186 TECHNOLOGY OF BACTERIA. 



fluid in which they are immersed, are invisible ; 

 but if endowed with active movement, they may 

 be detected by the disturbance they cause among 

 motionless objects which happen to lie in their 

 course. Thus the septic vibrio of Pasteur is so 

 slender and transparent as to be almost invisible ; 

 but when present in the blood of a septicaemic rab- 

 bit, its vigorous serpentine movements are marked 

 by a displacement of the blood globules, which it 

 moves as a serpent moves the grass in which it is 

 concealed. This septic vibrio I have found in the 

 blood of rabbits, victims of my experiments in 

 New Orleans during the summer of 1880. 



The use of staining reagents is indispensable 

 for the recognition of these extremely transparent 

 or extremely minute species. Their value has 

 recently been demonstrated by Koch, in a most 

 striking manner, by the discovery of a specific 

 bacillus in the lungs and sputum of patients suf- 

 fering with pulmonary consumption, which had 

 escaped the observation of pathologists and mi- 

 croscopists up to the time of his announcement 

 of its presence and peculiar color-reaction. 



3. STAINING BACTERIA. By far the most use- 

 ful staining reagents are the aniline dyes, first rec- 

 ommended by Weigert. Previously to the intro- 

 duction of this method, hasmatoxylin had been 

 used to some extent, but did not give very satis- 

 factory results, as " it does not stain rod-shaped 

 bacteria at all, and only colors the spherical so 



