SHIELDS OF ANIMALCULES. 143 



to which they belonged. To them must be traced 

 the polishing slate,* or tripoli of Bilin, so frequently 

 used in some of the arts. The size of a single one of 

 the infusoria, which forms this substance, amounts 

 on an average, and in the greater part, to one two 

 hundred and eighty-eighth of a line, which equals 

 one-sixth of the thickness of the human hair, 

 reckoning its average size at one forty-eighth of a 

 line. The globule of the human blood, considered 

 at one three hundredth, is not much smaller. The 

 blood globules of a frog are twice as large as one of 

 these animalcules. As the slate of Bilin is without 

 cavities, these animalcules lie closely compressed. 

 In round numbers, about 23,000,000 of animals 

 would make up a cubic line, and would in fact be 

 contained in it. There are 1,728 cubic lines in a 

 cubic inch ; and therefore a cubic inch would con- 

 tain, on an average, about 41,000,000,000 of these 

 animals. On weighing a cubic inch of this mass, 

 I found it, says an observer, to be about 220 grains. 

 Of the 41,000,000,000 of animals, 187,000,000 go 

 to a grain ; or the siliceous shield of each ani- 

 malcule weighs about one one hundred and eighty- 

 seven millionth part of a grain. 



The white calcareous earth, so common at the 

 bottoms of bogs and morasses, has its origin in the 

 ceaseless labours of some animalcules ; and the 

 "bog-iron ore" consists of the ferruginous shields 

 of others. The chalk-beds of England are many 

 * Polirschiefer. 



