242 UTILIZATION OF MINUTE LIFE. 



or cell, having a few green or coloured spots in its 

 interior. These curious beings are very small ; I 

 never measured any, but I find they require to be 

 magnified at least 640 times to be seen at all 

 distinctly. Some authors say they vary from 

 1 -24,000th to 1 -500th of an inch in size, according 

 to the species. In the opinion of Humboldt, the true 

 monad never exceeds 1 -3000th of a line in diameter. 

 He alludes probably to Monas crepusculum, the 

 smallest species. One single drop of water may 

 contain about 500,000,000 monades, a greater 

 number than our earth contains of human in- 

 habitants.* 



They effect their locomotion by means of cilia, 

 fine hair-like processes which cover the whole sur- 

 face of the animalcule's body, and which are con- 

 stantly vibrating, like those which are found on 

 several membranes of our own bodies. Such is the 



* Even in Leuwenhoek's time the excessive number of animal- 

 cules in some waters was noticed with, surprise ; but in his day the 

 microscopes were exeedingly defective. The eminent naturalist 

 Swammerdam, who published the results of his dissections in 

 1660, had to work with very imperfect glasses. Leuwenhoek, who 

 made known his curious and novel discoveries about 1677 (some 

 years before and after), laboured under the same disadvantages. 

 He actually ground his own lenses, in which art he excelled the 

 best opticians of the day. Most of his papers have been published 

 in the English " Philosophical Transactions." In a paper of his 

 published in the "Philosophical Transactions" for 1677, we are 

 struck by the ingenious method he employed to calculate the 

 number of animalcules present in a drop of water. 



