412 Mr Shaw, On an experiment in Ventilation. [May 10, 



theory, the warm air supplied by the Tobin T does pass to the 

 ceiling and that the ventilator V derived a large part of its 

 supply directly from that source, and that this Tobin was practically 

 useless as a means for renewing the air of the room. 



The theory of ventilation is no doubt applicable in cases like 

 the one mentioned, in which the source of heat heats sufficient air 

 to form a steady convection current, such as would be generated by 

 the hot air in the shaft of the Tobin T, or by means of a gas 

 flame, but with some hesitation I offer the following reasons for 

 regarding the diffusion of air surrounding persons in a room as a 

 much more complex phenomenon; which in a case like the one we 

 are considering would result not in a transference of the bad air 

 to form a layer along the ceiling leaving fresh air beneath, but in 

 a local circulation i.e. passing upwards for some short distance and 

 then sinking again over the space not occupied by the audience. 



The air which is respired does not rise upwards without mixing 

 with the surrounding. We may form some idea of the rate at 

 which mixing goes on from the fact, that, in breathing into a 

 cold room a cloud is formed which disappears almost instantly, 

 shewing that the air has become sufficiently mixed to take it 

 above the saturation point by the time the expiration is completed. 

 Assuming that 30 cubic inches are expired and that these contain 

 4 p. c. of carbonic acid and are saturated with moisture at the 

 temperature of 35" C. and breathed into an atmosphere of 15° C. 

 I find that in order to reduce it to air which is just saturated 

 its volume must have increased to nearly twice its original value. 

 Its temperature would then be 28° C. and its density - 96 of that 

 of the surrounding atmosphere, thus giving a lifting force on each 

 gramme of air of about 3 centigrammes; this would be very 

 rapidly diminished by further mixing. Moreover the moist air 

 would rapidly radiate heat through the drier surrounding air 

 and approach very nearly to its temperature, so that before it 

 had been raised any considerable distance its lifting force, due 

 to difference of temperature, would have become inappreciable. 

 It would have become generally diffused at some small height 

 above the people at the same, temperature as the surrounding air 

 and would be loaded with carbonic acid, which would render it 

 heavier, and thus unless mechanically removed it would tend to 

 sink and replace the colder air taken from below to supply its 

 place when it originally rose. 



In the Biological lecture-room the mechanical effects of the 

 ventilators and the cold Tobin T would interfere to some extent 

 with the establishment of the circulation in question, and the 

 effect may not be so readily perceived as in some other instances, 

 such as the lecture-room of the Cavendish Laboratory, which 

 furnishes a very good example. In that lecture-room a dozen 



