174 THE POPULAR SCIENCE MONTHLY. 



of cast-iron stoves, effects from which sheet-iron stoves, which are not 

 pervious to it, are free ; and it is one of the products of the combus- 

 tion of poor illuminating gas. It is, nevertheless, customary to meas- 

 ure the degree of insalubrity which any atmospheric medium has 

 reached by the quantity of carbonic acid it contains. This is found to 

 increase rapidly in school-rooms, hospital- wards, and assembly-rooms of 

 all kinds, but not nearly so rapidly, unless the room is extremely close, 

 as the gas is actually developed by the life-processes of the inhabit- 

 ants of the rooms. This fact indicates that, even in rooms regarded 

 as close, a considerable renewal of air is all the time going on by nat- 

 ural or spontaneous ventilation. 



Dr. Pettenkofer has made an ingenious use of the estimation of 

 the proportions of carbonic acid to measure the spontaneous ventila- 

 tion, or the speed with which the air gradually renews itself in rooms. 

 It is sufficient for this purpose to develop artificially in a room an ex- 

 actly ascertained quantity of the gas, and to determine by repeated 

 analyses the quantity of acid that disappears in a certain time. The 

 method is a good one, provided there is no opportunity for the acid to 

 be absorbed by fresh mortar. By gauging in this manner the ventila- 

 tion of a number of places, and then observing in the same places the 

 degree of alteration in the atmosphere resulting from the presence of 

 a given number of persons. Dr. Pettenkofer found that the atmos- 

 phere remained of a satisfactory quality when it was renewed at the 

 rate of sixty cubic metres an hour per head. The proportion of car- 

 bonic acid continued under these conditions to be less than a thou- 

 sandth. Experiments were made in a room with brick walls, and hav- 

 ing a capacity of seventy-five cubic metres. On the first day when 

 the temperature was 66° in the room and below the freezing-point out- 

 of-doors — the difference being nearly 36° — the rate of change (sev- 

 enty-four cubic metres) was sufficient to renew all the air in the room 

 in an hour ; with a good fire in the stove, the rate of ventilation was 

 raised to ninety-four cubic metres an hour. With paper pasted over 

 the joints of the doors and windows, it fell to fifty-four cubic metres. 

 On another day, when the difference between the inner temperature 

 and that out-of-doors was about seven degrees, the rate of ventilation 

 was only twenty-two cubic metres an hour ; and with a window half 

 open it was only increased to forty-two cubic metres ; thus an open- 

 ing of eighty square decimetres was of less effect upon ventilation 

 than the simple transpiration through the walls assisted by a difference 

 of about 36° between the outer and inner temperatures. A calcula- 

 tion based on these experiments indicates that a difference in tempera- 

 ture of 1° C. (1*8° Fahr.) causes to pass every hour about two hundred 

 and forty-five litres of air for each square metre of exposed wall-surface. 



The question of the volume of air needed by a man for free respi- 

 ration is a complex one, on which hygienists do not readily agree. The 

 answer to it must depend, not only on the exterior conditions in view. 



