140 
DR. P. F. FRANKLAND ON THE QUANTITATIVE ESTIMATION OF 
~ 7 . . n 7 / 147 moulds. 
Colonies m Jiasks. —(a) 62 j ^ ^ 
(6) 0. 1 * 
Control-tube, containing plug of sugared glass-wool, exposed 50 minutes. 
Colonies in flask, 0. 
10 litres of air yielded, therefore, in j 
No. 1 (glass-wool), 18 colonies. 
No. 2 (sugar-wool), 26 ,, 
The experiments recorded above show that when doubly plugged tubes are 
employed the organisms are almost invariably arrested by the first plug, and that 
the second or (b) plug only occasionally yields any, and then very few, colonies, 
although in all cases the second plug was constructed of a far more impervious 
material than the first plug, by introducing powder in addition to wool into its 
composition. 
The results obtained with the control-tubes, which were in each case simultaneously 
exposed, show that the number of organisms gaining access to the tubes, irrespectively 
of aspiration, is, in general, nil, or at most forms a very small fraction of the 
number collected by aspiration in the same time. The exposure of these control- 
tubes involves very little additional trouble, and should be invariably resorted to, and 
if any organisms are found in them a simple correction can be made, as has been done 
in the above experiments. 
As regards the most advantageous construction of the front or (a) plugs, it would 
appear that there is nothing to choose between simple glass-wool and sugared glass- 
wool as regards their power of arresting tire organisms, whilst the sugared wool leaves 
the gelatine-film in the flask rather clearer; but this slight difference is of no material 
consequence. 
Comparison of Flask-Method with Hesse’s Method. 
Having found that the results obtained by means of the new flask-method were 
very concordant inter se, it became of interest to institute a comparison between the 
results yielded by this method and those by Hesse’s apparatus under precisely similar 
conditions. 
There is an interest attaching to this comparison entirely apart from the question of 
which of the two processes is most reliable in the results which it yields, or of which 
is most advantageous for practical purposes, for by a comparison of the results obtained 
by the two methods it is possible to ascertain whether the living organisms suspended in 
air are in an isolated state, or whether they are present in aggregated masses. Hesse’s 
researches show that the suspended organisms of the air are specifically isolated, that 
is to say, that organisms of two or more different kinds are not found adhering 
together, for the individual colonies which result from their deposition in Ins tubes, or 
on the surface of any solid medium, are pure cultivations and not mixtures of organisms. 
