182 TEXT-BOOK OF BACTEKIOIXKSY. 



I)en5e of the food medium on which they thrive, and out of which 

 they take what they require to form the pigment. The appearance 

 of the latter is, therefore, dependent on the chemical qualities of 

 the substratum, which explains its variable behavior. In milk it 

 takes its origin in the casein and more closely resembles pure blue 

 in its appearance the more distinctly acid the reaction of the liquid 

 becomes, in consequence of the lactic-acid fermentation- 

 According- to the investigations of Scholl, the Bacillus cyano- 

 genus loses in long-continued, uninterrupted culture on our ordi- 

 nary gelatin more or less of its power to form coloring matter. It 

 becomes attenuated, like Bac. acid, lact., and then produces no pig- 

 ment in sour milk, even under the most favorable circumstances. 



Bactekia of Dbixkixg-water. 



Every one knows with what care, at the present day, we watch 

 over the water which we drink, and how we endeavor by periodi- 

 cally-repeated examination to know at all times what it contains. 

 These examinations are both chemical and bacteriological. The 

 latter are intended to inform us as to the micro-organisms present 

 in the water. The particulars as to the manner of conducting such 

 examinations will be considered later. "We wiU here only remark 

 that they have made us acquainted with a number of bacteria 

 which occur with a certain degree of regularity, and some of which 

 are distinguished by striking peculiarities. 



Although they are without any special importance, yet we will 

 dweU briefly on some of the best-known species. 



Baciix.t:s Violacets. 



In river- water we sometimes find a bacterium which produces 

 a beautiful -^-iolet pigment and which liquefies gelatin rather 

 quickly. 



This is Bacillus violaceus, a slender rod-ceU about three times as 

 long as broad, which is often found in moderately-long strings. It 

 has a very lively spontaneous motion and forms spores in the mid- 

 dle of its ceUs. 



On the glass plate its colonies at first appear to the naked eye 

 like Mttle air-bubbles inclosed in the gelatin. On closer examina- 

 tion it will be found that these bubbles are nothing but an even 

 liquefaction of the gelatin, which proceeds also in a downward 

 direction, and at the bottom of which the whitish culture lies. 

 Under the microscope the colonies appear as small irregular heaps, 

 with confused, fibrous, loose edges. The larger colonies which have 



