378 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1916. 



Only one practical use has been developed from this esthetic study 

 of the diatoms ; they have been long recognized as the most accurate 

 and satisfactory test objects for determining the perfection of micro- 

 scopic lenses and accessories, the ability of any microscope to render 

 visible the fine lace ornamentation which overspreads some of the 

 species being the best index of its optical excellence. As a conse- 

 quence of this, all microscopes are to-day tested vs^ith one or both of 

 two species of diatoms, Pleurosigma angulatuTn^ or AmpMpleura 

 2}elliicida. 



One of the oldest of the economic uses of diatoms has been that 

 of employing fossil diatom earth as a polishing powder, especially 

 in metal work. These organisms appeared geologically about the 

 middle of the Cretaceous period, and although, therefore, among 

 the later of the now existing forms of plant life, their prolific multi- 

 plication has resulted, during former periods of time, in the forma- 

 tion of enormous fossil beds composed of the silica remains of these 

 minute aquatic plants. Such beds are found all over the world, 

 famous deposits being located at Luneburg, Grermany; Bilin, Bo- 

 hemia; Sendai, Japan; Ananino, Russia; Oamaru, New Zealand; 

 Moron, Spain; Keene, N. H. ; Nottingham, Md. ; the coast of Cali- 

 fornia, and many other places of minor importance. The first con- 

 siderable fossil deposit of diatomaceous earth used was confused 

 with a polishing material called " rottenstone," mined at Tripoli, in 

 Africa, and it was therefore referred to in commerce by the same 

 name, " Tripoli powder," and is in fact so sold in drug stores at the 

 present time. Its high abrasive value comes from the fact that tlie 

 material, silica, has a high degree of hardness and the grain of the 

 diatom powder is so fine as to produce as a polish the highest luster. 

 Its extreme fineness of texture is shown by a computation made by 

 Ehrenberg, that in 1 cubic inch of the Bilin diatom earth there are 

 40,000,000 individuals. : <,!l,i -.lu/ . ; a- 



This abrasive quality of the diatoms has led to their use for other 

 purposes than metal polishing, as for example, for tooth powder. 

 One of the widely advertised tooth powder preparations upon the 

 market is composed entirely of diatomaceous earth. It can not be 

 said that this is a good material for the purpose, as the cutting 

 quality of this siliceous substance is too great to be used constantl}^ 

 upon the thin layer of enamel of the teeth. It is, however, interest- 

 ing to think that many of the users of this diatom dentifrice would 

 be amazed if they could see the thousands of exquisite gem-like organ- 

 isms lying upon their tooth brush and used as a toilet preparation. 



As a curious instance of perverted use, it might be well here to 

 mention the fact that diatomaceous earth was at one time extensively 

 eaten by the impoverished and half-starved tribes inhabiting the 

 remoter portions of eastern Europe and Asia. Generally the diatom 



