fsHAw] RELATIVE HUMIDITY 123 



The treatment of the subject can be made exceedingly elemen- 

 tary and be kept quite clear, if the term, relative humidity, is kept in 

 its proper place as a derived quantity of secondary importance. First 

 we require the simple conception that vapour exerts pressure propor- 

 tional to the amount present, and next, that this pressure increases or 

 decreases with the velocity of the molecules composing it. On these 

 sim.ple ideas the whole subject of humidity rests, and the quantities 

 vapour pressure and temperature should be both first and fundamental 

 in our analysis of humidity problems. Questions of vapour saturation 

 can be handled in an analogous manner to those of the saturation of 

 salt solutions; fortunately we do not find that concentrations of 

 salts are continually and primarily expressed as fractions of satura- 

 tion, although there would be nearly as much ground for doing so. 

 If the temperature of a liquid were usually expressed as a percentage of 

 its boiling point temperature, we would have a method no more con- 

 fusing than that which involves the continued emphasis of the fraction 

 of saturation, whether the question of saturation is pertinent to the 

 problem or not. 



In the more general problems of physics and chemistry dealing 

 with the moisture content of samples of gas or of any material, the 

 chief question is usually that of equilibrium of pressures across a 

 known boundary under varying conditions of temperature. If one 

 body (gas, liquid or solid) has an aqueous vapour pressure less than 

 that of another body in contact with it, there will take place in it, an 

 absorption of water until the two pressures become equal, and if on 

 the other hand its vapour pressure is greater, there will be excess 

 evaporation until the pressures are likewise equalised. If the quan- 

 tity, relative humidity, is brought into problems requiring such analy- 

 sis, it will require a reference to tables and a consideration of tempera- 

 ture and fraction of saturation to an extent which is both unnecessary 

 and confusing. 



In the use of dyes, and in the preparation of sugar, cotton, tobacco, 

 explosives, many chemicals and other commercial products, a know- 

 ledge and special consideration of humidity conditions with reference 

 to this problem, are often essential; and it has been the neglect of 

 some of the most elementary considerations, which has rendered the 

 duplication of some foreign industries less successful than frequently 

 was quite possible. This neglect has similarly been due largely to 

 this over emphasis of the fraction of saturation in its relation to 

 problems of evaporation and moisture content. There has been, for 

 example, a frequent failure to appreciate such obvious facts as that a 

 relative humidity which is suitable at one temperature, and under a 

 given rate of circulation of one of the media, may be entirely unsuit- 



