266 BACTERIAL EXAMINATION OF AIR 
in principle. It was reeommended in our 1909 report and again, after careful 
studies of altertive processes in 1912. 
One modification has been made in the apparatus since 1912 which is so 
clearly an improvement as to warrant immediate adoption. The layer of sand 
in the filters as previously used was supported on a perforated rubber stopper 
in a straight section of glass tubing, and the aspiration was applied to a small glass 
tube passing into the rubber stopper. It was clearly essential that the stopper 
should fit tightly, which meant that the apparatus must be sterilized in steam. 
Ruehle (1915) points out that the steam sterilization causes caking of the filter 
sand and thus introduced appreciable errors in analysis. Ruehle, therefore, 
made the suggestion that the rubber stopper should be eliminated and the small 
aspirating tube fused into the large one. 
We, therefore, recommend that for the study of bacteria in air there should 
be used a glass tube of 15 mm. in diameter and 70 mm. long with a smaller tube 
6 mm. in diameter and 40 mm. long fused into one end. On the shoulder of 
the joint between the tubes rests a plug of cotton supporting a layer of sand 
10mm.deep. The sand should be capable of passing a 100-mesh sieve but not a 
200-mesh sieve. The opposite or inlet end of the larger tube is stoppered by a 
cork stopper (which need not be exactly tight) perforated by a glass tube 6 mm. 
in diameter and 40 mm. long bent at an angle of 45° to prevent direct precipi- 
tation of dust particles into the filter tube. 
Five cu. ft. of air should be drawn through the filter by the use of an aspira- 
toc of known volume, preferably one of the double or continuous type, or by the 
use of some form of pump or meter. A convenient and ingenious sampling pump 
was made by Wallace and Tiernan of New York City for the study of Basker- 
ville and Winslow (1913) of the air of New York City schools. After filtra- 
tion, the sand should be shaken out into 10 c.c. of sterile water, and, after thor- 
ough shaking, aliquot portions of the water should be plated on ordinary nutrient 
agar. 
Browne (1917) has proposed a tube which he claims has special 
advantages over the aeroscope used in standard methods. This con- 
sists of the filter tube and dilution tube fused together at right angles. 
During filtration the sand is placed over the stoppered end. After fil- 
tration the sand is shaken into the other end and the filter plug with the 
bolting cloth is replaced by a sterile stopper. Ten c.c. of sterile water 
are added and the sand thoroughly shaken. From this decimal dilutions 
are plated on standard media. 
BIBLIOGRAPHY 
Baskerviuis, C. and Winstow, C. E. A. 1913. The Air of New York City 
Schools. Report of the Committee on School Enquiry Board of Esti- 
mate and Apportionment, City of New York. ITI, 611. 
