Sl;PTEMBEE 30, 1904.] 



SCIENCE. 



427 



size, and their possibilities of reproduction. 

 In the terms of the mieroseopist a half 

 micron is about the diameter of the smaller 

 kinds and twice this for the larger ones. 

 That is, from .0005 to .001 mm., or .00002 

 to .00004 inch. It requires 13 sheets of 

 the typewriting paper on which this is 

 written to measure in thickness one mm., 

 therefore there is room across the cut edge 

 of one sheet for about 77 of the larger or 

 144 of the smaller of these bacteria to lie 

 side by side. Of course a single organism 

 is absolutely invisible to the naked eye and 

 can not be seen without more than 100 

 times magnification. To study bacteria 

 effectively the best compound microscope is 

 often taxed to its utmost limits. 



Over and against the diminutive size may 

 be placed the extraordinary powers of mul- 

 tiplication of these minute creatures. In 

 the case of many common species any one 

 of the organisms may become two by a 

 simple process of self-division within thirty 

 minutes of time, when conditions are favor- 

 able. At this rate at similar intervals of 

 a half hour each, two become four, four 

 become eight, eight become sixteen, etc. 

 This continued for 48 half -hours, one day 

 and night, makes the enormous number of 

 281,470,000,000 individuals. This seems 

 incredible, but a little calculation which 

 can be made by any schoolboy will prove 

 the statement. As shown by the census of 

 1900, there were then in Illinois 4,881,450 

 people. The number of bacteria produced 

 in 24 hours from a single organism is, ac- 

 cording to this, 58,327 times as many as 

 the human population of Illinois, and is 

 equal to something like 185 times the niun- 

 ber of people in the whole world. 



We are not to take from this that such 

 a rate of multiplication ever actually con- 

 tinues for any very considerable time, for, 

 small as these single organisms are, there 

 would soon be room for nothing else on the 



earth. This rate of growth and self-divi- 

 sion only occurs while all the conditions 

 are favorable. Now, even if other things 

 so continued, the prodigiously increasing 

 numbers themselves would soon interfere 

 with further advance. There are limita- 

 tions here as there are to the number of 

 people in a house. Even if nothing else 

 prevents, their own excretions or other 

 products may arrest growth. For instance, 

 the souring of milk is for the most part 

 caused by a particular species of bacteria. 

 The individual organisms belonging to this 

 species increase very rapidly in fresh milk 

 at the proper temperature, aiid lactic acid 

 is produced. But the proportion of this 

 acid does not rise above eight tenths of one 

 per cent., for this amount becomes pro- 

 hibitive of further activity on the part of 

 the organism. Its numbers do not further 

 increase unless the acid is more or less 

 neutralized by some alkaline substance. 

 From some such cause our phenomenal 

 numbers are not often found as the product 

 of one bacterium in 24 hours, but the cal- 

 culations do show approximate facts and 

 do help us to explain observed results. One 

 bacterium, perhaps one thirty-thousandth 

 of an inch in diameter, far too small to be 

 seen by the unaided eye, can do compara- 

 tively nothing while several thousands of 

 them in a drop of broth may soon cause its 

 putrefaction. 



So much has been said of bacteria as 

 germs of virulent diseases, as agents of pol- 

 lution, as destroyers generally that we may 

 well think of them as invisible enemies 

 with which we must ever battle to live, or 

 to have anything to live upon. To-day we 

 are to discuss some of the benefits they 

 confer upon us, and we shall see that not 

 all good bacteria are dead bacteria. If 

 they, or some of them, are destroyers and 

 enemies to be feared, they, or some of them. 



