passes tightly the 

 glass stem of an air-thermometer, A A. The bottom of the 

 air-thermometer is attached by a piece of india-rubber tubing, 

 1 1, to a vertical glass tube, T T ; and the thermometer being 

 filled with dry air, some mercury is introduced into the tube 

 so as to stand at about the same height in the two limbs 

 when the air in the bulb b is at atmospheric pressure. The 

 height of the level of the mercury can be varied by turning 

 the nut, N, which causes the clamp, C, turning on the hinge H, 

 to squeeze the india-rubber tube more or less tightly, and so 

 to alter its internal capacity ; and in this way the level of the 

 mercury in the left-hand tube can be kept quite fixed, and 

 therefore the volume of air in the bulb b quite constant, while 

 its temperature is altered, the corresponding pressure being of 

 course measured by the difference between the levels of the 

 mercury columns in the two limbs. 



To perform the experiment, the box B B was first filled with 

 mercury and frozen by stirring carbonic-acid snow and ether 

 up with it ; and when sufficient of the mercury was frozen, 

 the bulb of the mercurial thermometer, 1 1, was introduced into 

 the pasty mercury, and the thermometer fixed in position by 

 means of the clamp, (7. The nut N was now turned by one 

 observer until the level of the mercury in the left-hand limb 

 came opposite a fixed mark on the tube which is only just 

 below the bottom of the box, when the height of the mercury 

 in the right-hand tube was read by a cathetometer made by 

 the Cambridge Instrument Company, and the position of the 

 mercury in the mercury-thermometer was read by a third 

 observer. In this way several series of simultaneous observa- 

 tions were taken, during the course of some weeks, of the 

 pressure to which the air in the air-thermometer had to be 

 subjected to keep its volume constant, as the mercury in the 

 box B B varied in temperature from about — 39° C. to 0° C. 

 Plotting these results, it was found that they lay in so nearly 

 a straight line that we may conclude that mercury expands 

 regularly below 0° C. as it is known to do above 0° C. \ and 

 that there is no critical point for mercury, as there is for water, 

 above the freezing-point. 



When the mercury freezes it contracts still further, as may 

 be seen from the following extract from page 6 of the second 

 volume of Nordenskiold's ' Voyage of the Vega/ which Mr. 

 Whipple has kindly looked up for us : — 



" When mercury freezes in a common thermometer, it con- 

 tracts so much that the column of mercury suddenly sinks in 

 the tube, or, if it is short, goes wholly into the ball. The 

 position of the column is therefore no measure of the actual 

 degree of cold when the freezing takes place." 



