90 
PROF. T. OARNELLEY, MR. J. S. HALDANE, AND DR. A. M. ANDERSON 
Sources of and Circumstances affecting the Number of the Micro¬ 
organisms in Vitiated Air. 
It has been shown by Hesse ( loc. cit.) that when a room is left quiet the micro¬ 
organisms settle out in a few hours, so that the air becomes comparatively free 
(cf. Tyndall’s experiments on sterilisation of air by subsidence). Hence it is clear 
that a certain amount of physical disturbance in a room is a condition necessary to the 
presence of micro-organisms in the air. It might naturally be supposed that the 
effects of physical disturbance would tend to obscure all other factors affecting the 
number of micro-organisms present in air. It is, therefore, necessary to consider first 
what, other things being equal, are the limits of the influence of ordinary physical 
disturbances on the number of micro-organisms. 
The first observation bearing on this point was made at the High School. A 
determination was first made with the class in the room under ordinary conditions. 
The boys were then told to stamp with their feet on the floor for a short time. This 
they did with particular vigour and gusto, raising a cloud of dust which diffused itself 
through the room. A second determination was then made. The first determination 
gave 11 per litre, and the second about 150. It will be noticed that, although the 
increase is very great, the number found barely reaches the average in the naturally 
ventilated Board schools. 
Such violent disturbances as that just described are, however, altogether excep¬ 
tional. What is of more importance is the effect of slighter disturbances, such as 
occur frequently. In the same school, on another day, the boys were allowed to go 
out during a determination. The number found per litre was 5, as compared with an 
average of 1'8 in five other rooms in which the classes were sitting. The difference 
due to the disturbance was small as compared with the differences caused by other 
factors (see below). Again, in the small Chemical Lecture-room the number was 
actually less (1'5 per litre) at the end of a lecture than at the beginning (3 per litre). 
The room had only been slightly disturbed before the lecture. Again, in the Large 
Lecture-room the average found on three separate occasions, after an hour of a 
crowded popular lecture, was only 4’7 per litre. All these determinations were made 
before the audience left. 
In the case of houses of four rooms and upwards, the rooms were classified according 
as the occupant rose from bed before the determination or not. The average for the 
former class was 12 per litre, for the latter 7 per litre. Again, in a set of observations 
on a block of 2-roomed houses, we started later than usual, and found that in three of 
the rooms visited the people were already stirring. The average in these rooms was 
76 per litre, while in the other two the average was 90. The difference was due 
mostly to one of the former houses, which was cleaner than the rest, giving a lower 
number (34). 
From the above results, taken in connexion with what follows, wey ma conclude 
