THE MICRO-ORGANISMS PRESENT IN THE ATMOSPHERE. 
115 
Advantages and Disadvantages of Methods in Vogue. 
Of the methods mentioned above, the two most generally in use at the present time 
are those of Miquel and of Hesse. The former finds most favour with those experi¬ 
menters who still work principally with liquid media, whilst Hesse’s method is 
preferred by those employing a solid material for the cultivation of micro-organisms. 
There are many objections to Miquel’s process, sufficiently obvious from the above 
description. Thus, in the first place, each individual experiment is attended with 
great expenditure of time, trouble, and material, for the water with which the glass- 
wool plug is mixed has to be divided amongst a large number (ten, thirty, fort} 7 , or 
more) of culture-tubes in one single experiment, so that the preparation of these alone 
for an extensive series of investigations must become an intolerable burden. A still 
weaker point in the method lies in the necessity of so selecting the quantity of water 
with which the glass-wool plug is mixed that, when divided into the given number of 
equal parts for inoculation into the culture-tubes, each part shall not contain move 
than one organism, for, as pointed out above, it is assumed in the subsequent estima¬ 
tion of the number of organisms present that each culture-tube which suffers alteration 
has succumbed through the introduction of a single organism. Should, therefore, the 
quantity of water taken for mixing be wrongly proportioned and the whole of the 
inoculated culture-tubes break down, the experiment is rendered worthless, as no 
calculation with regard to the number of micro-organisms can then be made. Again, 
the assumption that each tube suffering alteration does so in consequence of the 
introduction of a single organism must be accepted with much reserve, more especially 
as the difficulty of equally distributing the organisms in the water (and upon this 
equal distribution the process is absolutely dependent) is rendered particularly 
difficult through the presence of the suspended particles of glass-wool, and of this 
difficulty I shall furnish experimental proof later on. 
Owing to the above objections to Miquel’s process, and in consequence of the 
obvious advantages to be secured by working with a solid nutritive medium, I have 
myself adopted Hesse’s method for carrying out a number of experiments on the 
distribution of micro-organisms in air, the results of which were communicated to the 
Society in June last.* This method possesses the great advantage that the tubes, 
after preparation in the laboratory, can be transported to the place where the experi¬ 
ment is to be performed, and that there are no further operations requiring special 
appliances. In hot weather, however, there is considerable difficulty in carrying the 
tubes about, and I had to devise special precautions, which I have described in 
another communication, for preventing the melting of the gelatine film in experiments 
performed in direct sunshine. After a very short acquaintance with Hesse’s apparatus, 
however, I became convinced that it was a matter of great consequence in which 
direction the tube was placed with regard to the wind, and that, if the open extremity 
* ‘ Roy. Soc. Pi-oc.,’ vol. 40, p. 509. 
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