28 



SCIENCE 



[N. S. Vol. XXVIII. No. 705 



part of the chamber around the tube shows 

 strong coronas on exhaustion -while the other 

 half (toward the brass cap) is blank. Some- 

 tWng, consisting of very slow-moving par- 

 ticles, gradually diffuses radially out of the 

 aluminum tube. Of course it is difficult to 

 deny with assurance that merest traces of 

 emanation decaying within the aluminum 

 tube may not possibly account for the ac- 

 tivity; but what is remarkable in any case is 

 the existence side by side of a region with 

 nueleation and a region without it, in the 

 absence of anything like a partition. The fog 

 chamber itself must at all times be scrupu- 

 lously free from infection such as an emana- 

 tion would produce, and anything of this kind 

 is at once detected. 



Gael Barus 

 Beown Univeesitt, 

 Providence, R. I. 



A NEW METHOD OF ENUMERATING BACTERIA 

 IN AIR 



The development of accurate bacteriological 

 methods for the examination of air has not 

 attracted wide attention in recent years; and 

 this branch of bacteriology is far behind the 

 related subject of water bacteriology in its 

 technique and interpretation. 



Bacteriological examinations of air have 

 been carried out by most observers in one of 

 two ways, without much attempt at critical 

 control. The most primitive method consists 

 in the simple exposure of plates of nutrient 

 gelatin or agar for a more or less indefinite 

 period. The colonies developing, correspond 

 in a rough way to the bacterial flora of the air 

 above. The method, however, can not be con- 

 sidered a quantitative one, since the number 

 of bacteria which fall on the plate is not re- 

 lated to any particular volume of air and must 

 vary with all sorts of environmental condi- 

 tions. Nevertheless, this method is still used 

 in many investigations in which quantitative 

 results would be valuable; as in the important 

 work of Major Horrocks on the presence of 

 bacteria derived from sewage in ventilating 

 pipes, drains, inspection chambers and sewers.* 



' Proceedings of the Royal Society, Series B, 

 Vol. 79, No. 531, p. 255. 



The other method in common use is a modi- 

 fication of the sand-filter method of Pasteur 

 and Petri. It involves the filtration through 

 asbestos, sand, sugar, etc., of a measured vol- 

 ume of air; the washing of the filtering ma- 

 terial with sterile water; and the plating of 

 aliquot portions of the wash water in the 

 usual way. Pasteur used asbestos for his 

 filtering material; Sedgwick and Tucker 

 recommended finely powdered sugar; and Petri 

 and most recent observers have used sand. 

 Petri pointed out that the sand should be of 

 such fineness as to pass a .5-ram. mesh. In a 

 recent important study of the air of the New 

 York Subway Soper used both the plate 

 method and the sand-filter method. The sand 

 grains used were " about half a millimeter in 

 diameter " and the sand layer 5 cm. deep." 

 In discussing these methods, in another paper, 

 this author said, " as is well known, there is 

 no precise way to determine the numbers of 

 bacteria in air." ' 



I have been engaged for about a year in a 

 study of bacteria in sewer air; and relied at 

 first upon the sand-filter method. The re- 

 markable results, reported by Major Horrocks 

 in the paper to which reference has been made, 

 led me to revise the detail of my technique 

 with considerable care. In the course of the 

 investigation a modified method of air exam- 

 ination was developed which is here reported 

 in the hope that it may be of assistance to 

 others at work on similar lines. 



My aim was to combine the quantitative 

 results of sand filtration with the directness 

 and simplicity of the plate method. Hesse 

 did this after a fashion by slowly aspirating 

 air through a long roll-tube the walls of which 

 were covered with melted gelatin. There was, 

 however, a possibility in such an apparatus 

 that bacteria might be drawn through, without 

 settling out on the walls. My method is really 

 a modification of Hesse's with an increase in 

 the size of the culture vessel relative to the 

 sample of air. I use two liter-and-a-half 

 bottles arranged as shown in Pig. 1. On the 



' Technology Quarterly, XX., 58. 

 'Journal of Infectious Diseases, Supplement 

 No. 3, 1907, p. 82. 



