114 
DR. P. F. FRANKLAND ON THE QUANTITATIVE ESTIMATION OF 
with which the individual tube had become infected, and thus, in a measure, this 
apparatus foreshadows some of the great advantages to be obtained from the use of a 
solid culture-medium in which each individual organism gives rise to a separate 
colony. 
Freudenreich and Miquel* * * § have further elaborated processes for the quantitative 
distribution of micro-organisms in air, their researches having led them to adopt the 
following method, which has been largely used in the well-known experiments at the 
observatory of Montsouris. 
This method consists essentially in aspirating a definite volume of air through a 
plug of sterilised glass-wool, which is then agitated with a definite volume of sterilised 
water, so as to distribute the collected organisms in the latter. This water is then 
divided into a number of equal parts, each of which is added to a tube, flask, or bulb 
containing sterilised broth. The volume of water with which the plug is mixed must 
be so selected that, when divided into the given number of equal parts, the micro¬ 
organisms are so attenuated that only a 'portion of the total number of inoculated 
tubes become infected. If only a portion of the total number of tubes become thus 
infected, Miquel argues that the organisms were distributed at such wide intervals 
in the water that not every one of the parts into which the latter was divided 
contained even a single organism, and that, therefore, those broth-tubes which suffered 
alteration through the inoculation of such a part must have received only a single 
organism, and, thus, that the number of culture-tubes which break down is identical 
with the number of organisms communicated to the water by the plug. 
Koch t first applied a solid nutritive medium to the examination of air for micro¬ 
organisms by exposing glass dishes containing a nutritive solid surface, such as 
peptone-gelatine, potatoes, &c. Upon this surface the aerial microbes become 
deposited, and, after suitable incubation, give rise to colonies which can be counted 
and further examined. 
This method, which forms an entirely new departure in the examination of air, was 
further developed by Hesse, J who, by aspirating air through wide glass tubes of 
about 2 feet G inches in length and 1’5 inch diameter, coated internally with 
peptone-gelatine, found that, provided the current of air was not too rapid, practically 
the whole of the suspended organisms were deposited on the bottom of the tube in 
the first half or two-thirds of its length. The remarkable phenomenon of the rapid 
subsidence of organisms in comparatively still air, upon which this method depends, 
is similar to that previously observed by Tyndall § in his well-known sterile 
chambers coated with glycerine. 
* ‘ Annuaire de l’Observ. de Montsouris,’ 1884, p. 533. 
f ‘Mittheil. Kaiserl. Gesundkeitsamte,’ vol. 1, 18S1, p. 32. 
X Ditto, vol. 2. 
§ Tyndall, loc. cit. 
