42 PRINCIPLES OF CIIEMJSTRY 



plants, which, when fresh, contain from 40 to 80 per cent, of water by 

 weight. Animals contain about the same amount of water. In a 



the precaution being taken that a portion of the water remains in a liquid state; then 

 the volume of the moist gas is determined, from which that of the dry gas may be 

 calculated. In order to find the weir/lit <>r' the aq//ca//v ntjixntr in a pis it is necessary 

 to know the weight of a cubic measure at 0~ and 7(U) mm. Knowing that one cubic 

 centimetre of air under these circumstances weighs O'OOl'J'.Ki gram, and that the density 

 of aqueous vapour is 0'62, we find that one cubic centimetre of aqueous vapour at and 

 760 mm. weighs 0'0008 gram, and at a temperature t and pressure // the weight of one 



cubic centimetre will be O'OOOS x x ^ -- . We already know that v volumes of a ga^ 



at a temperature t pressure h contain v x -- volumes of aqueous vapour which satu- 

 rate it, therefore the weight of the aqueous vapour held in v volumes of a gas will bt 



V J1/ x 0-0008 x Ax - , or z; x O'OOOS x f x >27l! . 

 h 7GO 273 + t 7(50 878 + < 



Consequently, the weight of the water which is held in one volume of a gas is only 

 dependent on the temperature and not on the pressure. This also signifies that evapo- 

 ration proceeds to an equal extent in air as in a vacuum, or, in general terms (this is 

 Dalian's law), vapours and gases diffuse into each other as if into a vacuum. In a given 

 space there enters, at a given temperature, a constant quantity of vapour whatever be 

 the pressure of the gas filling that space. If the degree of moisture equals r then the 



weight of the vapour in v cubic centimetres will be y) = v x O'OOOS x J[ x J ' grams, 



7oO 



From this it is clear that if the weight of the vapour held in a given volume of a gas 

 be known, it is easy to determine the degree of moisture r= u-ono X / X )->' 

 On this is founded the very exact determination of the degree of moisture of air by the 

 weight of water contained in a given volume. It is easy to calculate from the preceding 

 formula the number of grams of water contained at all pressures in one cubic metre or 

 million centimetres of air saturated with vapour at various temperatures ; for example, 



at 80 /= 31-5, therefore p = 1000000 x 0*0008 x ^ x 27g + g( j or 2<)\S4 grams. 



The laws of "Mariotte, Dalton, and Gay-Lussac, which are here applied to gases and 

 vapours, are not entirely exact, but are approximately true. Were they unite exact, a mix- 

 ture of several liquids, having a certain vapour pressure, would be able to give vapours 

 of a very great pressure, which is not the case. In fact the pressure of aqueous vapour 

 is slightly less in a gas than in a vacuum, and the weight of aqueous vapour held in a 

 gas is slightly less than it should be according to Daltoifs law. as was shown by the ex- 

 periments of Eegnault and others. This means that the tension of the vapour is less 

 in air than in a vacuum, which also is the reason why the weight of vapour is less than 

 the theoretical weight. The difference between the pressure of vapours in air and in a 

 vacuum does not, however, exceed ^ of the total pressure of the vapours, and therefore 

 in practice the application of Dalton's law may be followed. This i/rcm/trnf in rajtour 

 tension which occurs in the intermixture of vapours and gases, although small, indicates 

 that there is then already, so to speak, a beginning of chemical change. The essence of 

 the matter is that in this case there occurs as on contact (see preceding footnote) an 

 alteration in the movements of the atoms in the molecules, and therefore also a change 

 in the movement of the molecules themselves.! 



In the uniform intermixture of air and other gases with aqueous vapour, and in tin- 

 capacity of water to pass into vapour and form a uniform mixture with air, we may 

 perceive an instance of a physical phenomenon which is analogous to chemical phe- 

 nomena, forming indeed a transition from one class of phenomena to the other. Between 

 water and dry air there exists a kind of affinity which obliges the water to saturate the 



