158 POPULAR SCIENCE MONTHLY 



a glass jar to avoid contamination from the air. In order to estimate 

 the number of bacteria present, 0.5 c.c. of this wash water were 

 diluted with 10 c.c. of physiological salt solution and three series of 

 agar plates were poured, using 0.2, 0.3 and 0.5 c.c. of this dilution. 

 The wash fluid was then measured and centrifugated, the coarse 

 particles of dirt and paper thus thrown down were filtered off and the 

 fluid again centrifugated. With the sediment of bacteria and fine dirt 

 obtained in this manner, I inoculated three bouillon tubes in order to 

 look for streptococci and other forms that might develop. Ten tubes 

 of serum were smeared for the detection of Bacillus diphtheria 7 , while 

 the rest of the sediment was then carefully injected subcutaneously into 

 a guinea pig. The animals were closely observed for several days so 

 that in case of a fatality an autopsy could be performed immediately 

 after death and the lesions thus more correctly observed. All media 

 used for this experiment were prepared according to the standard 

 method of the American Public Health Association, with a final re- 

 action of + 0.8 (0.8 per cent. acid). 



At first I intended to make a study of only twelve bills, since it 

 required a full week to complete the study of each bill; but the ap- 

 pearance in the daily press of accounts of the present agitation for 

 clean money led me to make a total of twenty-four examinations. In 

 twenty of these all the sediment was injected into guinea pigs, in order 

 to allow the development of tuberculosis or septicaemias that might be 

 caused by the bacteria in the sediment; in the other four, the sedi- 

 ment was used for smearing serum plates to detect Bacillus diphtheria. 



It was surprising to find the flora so constant, Staphylococcus 

 pyogenes alius being by far the most common form present, with 

 various members of the subtilis group next; Staphylococcus pyogenes 

 aureus was found on all bills, but not in large numbers. The follow- 

 ing were found on some bills, but always few in number; Staphylococcus 

 cereus albus, a streptothrix, Sarcina lutea, streptococci and in one in- 

 stance, Bacillus xerosis. Other chromogenic bacteria were frequently 

 found but not identified. 



The numbers of bacteria present on the bills ranged from 14,000 up 

 to 586,000, with an average for twenty-one bills of 142,000. There 

 seemed to be no connection between the amount of dirt and the 

 number of bacteria present; the cleanest-looking bill that I used had 

 next to the highest count (405,000), while the bill that looked the 

 dirtiest had but 38,000. When a bill has been in circulation for a short 

 time and has become somewhat cracked, and its peculiar glaze worn off, 

 the bacteria very easily cling to it without the presence of dirt and 

 grease. 



All inoculations gave negative results, the time limit being placed at 

 from six to seven weeks. All of the guinea pigs showed more or less 

 local reaction, with swelling of the lymph glands of the groin, but 



