BACTERIA. 7 



ual consisting of a single cell. (Fig. 6.) 

 Tliey are often united into chains or masses. 

 A bacterium multiplies by dividing into two 

 individuals, these two individuals again di- 

 viding, and so on in a geometrical progres- 

 sion. A simple calculation will demonstrate 

 how enormous must in a few days be the in- 

 crease if this progressive breaking in two 

 continues unmolested at intervals of an hour 

 or two. Professor Cohn calculates that 

 from one of these minute organisms suffi- 

 cient numbers will have been reproduced in 

 five days to fill full the oceans of the world ! 

 Ordinarily these plants are not more than 

 55V0 of an inch in thickness, while many are 

 much smaller. Indeed, it is highly probable 

 that there are many species so minute that 

 our best microscopes have not yet revealed 

 them. Of the ordinary kinds an aggrega- 

 tion of from one hundred to three hundred 

 placed side by side would not exceed in 

 length the thickness of this paper. Most 

 bacteria, and perhaps all, have the power of 

 moving spontaneously. They whirl, quiver, 

 move slowly and steadily, or perhaps dart 

 rapidly across the field of the microscope. 

 In color they are usually white, although 

 some species possess beautiful tints of red. 



