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 different 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 which would make us independent of our subjective estimate. 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 depends 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 simultaneously and proportionately, in consequence of respiration and perspiration; 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 carbonic 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 measure 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 changes 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