Bac'dlarieoi 271 



analogous to the essential oils, and the foulness could be prevented 

 by storing water in the dark. It has been suggested that the 

 immense beds of petroleum which exist in certain regions have 

 had their origin from the oil secreted in the protoplasm of 

 Diatoms 1 . 



Miquel has made many interesting experiments on the cultiva- 

 tion of Diatoms, and he finds that, although they thrive in water 

 at freezing point, they cannot withstand being frozen. Their 

 vitality is destroyed at 15 C., and a temperature above 45 C. is 

 rapidly fatal. Desiccation is also fatal. The yellow rays of light 

 are the most favourable for cultivation, so that cultures should be 

 placed under yellow glass. 



Large numbers of fossil Diatoms are known. Not only are 

 these minute plants actively engaged at the present time in 

 forming oceanic and lake deposits, but the numerous Diatumaceous 

 Earths are a proof of their activity in former ages. These earths 

 are of a white or grey colour, often so soft and friable as to crumble 

 readily between the fingers, and they are composed almost entirely 

 of the siliceous valves of Diatoms. They may have had a marine 

 or a freshwater origin, and most of the forms contained in the 

 deposits belong to genera, and many of them to species, now living. 

 The deposits have been formed in past times at various periods of 

 the earth's history, but they appear to be principally associated 

 with rocks belonging to the Tertiary formations. Some of them 

 are of economic importance, being used as polishing powders 

 ("Tripoli"), as non-conducting materials, as absorbents for nitro- 

 glycerin in the manufacture of dynamite (" Kieselguhr "), as a 

 dentifrice, and for other purposes. Although many species usually 

 occur in deposits of this nature, one is usually predominant, form- 

 ing the great mass of the material. In some parts of the world, 

 such as in China, Japan, Siberia, Lapland, and other countries, 

 certain earthy deposits of a diatomaceous origin are mixed with 

 meal to make a kind of flour. The best known deposits in the 

 British Islands are those at Dolgelly in Wales, and at Toome 

 Bridge in Antrim, Ireland. The deposit at Biln, in Bohemia, 

 which is about 14 ft. in thickness, was estimated by Ehrenberg to 

 contain some 40,000,000 of the frustules of Diatoms in every cubic 



1 Kramer & Spiller in Berichte Deutsch. Ckern. Gesellscb. xxxii, 1899. 



