Mabch 31, 1911] 



SCIENCE 



489 



vessel is the sum of the pressures exerted by 

 the individual gases composing the mixture; 

 thus, air is a mixture of oxygen, nitrogen, 

 CO,, superheated water vapor and many other 

 so-called rare gases, and the atmospheric pres- 

 sure is the sum of all the pressures exerted 

 individually by all these gases. 



Take again the closed vessel containing 

 ■water and air; the quantity of water vapor 

 per cubic foot of space above the water, as 

 well as the pressure exerted by this water- 

 vapor, conditions having become constant, will 

 always be the same for the same temperature, 

 but will rapidly increase with rise of tempera- 

 ture. The following tabulation shows the 

 maximum capacity of air (or other gas or 

 better simply of space) for water vapor at dif- 

 ferent temperatures, and also the pressures 

 exerted by the water vapor at these maximum 

 capacities, the values being taken directly 

 from the steam tables of Marks & Davis, 

 Table I. 



The extent to which what Dr. Gulick calls 

 the " thirst " of air for more water vapor is 

 unsatisfied is the difference between the maxi- 

 mum capacity and the quantity actually pres- 

 ent. The rapidity with which moisture is 

 evaporated from a moist surface, such as the 

 skin, probably depends, first, on the " thirst " 

 of the air; second, upon the velocity with 

 which the air moves across the moist surface; 

 and third, upon the physical condition of the 

 moist surface, particularly with respect to 

 temperature. Relative humidity is ratio of 



the quantity of water vapor actually present 

 to the maximum quantity that could be pres- 

 ent at a given temperature. Relative humid- 

 ity will not of itself determine air " thirst," 

 as this " thirst " will be different for the same 

 relative humidity at different temperatures. 



Dr. Gulick asks, " Does any one know why 

 delicate children and tuberculous persons get 

 well out of doors but fail to do so indoors." 

 I am not a physician but do believe the air 

 " thirst " is certainly, as a rule, far greater in- 

 doors than out of doors, and that this " thirst " 

 continually absorbs moisture from the skin, 

 tending to keep it cold and dry and chilling 

 it in local spots. 



William J. Crowell, Je., Ch.E. 



To THE Editor of Science : The capacity of 

 gases for carrying vapors of volatile liquids 

 and the capacity of various volatile liquids to 

 take the vapor form is a subject to which the 

 writer recently has had occasion to devote con- 

 siderable thought in connection with certain 

 technical processes. Possibly some of the con- 

 fusion which it seems exists in Dr. Gulick's 

 mind may be dispelled by the following state- 

 ments which I think can be readily verified. 



I will attempt to answer the questions pro- 

 pounded by Dr. Gulick on page 32Y of the 

 March 3, 1910, number of Science. 



1. Water vapor, I believe, is the correct term 

 to be used in a discussion of the hygienic ef- 

 fect of atmospheric humidity since the word 

 steam sometimes conveys the meaning of the 

 cloud of minute water particles resulting 

 from condensation of water vapor in the pres- 

 ence of another gas (air). 



2. Water vapor then (the amount of which, 

 compared with the total capable of being car- 

 ried, in the air at any temperature represents 

 its humidity) does " act strictly in accordance 

 with the ordinary laws governing the move- 

 ment of gases." 



3. Humidity in the air is a term used to ex- 

 press the amount of water vapor in the air 

 usually expressed in terms of the percentage 

 of the total amount capable of being carried 

 at that temperature. 



4. Water vapor (as long as it remains in 



