DIATOMS. 117 



' nearly allied, by the fact that they are always of a 

 light pea-green colour, whereas diatoms are usually 

 of a dull olive-brown. Again, diatoms are found in 

 fresh, brackish, and salt water alike, whereas we have 

 seen that the desmids are confined to fresh water. 

 Another most important difference is the fact that 

 diatoms have the power (which desmids have not) of 

 secreting a siliceous or glassy film on their exterior, 

 like that which coats the outside of straw. This glassy 

 film remains perfect after the diatom is dead, and is 

 called the "frustule." It is divided into two parts, 

 like the body and lid of a pill-box, so that the same 

 species has a very different appearance according to 

 the side which is looked at. These glassy cases or 

 frustules are indestructible, and often accumulate to 

 an extraordinary depth. They form a considerable 

 part of the black mud laid bare at low water in tidal 

 rivers or estuaries they compose the greater part 

 of similar material in our ponds and ditches. They 

 will accumulate on the bottom of an old aquarium, 

 where they may always be obtained. A little and 

 patient treatment with hydrochloric, sulphuric, and 

 nitric acids, to get rid of the soluble organic matter, 

 at length displays these glassy sheaths or frustules in 

 all their beauty, adorned with dot and line and curve, 

 in the most extravagant and even luscious style of 

 ornamentation. We have frequently thought that 

 these diatom ornamentations might be studied to 

 some purpose by jewellers and others interested in 



