THE MICRO-ORGANISMS PRESENT IN THE ATMOSPHERE. 
149 
performed in-doors, or on very calm days out-of-doors, whilst when performed in a 
disturbed atmosphere, more especially when the direction of the wind is variable, or, 
what amounts to the same thing, when the number of colonies in the control-tube, 
which I employ with Hesse’s apparatus, amounts to a considerable fraction of the 
number found in the Hesse tube through which air has been aspirated, the results 
obtained by Hesse’s method are markedly in excess of those obtained by the flask 
method. 
The only conclusion with regard to Hesse’s method which can be drawn from the 
issue of these experiments is this, that in undisturbed air it yields results which can 
lay claim to a fair degree of accuracy, but that when the atmosphere is disturbed, 
more especially by currents variable in direction, the results are often very considerably 
above the truth. For practical purposes, therefore, it may be taken that when a 
control-HESSE-tube, such as I have employed, yields practically no colonies the result 
by Hesse’s method is accurate, but that when such a tube yields a considerable 
number of colonies the result is too high. 
The fact that the flask method and Hesse’s method yield practically coincident 
results when the latter (Hesse’s) is employed under circumstances in which it is 
reliable (i.e., in calm air) gives a clear and unmistakable answer to the question, which 
I have already referred to, as to whether the suspended organisms in the air exist 
there in an isolated condition, or in aggregates of greater or less magnitude. 
The coincidence between the results obtained by these two methods is only 
explicable, in fact, on the supposition that the aerial micro-organisms are present as 
isolated individuals, for, were they present in aggregates, the results yielded by the 
flask-method would be in excess; and if the magnitude of the aggregates was great, 
in enormous excess of those obtained by Hesse’s method, as in the process of 
agitation with the liquid gelatine to which they are subjected in the flask-method, 
these aggregates would necessarily become broken up into lesser aggregates, at any 
rate, if not into the ultimate individuals of which they are composed. 
Hesse, in his well-known paper “ Ueber quantitative Bestimmung der in der Luft 
enthaltenen Mikroorganismen ” (‘ Mittlieilungen a. d. Kaiserl. Gesundheitsamte,’ 
vol. 2, 1884, p. 187), draws the following conclusion with regard to this point :— 
“ The conspicuous fact that the extreme colonies formed in the tube are moulds 
indicates that the mould-germs present in the air are on the average lighter than the 
bacterial germs, and leads to the conclusion that the aerial bacteria are not present 
as isolated individuals, but as aggregates of individuals or adhering to carriers in 
such a manner that on the average they weigh heavier than the mould-germs.” * 
* “In beiden Rohren sind die aussersten Colonien Pilzcolonien. Diese .... auffallig kervortretende 
Erscbeinung zeigt, dass die in der Luft enthaltenen Pilzkeime durchschnittlich leichter sind als die 
Bacterienkeime, nnd fiihrt zn dem Schlnsse, dass die Bacterien nickt als einzelne Individuen isolirt in der 
Luft entkalten sind, sondern als Haufcken von Individuen, oder an Tragern haftend derart, dass sie 
durchschnittlich etwas schwerer wiegen als Pilzsporen.” 
