ON THE CARBONIC ACID, ORGANIC MATTER, ETC., IN AIR. 
91 
that the effects of minor differences as regards physical disturbance, such as cannot 
well be avoided in making observations in a number of rooms, are not in themselves 
sufficient to obscure the influence of other important factors. Nevertheless we always 
endeavoured to examine each class of rooms under conditions as nearly as possible 
the same as regards physical disturbance. In examining dwelling-rooms, it was, of 
course, impossible to avoid the disturbances due to our own presence in the room. 
But these disturbances must have been tolerably uniform, and we knew (see below) 
that our own persons did not act to any appreciable extent as a source of con¬ 
tamination. 
In the cases hitherto referred to, the effects have been studied of an increase of 
physical disturbance apart from the simultaneous introduction of any other factor not 
previously in operation. A new factor, previously latent, may, however, be brought 
into operation by physical disturbances, as for instance when some object specially 
likely to give off bacteria is disturbed. For example, one of the observations at the 
Infirmary in the early morning was made immediately after the making of a number 
of beds in the ward. The number of micro-organisms found was 28 per litre, as 
compared with an average of 2*8 per litre in the other wards.* It was, of course, to 
* We may refer here to an interesting phenomenon observed in connexion with the jelly in the tubes. 
It had been found that in some of the tubes crystals tended to make their appearance on the surface of 
the jelly after the tube had been used, and that these crystals were much more numerous and much 
smaller near the perforation in the cap, becoming fewer and larger towards the other end (see Plate 6, 
fig. 4). They thus resembled in their distribution the colonies of bacteria. We might represent 
diagrammatically their number and distribution as compared with the colonies of bacteria by a diagram 
such as the accompanying’. The inner shaded triangle represents, as regards number and distribution, 
the bacteria; the large triangle the crystals. These crystals only appeared in tubes made of inferior 
glass, apt to dissolve and crack on the surface. We have not as yet determined their composition. 
Probably they were composed of phosphate of lime, and were due to the lime dissolved from the glass 
combining with the phosphoric acid contained in the meat juice. 
Of the tubes used at the Infirmary, four showed these crystals. The crystals were counted, and the 
corresponding numbers of crystals and colonies per litre were as follows :— 
Crystals. Colonies. 
Ward 10 (5 p.m.) . 14 . 0 
„ 10 (5’30 a.m.) . 44 . . 0 
„ 12 (3-30 a.m.) .115 . . 4 
Accident Ward (5 a.m., after some of the beds had been 
made). 1600 . . 28 
In the two tubes first in the list, all the crystals were very large, like those near the cork in the last tube. 
Taking the numbers in the first three tubes together, the ratio of colonies to crystals is 1 : 43, while the 
ratio in the last tube is 1 : 57. The ratio of crystals to colonies thus appeared to correspond roughly. 
The formation of the crystals was evidently determined by solid particles falling on the jelly, just as 
N 2 
