444 Dr. P. F. Franklaiid. On the Estimation of [Dec. 9, 



describes the process of Freudenreich and Miquel for the quantita- 

 tive estimation of the bacteria in air, and points out the great advance 

 which has been made upon their method in the adaptation by Koch, 

 and later by Hesse, of a solid nourishing medium for their investiga- 

 tion. In reviewing these different processes he draws attention to 

 the advantages and disadvantages attending them, and proceeds to 

 describe a new method which he has devised, and in which he has 

 endeavoured to overcome some of the objections to which the others 

 are open. 



The first experiments consisted in aspirating a definite volume of 

 air through plugs of either glass-wool or sugared glass-wool, and 

 mixing them by violent agitation with a definite volume of either 

 broth or sterilised distilled water. 



A portion of this liquid was then added to gelatine-peptone, and 

 plates were poured in the usual manner. 



In this way a much larger volume of air was capable of being 

 examined than was possible by Hesse's method, whilst at the same 

 time the solid medium, with its advantages, was retained. The 

 experiments, however, show that although in many cases the 

 results of two or more plates poured from the same mixture were 

 fairly uniform, yet discrepancies did occur, and were sometimes 

 very considerable, pointing to the fact that the organisms had 

 not become evenly distributed throughout the liquid ; also as only a 

 portion of the air aspirated was capable of being examined, a very 

 much larger volume of air had to be used in order to give a sensible 

 result. 



This objection, which applies equally to Miquel's method, which 

 rests upon the assumption that it is possible to equally apportion out 

 into a series of flasks or tubes the organisms contained in such a 

 liquid, led the author to abandon the plate process, and to devise a 

 method which should enable the whole volume of air aspirated to be 

 examined. 



The method consists essentially in aspirating a known volume of 

 air through a glass tube, containing two sterile plugs, consisting 

 either of glass-wool alone, glass-wool and fine glass-powder, glass- 

 wool coated with sugar, or sugared glass-wool and fine sugar-powder. 

 The plugs are so arranged that the first one through which air is 

 drawn is more pervious than the second. After a given volume of 

 air has been aspirated, the two plugs are transferred respectively to 

 two flasks, each containing melted sterile gelatine-peptone, and 

 plugged with sterile cotton- wool stoppers. The plug is carefully 

 agitated with the gelatine, so as to avoid any formation of froth, and 

 when the plug has become completely disintegrated and mixed with 

 the gelatine, the latter is congealed, so as to form an even film over 

 the inner surface of the flask. On incubating these flasks at a tempe- 



