1856.] and the Means of Determining its Amount. 237 



We may know exactly the quality or quantity of bread and water that 

 we want : we may see them measured or weighed, but how to get 

 possession of them may still present many difficulties. So with our 

 want of fresh air, the question. How much we want, and how to 

 know when we have got enough, and no more than enough, is totally 

 different from the question. How to get what air is wanted. It is 

 not my intention to say anything now on the means of ventilation. 

 I intend to bring before you the two other questions — partly, 

 because I think that some error exists as to what is wanted, and 

 much is required to be done by ^ood experiments to perfect our 

 knowledge on this subject ; and partly, because the means of de- 

 termining what air we have got is nowhere clearly stated. Usually 

 the various sensations of individuals constitute the test of purity or 

 impurity ; more rarely the rate of the passage of the air in or out 

 of the room is determined ; and most rarely the chemical analysis 

 of the air gives the amount of impurity which it contains. In- 

 dependently of any of these methods it has been thought that 

 measuring the cubic space in which we are breathing might suffice 

 to tell us what air we have, and to let us know what air we want ; 

 but it can be proved, in few words, that by the yard measure such 

 questions cannot be determined. 



If a fish were confined under water in a glass tube open at the 

 two ends, the time during which the fish would live in the tube 

 would not depend on the cubic contents of the tube, but on the 

 quantity of water caused to pass through the openings. So the 

 cubic contents of a room will give no more information than the 

 cubic contents of the glass tube. The rate of passage of the air, 

 (or rather the rate and quantity of air which passes in,) which 

 depends on the size of the openings, and on the difference of tem- 

 perature within and without the room, is the important question. 

 For the cubic contents which are enough or too much when one 

 amount of ventilation exists, are quite insufficient when the ventila- 

 tion is less ; that is, when the expired air is not sufficiently removed. 

 Moreover, in a room which is constantly inhabited, the cubic space 

 soon after the room is occupied ceases to be of importance, being 

 entirely lost in comparison with the importance of the change of 

 air or ventilation of the room. On the cubic space depends only 

 how soon change of air will become requisite, but it does not at all 

 influence the amount of change required. For example, if a single 

 man constantly inhabit the largest room ; if it be perfectly closed, 

 he will be poisoned in it just as certainly as in the smallest room, 

 the difference will only be in the time required ; and whether in 

 the small room or in the large room to live healthy he would require 

 exactly the same amount of ventilation. The following table will 

 prove that the cubic space actually given to persons in different 

 circumstances is so different that no general rule can be true. 



