NATURAL PHILOSOPHY. 145 



readily constructed to exhibit the vapor tension of water of given 

 temperature. Conceive a globe, completely exhausted of air, to con- 

 tain 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 ex- 

 plode 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 Mont Blanc), the globe will 

 have similar tendencies according as its temperature is above or be- 

 low 82 Centigrade. And so, for higher and lower pressures of the 

 atmosphere without, there will be required higher or lower tempera- 

 tures of the water globe to equilibrate from within the pressure from 

 without. Such a table, expressing the pressure of pure vapor arising 

 from water of given temperature, has been constructed with extreme 

 accuracy by Mr. Regnault ; 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 dilat- 

 able, including water in sufficient quantity. If the temperature 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 main- 

 tained, the vessel will enlarge until the incumbent air is so compressed 

 in space as to exert exactly the pressure of the vapor upon the 

 external surface of the vessel. The pressure which now obtains 

 within the globe is that due to the temperature of the water, accord- 

 ing to the value assigned in the table before mentioned. M. Lament 

 assures us, from experiments, that the pressure will maintain this 

 value if the walls of the including vessel be now removed. Dr. 



^j 



Dalton, however, deduced from his experiments a different rule. On 

 removal of the partition supposed to separate the gaseous fluids, more 

 aqueous vapor 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 ex- 

 pand into the space engaged by aqueous vapor as if it were a vacuum, 

 and a pressure will result, 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 distilla- 

 tion taking place. Yet the pressure within the globe and receiver 

 M. Lamont found to be compounded of that of the heated air and 

 of the aqueous vapor with which the aerial space of the globe was 

 saturated (by convection). This experiment is proof sufficient, in 

 the opinion of M. Lamont, that the particles of vapor and of air 

 are not, as in the theory of Dalton, indifferent one to another as 

 grains of dust, but exert upon one another a mutual and permanent 

 reaction. Yv'hereas the aqueous particles are incapable, at low tem- 

 peratures, of exerting those pressures by which they might assume 

 among the particles of air positions of equal and independent action, 

 M. Lamont advocates a view that an atomic combination arises be- 



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