PUTKEFACTION AND INFECTION. 53 



These waxed daily larger, and finally formed a thick 

 layer on the top of every column. The liquid, mean- 

 while, changed from a pale sherry to a reddish-brown 

 colour. To me the experiment was impressive in the 

 highest degree. 



On the 27th of September I provided myself with a 

 microscope having a magnifying power of 1200 diame- 

 ters. Under its scrutiny the turbidity of the liquid 

 immediately resolved itself into swarms of Bacteria in 

 active motion. Cohn correctly explains the turbidity. 

 The index of refraction of the Bacterium being slightly 

 different from that of the surrounding medium, a scat- 

 tering of light is the consequence. This scattering, 

 however, and the opalescence it produces, are practically 

 independent of the motions of the Bacteria. 



Since the date here referred to the exposed liquid has 

 been frequently examined, both with the eye and with 

 the microscope. To the former it is thickly turbid, to 

 the latter it is swarming with life. Its smell is putrid. 

 All this time the protected tubes exhibit a liquid per- 

 fectly unchanged in appearance. For four months it 

 has remained as transparent and of as rich a colour as 

 the brightest Amontillado sherry. 



On the 1st of October another experiment similar in 

 principle to that just described was begun. Fresh urine 

 was employed, and a much smaller case. The capacity 

 of the latter was 45 1 cubic inches ; and three test-tubes, 

 instead of twelve, were passed air-tight through its bot- 

 tom. Like those in the larger chamber they were filled 

 by a pipette, and boiled for five minutes in a bath of 

 brine. Beside them were placed three other tubes con- 

 taining the same liquid treated in exactly the same way, 

 but exposed to the common air. On the 5th all the 

 exposed tubes were turbid, and found by microscopic 

 examination to be swarming with Bacteria. The colour 



