176 THE POPULAR SCIENCE MONTHLY. 



carbonic acid is from that time constant ; we obtain it simply by 

 assuming that the acid disengaged is distributed through the volume 

 of air introduced by the ventilation. This proportion-limit is, then, 

 independent of the disposable cubic space. A ration of forty cubic 

 metres of air, for example, with a production of twenty litres of car- 

 bonic acid, to which are added the sixteen litres of acid contained in 

 the forty cubic metres of fresh air, gives the proportion of 0*0009, 

 whatever may be otherwise the disposable space. The capacity of the 

 inclosure plays no other part than that of delaying the moment when 

 the constant regime is established ; the space acts as a reservoir which 

 is gradually filled till it contains the same proportion of acid as the cur- 

 rent of air that traverses it ; but, once saturated, it intervenes no more 

 in the course of the phenomenon. The advantage of a considerable 

 cubic space consists, then, chiefly in the fact that it retards the approach 

 of the moment when the alteration of the air attains the limit which 

 it will not pass. This consideration becomes of some importance in 

 fixing the size of rooms that are to be occupied only for a definite 

 number of hours at a time ; for it will be always possible to arrange 

 matters so that the proportion-limit shall not be reached before the 

 end of the contemplated time. 



Let us suppose, for example, that the ventilation can supply six 

 cubic metres of fresh air per person per hour. This is the ration of 

 air which, according to Peclet, might be sufficient in case of extremi- 

 ty, because six cubic metres of air, half saturated at 60°, can absorb 

 the thirty-five or forty grammes of vapor given out by transpiration. 

 The fresh air containing already a proportion of 0*0004 of carbonic 

 acid, to which respiration adds 0*0033, we find that the proportion- 

 limit will be 0*0037. This limit will be almost reached and the regime 

 will be constant when the air has been renewed three times, for the 

 proportion of air will then exceed 0*0035. If the allotted space is only 

 one cubic metre, as we know happens sometimes to be the case in 

 theatres and other assembly-halls, a half an hour will be long enough 

 to bring about this state of things ; if the cubic space is increased to 

 ten cubic metres, five hours will be required, and ten hours if it is in- 

 creased to twenty cubic metres, to reach the same degree of alteration. 

 Such, then, would be the effect of a ventilation at the rate of six cubic 

 metres an hour, according to the capacity of the building. By rais- 

 ing the ration of air to thirty cubic metres, the proportion-limit be- 

 comes O'OOll, and we may assume that this has been reached when 

 the air has been renewed twice (the real proportion being then 0*0010). 

 This will happen at the end of four minutes in a space of one cubic 

 metre, after forty minutes in ten cubic metres, etc. But the pro- 

 longation of time obtained under these circumstances is not of the 

 same importance as in the preceding case, for the limit of 0*001 is a 

 characteristic of respirable air. With so energetic a ventilation as 

 this, the consideration of cubic space becomes a minor affair ; but it 



