1886.] 



Micro-organisms in the Atmosphere. 



445 



rature of 22° C, in the course of four to five days the colonies 

 derived from the organisms contained in the plugs make their 

 appearance, aud can be readily counted and further examined. 



A large number of experiments were made with a view of testing 

 the accuracy of the process. For this purpose experiments were 

 conducted, using sometimes single plugs, and sometimes double, and 

 it was almost invariably found that all the organisms were deposited 

 on the first plug, the second plug in the very exceptional cases when 

 it did yield anything, containing rarely more than one organism. 



In connexion with Hesse's method, it was found that in experiments 

 performed in the open air, when a blank Hesse tube was exposed side 

 by side with the one through which air was being aspirated, a number 

 of organisms also became deposited in the blank tube, thus intro- 

 ducing an important source of error in the quantitative results 

 obtained by Hesse's process. In the flask method, on the contrary, 

 ,such blank tubes rarely contained any organisms, and in no case more 

 than a vanishing proportion of those present in the other tube. 

 This shows that, whereas in Hesse's apparatus any disturbance of the 

 air during the experiment vitiates the accuracy of the result, in the 

 flask method such disturbances are immaterial. 



On the other hand, in the absence of aerial currents, there was 

 a remarkable concordance between the results obtained by Hesse's 

 method and by the " flask method." This is important, not only as 

 showing the quantitative accuracy of the new method, but by clearly 

 demonstrating that the organisms present in the air exist in an isolated 

 condition, and not in aggregates, as suggested by Hesse. It will be 

 remembered that the plug is violently agitated with the gelatine- 

 peptone, during which operation such aggregates would undoubtedly 

 be broken up wholly, or, any rate, partially ; it would, therefore, be 

 reasonable to expect that the "flask method" would yield a larger 

 number, and possibly a far larger number of colonies than those 

 formed in Hesse's tubes, but as, on the contrary, the numbers agree, 

 under the circumstances described, in so remarkable a manner, it 

 points to the fact that they exist in an isolated condition. 



The paper is illustrated with drawings and photographs. 



The following are the principal advantages which the author claims 

 for the " flask method." 



1. The process possesses all the well-known advantages attaching 

 to the use of a solid medium. 



2. The results, as tested by the comparison of parallel experiments, 

 can lay claim to a high degree of quantitative accuracy. 



3. The results, as tested by control experiments, are not appre- 

 ciably affected by aerial currents, which prove such a disturbing factor 

 in the results obtained by some other methods. 



4. The collection of an adequate sample of air occupies a very 



