TEXT-BOOK OF BACTERIOLOGY. 171 



to remove a portion of this moist layer with the platinum needle, 

 \ve find that it is held together by the agglutination of the inti- 

 mately-joined bacteria. Thus one can draw out threads a foot 

 long, which are only held together by the shining, swollen envel- 

 opes of the separate rods. 



It is remarkable that the surface of the potato itself, under the 

 influence of the bacterial mass growing upon it, often takes a 

 slightly-red, sometimes a decided red, color, which extends deep 

 into the substance. 



BACILLUS SUBTILIS. 



The hay bacillus (Bac. subtilis Ehrenberg) is one of the most 

 widely distributed and frequently occurring of all bacteria. As its 

 cells appear in the form of very large and clearly-recognizable rods, 

 it of course early attracted attention and became a subject of 

 study. F. Cohn observed the formation of spores in it, and a whole 

 series of facts, which were afterward found applicable to the bac- 

 teria in general, were first noticed in connection with this particu- 

 lar species. 



The spores of Bacillus subtilis are found in the air and in the 

 water; the upper layers of the soil, the dust of dwelling-rooms, the 

 faeces of men and animals, putrescent fluids, etc., all contain them 

 in rich quantities. It has been called "hay bacillus/' from its being 

 regularly found in hay and in vegetable infusions of all kinds. If 

 we cut some dry grass into small portions, put them into a flask, 

 add a moderate quantity of distilled water, close the vessel with a 

 pledget of cotton-wool, and boil for about a quarter of an hour, the 

 greater part of the germs contained in it are destroyed. Only 

 those of Bacillus subtilis, in consequence of their high power of re- 

 sistance, remain alive and capable of development, and after two 

 or three days a whitish covering forms on the surface of the fluid. 

 This covering or film consists of a luxuriant growth of hay bacilli. 



If we examine a small portion of it under the microscope, we see 

 large rod-like cells, somewhat slender, about three times as long 

 as they are broad, with slights-rounded corners and perfectly 

 homogeneous, bright, translucent contents. The hay bacillus has 

 a strong proclivity to form groups. Isolated cells are seldom ob- 

 served, and long threads crossing the whole field of the microscope 

 are by no means rare. This is the immediate consequence of its 

 energetic manner. of growth. Attentive observers assert that a 

 cell can, under favorable circumstances, divide and become two new 

 ones by transverse segmentation within half an hour, and that this 



