l886.] NEW-YORK MICROSCOPICAL SOCIETY. 123 



place upon it some of the bacteria to be studied, using therefor 

 a needle or platinum wire sterilized by passage through a flame. 

 The prodigiosus develops in about two days, and can be identified 

 by its deep red color, which, as far as we know, is peculiar to 

 this bacterium. Some bacteria are green, some are white, some 

 are yellow, etc., and by means of their colors one kind can be 

 distinguished from another. 



An important step in advance was taken when Dr. Koch hard- 

 ened, by the addition of gelatine, the bouillon and other trans- 

 parent media in which he cultivated bacteria. I have here a 

 culture so prepared. The transparency of the entire mass allows 

 easy observation of the colors by which the different bacteria 

 can be identified. Sometimes, instead of gelatine, isinglass or 

 agar-agar is used. The gelatine or other hardening agent must 

 be sterilized before it is used. Koch devised the method of cul- 

 ture in gelatine thinly spread on pieces of glass, which possesses 

 many advantages. He also contrived a plan for counting bac- 

 teria, as follows : Place over the glass plate upon which is the 

 gelatine containing bacteria, a plate of glass ruled to a scale in 

 minute squares. The number found in three or four squares 

 can be used for computing the sum total in a specified space. 

 Uncertainty, however, attends methods of ascertaining the pro- 

 ductiveness of bacteria, and for a variety of reasons. Some do 

 not grow in gelatine, some live in blood-cells only ; and while 

 half a million of a certain species may, after cultivation, be 

 found in the space of a cubic centimetre, perhaps another 

 species with which the gelatine was simultaneously inoculated 

 may not multiply at all in that medium, leaving us, as to the 

 productiveness of that species, entirely in the dark. So, too, 

 does uncertainty follow the study generally of these organisms. 

 Some species will not live in air or in oxygen. Cultures under 

 observation are very liable to become infected with atmospheric 

 germs. The vessel used may not have been sterilized, the hand 

 the knife, anything employed in manipulating during the pro- 

 cess of cultivation, may convey outside bacteria to the culture, 

 and render the experiment nugatory, or void of satisfactory 

 results. 



The bacteria found in drinking-water may or may not be 

 harmless to man. They may be very abundant, yet not dele- 

 terious, or they may be few in number and may consist, in part. 



