THE GASEOUS PORTIONS OF THE ATMOSPHEEE. 



19 



high barometer ; which, as the material that constituted that 

 moisture adds to the weight of dry air, seemed very surprising 

 and even contradictory. It also authorises us to correct an 

 error that is to be found in most of our meteorological works, 

 namely, that moist, is lighter than dry air ! There can be no 

 doubt, that, considered independently of temperature, any 

 vapour passed into dry air adds to the total weight of the 

 mixture as measured by the barometer. The whole of the 

 gases on the globe have a certain weight, be it more or less, 

 and all the aqueous matter that is diffused through them has 

 also a certain weight ; and when we speak of, or compute, the 

 total weight of the atmosphere, we have to add the latter to 

 the former. Temperature being the same, to say that moist 

 air is lighter than dry air, is to say that the whole is less than 

 a part. It is true that the specific gravity of aqueous vapour, 

 considered separately, is less than that of the atmospheric 

 gases, but when mixed with them the vapour adds as much 

 weight to the gases as it possesses. The vapour penetrates 

 into, and exists within the gases, just as much as the gases 

 exist in the vapour, each having its separate weight ; and 

 when both are in a state of rest, each presses separately 

 and independently on the surface of the earth, and against the 

 mercury of a barometer which may be placed on that surface. 

 Some meteorologists, indeed, from the^- language, seem to 

 think that when vapour first penetrate<!> the gases, they are 

 swelled by it, and thus become permanently lighter, and this 

 idea is countenanced by the temporary efiect that is produced 

 when vapour first enters the gases. Vapour, discharged suddenly 

 against a mass of the gases, impinges on them, and when it 

 penetrates them and is pressing further through them, it no 

 doubt tends to force them forward or upward, as the case may 

 be. This is shewn by steam being used to blow out air from 

 a vessel that contains it, but this is no more than what one 

 gas will do with another. Nitrogen will act thus on oxygen, 

 and oxygen on nitrogen; but when these gases have fully 



