150 



SCIENCE 



[N. S. Vol. XXXIV. No. 



ogy in the University of Alabama to take the 

 place recently vacated by Mr. James J. Dur- 

 rett. 



Dr. F. B. Dains has resigned the professor- 

 ship of chemistry in Washburn College to 

 accept an associate professorship of chemistry, 

 in charge of organic chemistry in the Uni- 

 versity of Kansas. 



Professor Karl Pearson, F.R.S., has been 

 appointed to be the first occupant of the chair 

 of eugenics in the University of London, es- 

 tablished by the legacy bequeathed for that 

 purpose by the late Sir Prancis Galton. 



Dr. Erhaed Schmidt, professor of mathe- 

 matics at Erlangen, has been called to Breslau. 



DISCUSSION AND COBEESFONDENCE 



THE AIR WE BRtATHE IN BUILDINGS 



To THE Editor of Science : In a recent num- 

 ber of Science Dr. Gulick asks several ques- 

 tions with regard to the behavior of aqueous 

 vapor in the air, and particularly as to the 

 reason why air when heated becomes drier. 

 All of his questions could be answered by any 

 competent physicist, or could be resolved by 

 reference to any good text-book of physics or 

 of meteorology. But unfortunately, in these 

 days of over-specialization, the language is apt 

 to be too technical, or in the text-books the 

 information too scattered, to be readily found 

 and comprehended by the general reader. 

 Hence the following explanations may be of 

 some use to him and others in a like position. 



There are two popular misconceptions, 

 which it is necessary first to dispel. To begin 

 with, few people seem to understand why 

 water is wet. They think, moreover, that be- 

 cause water is wet, the same is true of ice and 

 of aqueous vapor. Now this is not the case. 

 Both ice and aqueous vapor are themselves 

 dry. They hecome wet, only when they turn 

 to water, ice when it melts, aqueous vapor 

 when it condenses. Hence of the three water 

 is alone wet, and all real moisture is due to the 

 presence of water. So dry is aqueous vapor 

 that it will dry any moist object that it comes 

 in contact with, just as would superheated 

 steam or a dry gas, which in fact are only 



other names for the same thing. Only, we 

 give the name superheated steam to the vapor 

 when the temperature and pressure are much 

 above those of the atmosphere, as in the case 

 of a steam boiler. Of course we must distin- 

 guish between the vapor itself, which is a true 

 gas, dry and transparent, and the cloud or mist 

 into which it condenses, on issuing from a 

 locomotive. Hence it is, strictly speaking, in- 

 correct to talk of the moisture or humidity in ^ 

 the air. There never is any moisture or hu- 

 midity in the air, unless it be such cloud or 

 mist. The described fallacy therefore consists 

 in identifying things which are different, and 

 distinguishing things which are the same — 

 identifying moisture, humidity and water 

 vapor — and distinguishing water-vapor, super- 

 heated steam, and dry gas — which are the 

 same. 



The second misconception consists in speak- 

 ing of the air as moist or dry — an error not 

 likely to be dispelled by the language of the 

 text-books, which include sections on the 

 " Hygrometric Condition of the Atmosphere."' 

 Dr. Gulick falls victim to this misconception 

 when he seeks to explain the apparent drying^ 

 of the air on heating as due to some action of 

 the air on the contained moisture. Thus he- 

 says (p. 327), that on heating the air from 

 32° to 70° — " It appears that one of two things 

 must have happened — either the heat must 

 have contracted the existing moisture or the- 

 capacity of the air for moisture has been 

 vastly increased by the rise in temperature." 

 As a matter of fact neither happened, and, 

 moreover, the air had nothing whatever to do 

 with the matter. The same thing would have 

 occurred if the air had been entirely absent, 

 +1t» aoueous vapor alone present. That is to 

 say, aqueous vapor which at 32° seemed rela- 

 tively moist, would become apparently drier if 

 heated to 70°, whether the space filled by it 

 were simultaneously occupied by air or not. 

 This independence of the substances was first 

 deduced theoretically by Dalton, afterwards- 

 established experimentally by Regnault, at 

 least with a high degree of approximation. 

 Hence it is a change in the condition of the- 

 aqueous vapor, not of the air, to which the- 



