-1859] WRITINGS OF JOSEPH HENRY. 233 



perature of the saturated air be 75°, we would find opposite 

 this, in table B, (p. 225,) the weight of 9-37 grains; and by 

 merely knowing the temperature at other times and at other 

 places we would be able to determine the relative quantity 

 of the vapor under these different circumstances and to form 

 a judgment as to the dryness or humidity of different local- 

 ities; but since there is a constant resistance to the diffusion 

 of vapor through the atmosphere it follows that the air is 

 seldom at any time or in any place entirely saturated. It 

 is on the contrary in the condition of air filling a vessel into 

 which less water has been injected than that necessary to 

 furnish sufficient vapor to fill the interstices between the 

 atoms at the given temperature. 



We have been provided by Dalton with a very simple 

 process by which the amount of vapor in a given portion of 

 air which is not saturated can be determined. For this pur- 

 pose it is only necessary to procure a bright metallic tum- 

 bler, the thinner the sides of which the better, and partly 

 filling this with water at the temperature of the air and 

 gradually adding colder water, stirring the mixture all the 

 while with the bulb of a delicate thermometer, note the tem- 

 perature at the moment when dew begins to be deposited on 

 the outside. This temperature is called the dew-point, from 

 which we determine by the tables the tension and the amount 

 of vapor in the surrounding atmosphere. To render this 

 clear, suppose the amount and tension of vapor in the atmos- 

 phere to be that which would be produced by a temperature of 

 60°, the temperature of the air at the time of the experiment 

 being 70°, the atmosphere in this case would not be satu- 

 rated ; but if we should gradually cool it down to the tem- 

 perature of 60°, it would then be saturated, and the least 

 diminution of temperature below this degree would cause a 

 precipitation of vapor in the form of mist or dew, and this 

 is what really takes place in regard to the vapor which 

 immediately surrounds the sides of the tumbler. The in- 

 troduction of cold water into the tumbler cools the surface, 

 which in turn cools the air immediately around it, and when 

 the diminution of temperature reaches the point at which 



