August 4, 1911] 



SCIENCE 



151 



apparent drying is due, and it is the nature of 

 this change which I must now endeavor to ex- 

 plain. 



I have said that aqueous vapor is always 

 dry. How then can it be at times apparently 

 drier than at others? The reason is that we 

 judge of the wetness or dryness of a place by 

 the rate at which evaporation occurs therein. 

 This depends upon the elevation of the tem- 

 perature of the vapor above its dew-point, or 

 that temperature at which it would condense. 

 Suppose we had a hollow vessel enclosing a 

 perfect vacuum. Now introduce a small 

 amount of aqueous vapor at 32° temperature. 

 The vapor will immediately expand until it 

 fills the whole space, and by the heat vibration 

 of its molecules will exert a certain pressure 

 against the sides of the vessel. If now we 

 introduce some more vapor, the latter will 

 likewise expand and the pressure will be in- 

 creased. But at 32° the vibrational energy of 

 the molecules is limited. If we keep on add- 

 ing vapor we shall presently so increase the 

 density that this energy can no longer keep 

 the molecules separate. Some of the vapor 

 will condense. There is then a maximum 

 density or pressure, which, so long as the 

 temperature remains at 32°, can not be ex- 

 ceeded. The vapor is then said to be satu- 

 rated. Suppose, when we arrive at this point, 

 we raise the temperature to Y0°. The heat 

 energy of the molecules is thereby increased, 

 and we shall find that we can now put in con- 

 siderably more vapor before the limiting den- 

 sity and pressure are reached. Hence the 

 latter rise with the temperature, or, what is 

 the same thing, the dew-point or boiling-point 

 increases with the pressure. Now a moist 

 body must be considered as a source of aqueous 

 vapor. If such a body is put into our saturated 

 vapor at 32° no evaporation from it will take 

 place. If the temperature is raised to 70° the 

 vapor becomes superheated, and more vapor is 

 required to saturate it. The moist body be- 

 comes the source of that vapor. Evaporation 

 takes place the more rapidly, the greater the 

 degree of superheat, or in direct proportion as 

 the amount of vapor actually present is in 

 defect of that required for saturation. The 



ratio of the former to the latter is technically 

 known as the relative humidity. Thus the ap- 

 parent dryness of a place depends solely upon 

 the condition of the aqueous vapor therein, 

 and not at all upon that of the air. 



Whenever air is heated for a building it 

 should be moistened; whenever cooled, it 

 should be dried. This is generally appreci- 

 ated, but unfortunately the arrangements pro- 

 vided are usually inadequate. A very consid- 

 erable amount of moistening is required. The 

 Sturtevant Company, however, manufactures 

 a heating apparatus in which steam is blown 

 into the hot air current from one of their fans, 

 by means of a nozzle which finely atomizes 

 the steam. This insures good mixing. It is 

 found that with such an apparatus a much 

 lower temperature suffices for comfort, and is 

 also more healthful. In Europe, where the 

 winters are, in general, moister than ours, 

 lower house temperatures are habitual. I 

 spent three winters in Italy, and can vouch 

 for the fact that when the temperature in my 

 study reached 65°, I found it uncomfortably 

 warm. A certain Italian lady, who considered 

 55° in her own country a comfortable temper- 

 ature and 60° too warm, finds 70° in this 

 climate insufiicient. 



The fact that air when cooled increases in 

 dampness is much more noticeable. It is a 

 serious impediment to the use of refrigerating 

 apparatus for cooling houses. Air which at 

 90° has a relative humidity of only 65 per 

 cent, becomes saturated at 70°. If air thus 

 cooled were admitted to a room, moisture 

 would condense on the walls. Such conditions 

 would naturally be very disagreeable. 



Though the above explanations are only a 

 rehash of well-known principles, I hope they 

 may be of some use. In return I wish some 

 one would explain to me just what is the 

 danger of the open window. Why is a little 

 sneaking draught in the house a source of 

 colds and grippe, while a high wind out-of- 

 doors a pleasure and a benefit? This is a 

 problem that has long puzzled me, but perhaps 

 it is a foolish question. 



M. Mott-Smith 



Waterville, Me. 



