348 



Journal of Agricultural Research 



Vol. IV. No. 4 



at the junction of the large and small tubes. The upper stopper is re- 

 placed by a cork stopper, a construction that permits the aeroscope to be 

 sterilized by dry heat instead of by steam. After this tube was devised 

 it was discovered that Koning (i6) had used one of similar construction, 

 although he used a coarser sand in the filter. In this work the sand 

 used in the standard aeroscope and in the modified standard aeroscope 

 was of such a size that it passed a loo-mesh sieve but 

 was retained by a i6o-mesh sieve. 



Rettger's aeroscope may be described in his own 

 words (22, p. 462-463): 



The entire special apparatus [fig. 3] consists of a glass tube with 

 a small round bulb at one end. The bulb has 8 or 10 small per- 

 forations, which serve the purpose of allowing the air to pass 

 through at a rapid rate and yet divide the gas to such an extent 

 that every particle of it is brought into close contact with the 

 filtering fluid. This glass tube or aeroscope is fitted into a small, 

 thick-walled test tube by means of a rubber stopper, which also 

 bears, besides the aeroscope, a short glass tube bent at right 

 angles. The upper end of the aeroscope is bent at an angle of 

 about 45°, in order to prevent bacteria and particles of dust 

 from falling into the open end of the tube, and still permit of 

 the tube being drawn through the stopper without difficulty. 



Five c. c. of physiological salt solution are used as 

 the filtering agent. This is plated in aliquot portions, 

 after drawing a measured quantity of air through the 

 aeroscope. 



EXPERIMENT.^L DATA 



Comparison of Various T\-pes of Aeroscopes 



The problem of demonstrating which of two aero- 

 scopes is the more accurate is attended with certain 

 practical difficulties. It is not practicable to set up two 

 aeroscopes side by side and determine which is the 

 more reliable, since the bacterial quality of the air 

 is not uniform enough to make it possible to secure 

 strictly comparable results. For this reason some authors have frankly 

 given up this procedure and have resorted to determinations of the filtering 

 efficiency of each filter independently of the other. In these efficiency tests 

 the aeroscopes are set up in tandem — that is, end to end — so that the air 

 passes through one filter and then through the other. The percentage 

 efficiency of the aeroscope is determined by calculating the percentage of 

 the total number of bacteria that appear in the first aeroscope. Percent- 

 ages obtained by two different aeroscopes in this way ought not to be com- 

 pared unless they have been calculated from numbers of approximately 

 equal size. The fairest way of making these comparisons is to carry on the 

 two kinds of tests simultaneously. This was not realized in this work until 

 contradictory results had been obtained by the use of the tests separately. 



Fig. .-?. — Rettger's 

 aeroscope. 



