100 Mr. C. Tomlinson's Experimental Examination 



and the solution left to cool, the great differences in tempera- 

 ture between the bottom and top layers has not, so far as I 

 know, been hitherto observed*. It is known in a general way 

 that there is a difference, so that, in comparing thermometers in 

 hot water, means are taken to keep it stirred; but it has not, I 

 believe, been suspected that in a liquid column 7 inches high and 

 1J inch in diameter, supported in the air, as before described, 

 there may be a difference of 20° and upwards in the case of water/ 

 and of 30° and upwards in the case of certain saline solutions be- 

 tween the bottom and the top of the column. To show the reality 

 of the difference, the lower thermometer has been raised repeatedly 

 to the level of the upper one, and the two readings corresponded. 

 In the experiment with nitre the maximum difference was 13° in 

 favour of the upper thermometer, but in the course of a few 

 minutes this difference was reduced to 6°; soon after which a 

 few short crystalline shoots were observed at the bottom, and a 

 little after crystals were seen at the surface; the lower thermo- 

 meter now marked a higher temperatm-e than the upper one, and 

 went on increasing until the maximum difference in its favour 

 was also 13°. As soon as crystallization set in, the tempera- 

 tures declined slowly as compared with the cooling before that 

 event. From the removal of the lamp to the crystallizing-point, 

 the solution cooled down 88° in 16 minutes, viz. from 228° to 

 140°. From the crystallizing-point to the conclusion of the 

 experiment, the cooling was 66° in 85 minutes, viz. from 140° to 

 74°, and that in a room without a fire, where the temperature 

 was 49°, and the solution was within about 2 feet of the window. 

 A similar experiment was tried with sal-ammoniac, the details 

 of which are given in the following Table : — 



cate two different temperatures corresponding to the pressures in these 

 positions : the vapour formed at the bottom of the vessel, experiencing a less 

 degree of pressure in proportion as it ascends, dilates and cools until the 

 moment when, attaining the surface of the liquid, its elastic force becomes 

 equal to the pressure of the atmosphere. The temperature of the emerging 

 vapour, or, what is the same thing, the last liquid layer, is exactly that of 

 boiling under a given atmospheric pressure." 



* Count Rumford, in one of his essays to prove the non-conductibility 

 of water for heat, pointed out how a column of water with a lump of ice 

 attached to the bottom might show various temperatures between 40° and 

 180° F. Saussure also noticed in Lake Lucerne that the water at a depth 



