Lowering the Freezing-Point of Water. 125 



ment with those of a thermometer of Crich ton's with an ivory 

 scale, which has divisions corresponding to degrees Fahren- 

 heit of about ~5 tn °f an mcn each, £ found that the range of 

 the aether thermometer is about 3° Fahrenheit ; and that there 

 are about 212 divisions on the tube corresponding to the in- 

 terval of pressure from 31° to 34<°, as nearly as I could discover 

 from such an unsatisfactory standard of reference. This gives 

 7T of a degree for the mean value of a division. From a 

 rough calibration of the tube which was made, I am convinced 

 that the values of the divisions at no part of the tube differ by 

 more than ^th of this amount from the true mean value ; 

 and, taking into account all the sources of uncertainty, I think 

 it probable that each of the divisions on the tube of the aether 

 thermometer corresponds to something between -^ and T j of 

 a degree Fahrenheit. 



With this thermometer in its glass envelope, and with a 

 strong glass cylinder (CErsted's apparatus for the compression 

 of water), an experiment was made in the following manner : — 

 The compression vessel was partly filled with pieces of clean 

 ice and water : a glass tube about a foot long and y^th of an 

 inch internal diameter, closed at one end, was inserted with 

 its open end downwards, to indicate the fluid pressure by the 

 compression of the air which it contained; and the aether 

 thermometer was let down and allowed to rest with the lower 

 end of its glass envelope pressing on the bottom of the vessel, 

 A lead ring was let down so as to keep free from ice the water 

 in the compression cylinder round that part of the thermo- 

 meter tube where readings were expected. More ice was 

 added above ; so that both above and below the clear space, 

 which was only about two inches deep, the compression cy- 

 linder was full of pieces of ice. Water was then poured in 

 by a tube with a stopcock fitted in the neck of the vessel, till 

 the vessel was full up to the piston, after which the stopcock 

 was shut. 



After it was observed that the column of aether in the ther- 

 mometer stood at about 67°, with reference to the divisions on 

 the tube, a pressure of from 12 to 15 atmospheres was applied^ 

 by forcing the piston down with the screw. Immediately the 

 column of aether descended very rapidly, and in a very few 

 minutes it was below 61°. The pressure was then suddenly 

 removed, and immediately the column in the thermometer 

 began to rise rapidly. Several times pressure was again sud- 

 denly applied, and again suddenly removed, and the effects 

 upon the thermometer were most marked. 



The fact that the freezing-point of water is sensibly lowered 

 by a few atmospheres of pressure was thus established beyond 



