70 DIATOMS. 



it. It is certain that it is composed of more than one 

 layer, and in some species of at least three layers, dif- 

 fering from each other in thickness and in marking. 

 The subject is a little too complicated to deal with in 

 this paper, and as no one knows absolutely much about 

 it, it would be unprofitable to give the views of eminent 

 microscopists who fail to agree among themselves. Dia- 

 toms are all small— the smallest about the size of the 

 human blood corpuscle, which measures ^ inch in di- 

 ameter. 



The largest I have ever read of, larger than any I have 

 ever seen, measures ^V of an inch in diameter or 100 

 times the diameter and 10,000 times the area of the 

 smallest. (The head of a pin measures about yV of an 

 inch.) 



They are as widely distributed as water itself and 

 may be found wherever it is a little wet. I don't mean 

 that you will always find them wherever it is wet ; but 

 if you have a combination of sunlight and water, it is 

 possible for them to grow. They love the sun, these 

 little plants, and it is hardly worth while to look for 

 them in dense shade. They grow in clear water and in 

 stagnant ; in salt water and in fresh water ; in cold wa- 

 ter and in hot. 



They have been found in thermal springs so hot that 

 no other living thing could exist there, and they swarm 

 in the waters of the polar seas, so that, washed up by 

 the waves, they frequently discolor the ice pack for miles. 



They have been found on a brick wall which was kept 

 constantly damp by the exhaust pipe of an engine and 

 in the moist climate of England have been found grow- 

 ing on the trunks of trees. A definition of a diatom 

 then would be something like this: A minute water 

 plant consisting of a single cell with a coating of silex 

 more or less elaborately sculptured. They belong to the 

 sub class algse which includes the sea weeds and the 



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