226 How to prepare Specimens of Diatomacem for 



impart at the present time, he will draw upon it pretty freely, 

 supplementing it to such a degree as later investigations warrant, 

 or as may seem desirable. 



Although most of the published treatises on the use of the 

 microscope in general profess to give directions for mounting objects 

 in such a 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 Diatomacefe. In books, generally, when the preparation of these 

 organisms is treated of, it is usually the fossil deposits which are con- 

 sidered, 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 Diatomaceae, 

 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 properly a considerable amount of knowledge of their 

 natural bistory is plainly necessary. 



The Diatomaceae 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 descrip- 

 tion can seldom be used for the purpose of exhibiting such charac- 

 ters, 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. l)ut 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 forms occurring at a particular station. Then, again, 

 the fossil as well as the semi-fossil deposits and guanos may be 

 cleaned and mounted as obtained ; but even then it may become 



