NATURAL HISTORY OF THE DIATOMACEyE. 487 



manner as to preserve them for almost any length of time, and at the 

 same time exhibit their characters to the best advantage, and although 

 we have in the English language at least three books treating specially 

 of this subject of the preparation of microscopic objects, yet hardly any 

 one of these volumes gives any concise, practical, and, at the same time, 

 reliable descriptions of the best methods of collecting, preparing, and 

 mounting specimens of diatomaceae. In books, generally, when the 

 preparation of these organisms is treated of, it is usually the fossil 

 deposits which are considered, and even such directions as relate to 

 these are for the most part meagre and unsatisfactory; and, when the 

 specific and special directions are, as is often the case, copied from one 

 book into the other without having been tested by the copyist, any 

 faults they may have possessed, as originally written, are merely repeated 

 and not eliminated. To prepare and mount specimens of diatomacese, 

 for the purpose of sale alone, is one thing, and to prepare and mount 

 them, so as to preserve and exhibit their natural characters and fit them 

 as objects of scientific study, is another and very different thing. The 

 latter can only be attained after considerable practice, and to do it prop- 

 erly a considerable amount of knowledge of their natural history is 

 plainly necessary. 



The diatomacese should always be prepared and put up for a special 

 purpose, that of exhibiting characters peculiar to genera and species ; 

 and to do this those characters must of course be known. Muds, 

 guanos, dredgings, and gatherings of that description can seldom be 

 used for the purpose of exhibiting such characters, and when they can, 

 in exceptional cases, be so employed, it is when the forms they contain 

 are selected out in the manner to be described hereafter. Gatherings, 

 likewise, which contain many species in a mixed condition, should, as a 

 general thing, be rejected unless there be present something of special 

 importance, such as rare species, or some large and fine or distorted 

 forms of common species. But even in such cases it will be found best 

 not to mount the gatherings as collected, but to select out the forms 

 desired and place them upon slides by themselves, and in such media as 

 will exhibit their peculiarities to the best advantage. Of course it may 

 be desirable to study the geographical distribution of the diatomaceae; 

 and then mixed gatherings become of value as exhibiting the number of 



