130 BACTERIA IN AIR, SOIL, AND WATER. 



point which demands further inquiry, especially by the periodic 

 investigation of examples of different classes of soils. The 

 method is to take I c.c. of such a soil emulsion as that just 

 described, add it to 10 c.c. of gelatin, heat for ten minutes at 

 80 C. to destroy the non-spored bacteria, plate, incubate, and 

 count as before. 



Besides the enumeration of the numbers of bacteria present 

 in a soil, an important question in its bacteriological examination 

 lies in inquiring what kinds of bacteria are present in any 

 particular case. Practically this resolves itself into studying the 

 most common bacteria present, for the complete examination of 

 the bacterial flora of any one sample would occupy far too much 

 time. Of these common bacteria the most important are those 

 from whose presence indications can be gathered of the con- 

 tamination of the soil by sewage, for from the public health 

 standpoint this is by far the most important question on which 

 bacteriology can shed light. 



Bacillus mycoides. This bacillus is 1.6 to 2-4/x in length and about .g/x 

 in breadth. It grows in long threads which often show motility. It can be 

 readily stained by such a combination as carbol-thionin, and retains the dye 

 in Gram's method. All ordinary media will support its growth, and in surface 

 growths on agar or potato spore formation is readily produced. Its optimum 

 temperature is about 18 C. On gelatin plates it shows a very characteristic 

 appearance. At first under a low power* it shows a felted mass of filaments 

 throwing out irregular shoots from the centre, and later to the naked eye these 

 appear to be in the form of thick threads like the growth of a mould. They 

 rapidly spread over the surface of the medium, and the whole resembles a 

 piece of wet teased-out cotton wool. The gelatin is liquefied. 



Cladothrices. Of these several kinds are common in the soil. The 

 ordinary cladothrix dichotoma is among them. This organism appears as 

 colourless flocculent growth with an opaque centre, and can be seen under the 

 microscope to send out into the medium apparently branched threads which 

 vary in thickness, being sometimes 2 /*. across. They consist of rods enclosed 

 in a sheath. These rods may divide at any point, and thus the terminal ele- 

 ments may be pushed along the sheath. Sometimes the sheath ruptures, and 

 thus by the extrusion of these dividing cells and their further division the 

 branching appearance is originated. Reproduction takes place by the forma- 

 tion of gonidia in the interior of the terminal cells. These gonidia acquire at 

 one end a bundle of flagella, and for some time swim free before becoming 

 attached and forming a new colony. Houston describes as occurring in the 

 soil another variety, which with similar microscopic characters appears as a 

 brownish growth with a pitted surface, and diffusing a Bismarck-brown pigment 

 into the gelatin which it liquefies. 



A few experiments made with an ordinary field soil will, however, famil- 



