Professor Lamont' s New Theory of Atmospheric Vapour. 369 



tension of the vapour emitted by the water at boiling tem- 

 perature, a table is readily constructed to exhibit the vapour 

 tension of water of given temperature. Conceive a globe, com- 

 pletely exhausted of air, to contain a quantity of water, not 

 too small, and to be raised to a certain temperature in the open 

 air ; this globe will have a tendency to explode or to collapse, 

 according as the temperature is above or below 100° centigrade. 

 If the atmospheric pressure about the globe be only one half of the 

 ordinary pressure of the atmosphere at the sea level (as, for 

 instance, on the summit of Mout Blanc),' the globe will have 

 similar tendencies according as its temperature is above or 

 below 82° centigrade. And so for higher and lower pressures 

 of the atmosphere without, there will be required higher or 

 lower temperatures of the water globe to equilibrate from 

 within the pressure from without. Such a table, expressing the 

 pressure of pure vapour arising from water of given tempera- 

 ture, has been constructed with extreme accuracy by Mr. Reg- 

 nault; and it results, that by exceedingly rapid methods of 

 exhaustion, water may be made to freeze in the very act of 

 boiling, ice or snow being forms of water which do not in the 

 least interrupt the regular march of the numbers of the table. 



Conceive again a sealed globe to enclose perfectly dry air of 

 a given temperature and pressure, and likewise a vessel freely 

 dilatable, including water in sufficient quantity. If the tem- 

 perature of the globe be high, and the pressure of the air within 

 it be small, the water so included will boil, and the temperature 

 being exactly maintained, the vessel will enlarge until the in- 

 cumbent air is so compressed in space, as to exert exactly the 

 pressure of the vapour upon the external surface of the vessel. 

 The pressure which now obtains within the globe is that due 

 to the temperature of the water, according to the value assigned 

 in the table before mentioned. M. Lamont assures us, from 

 experiments, that the pressure will maintain this value if the 

 walls of the including vessel be now removed. Dr. Dalton, 

 however, deduced from his experiments a different rule. On 

 removal of the partition supposed to separate the gaseous fluids, 

 more aqueous vapour will be generated in proportion to the 

 space occupied by the air, it will cross the boundary and fill 

 that space as if it were a vacuum, the air at the same time will 

 cross the boundary and expand into the space engaged by 

 aqueous vapour as if it were a vacuum, and a pressure will re- 

 sult, the sum of that due to the temperature of the water and 

 that of the air originally enclosed. 



M. Lamont has found that a globe connected with an iced 

 receiver by a tube one line in diameter may for two hours be 

 occupied by water at a temperature of 100° Fahrenheit without 

 signs of distillation taking place. Yet the pressure within the 



