210 THE POPULAR SCIENCE MONTHLY. 



of proportion to the use which has been made of it already for the 

 purposes of respiration and evaporation. Besides, smelling is a very 

 subjective sensation, of very ditterent excitability in different persons. 

 Although generally a certain rule for judging the air of a room may 

 be based on its smell, the decision, in doubtful cases, will always be a 

 subjective one. It would be a different thing if we could lay hold of 

 the smelling particles in the air of the room, and measure or weigh 

 them, and compare them with the volume of air they were taken from ; 

 but we have no method of doing this; everything is left to our 

 noses. 



For this reason I considered it indispensable to look about for 

 some means wliich would make us independent of our subjective esti- 

 mate. I started from the excretion of carbonic acid, as it takes place 

 from the living human body ; its quantity in the air can be ascertained 

 easily and accurately. There is some in the open air, although very 

 little ; the question was, therefore, to find out its increase in a number 

 of inhabited rooms, with notoriously good and notoriously bad air, 

 and to draw a comparison. The correctness of this proceeding de- 

 pends on the supposition that there are no other sources of carbonic 

 acid but the inmates, that there are no burning flames, or tobacco- 

 smoking, etc. I will not say that I consider the detected carbonic acid 

 as the principal drawback to such air; it is, in my mind, the measure 

 only for all the other alterations which take place in the air simultane- 

 ously and proportionately, in consequence of respiration and perspi- 

 ration ; its increase shows to what degree the existing air has been 

 already in the lungs of the persons present. All other functions in 

 which the air participates keep in some proportion to the respiration. 



A series of examinations have resulted in the conviction that one 

 volume of carbonic acid in 1,000 volumes of room-air indicates the 

 limits which divide good from bad air. This is now generally adopted 

 and practically proved, always provided that man is the only source 

 of carbonic acid in the space in question. 



Suppose there is a known source of cai-bonic acid: the determining 

 the amount of it in a room can also be used for measuring another 

 element, which would otherwise defy calculation I mean the amount 

 of ventilation of a closed space of definite construction. Imagine to 

 yourselves a room with its walls, windows, and doors, its numberless 

 penetrable places through which the air holds ingress and egress. It 

 is impossible to measure the velocity of the air at each crack, to meas- 

 ure each little hole, the diameter of each pore, even if one had the 

 means of measuring such minute velocities and sections; yet still we 

 should like to know how much air clianges in a given space, and under 

 different external circumstances. The only way appeared to me to be 

 to mix the air of the room in question with carbonic acid to a certain 

 degree, then to break off this mixing, and to observe the decrease of 

 the acid in proportion to the air in definite times. Knowing the 



