NATURAL HISTORY OF THE DIATOMACE^E. 499 



divide the clean gathering, consisting of a white sediment of large and 

 small diatoms along with fine sand, all mixed up together into densi- 

 ties. Of course, if some of this sediment were to be mounted in this 

 condition, extremely unsightly slides would be procured ; so it is best to 

 separate the finer from the coarser diatoms, and these in turn from the 

 sand. This is accomplished by what is known as elutriation, or, separat- 

 ing into densities after the manner already described. Then slides may 

 be mounted from each of the densities in the following manner. A slide 

 is thoroughly cleaned, and a good sized drop of water placed upon the 

 centre portion. A little of the diatom sediment is then taken up in a 

 dip-tube, and the point of the tube brought just into contact with the 

 drop. As soon as a few diatoms have run out of the dip-tube, it is 

 removed. Then a small splinter of wood or stiff bristle is used to dis- 

 seminate the diatoms through the drop of water in such a way that they 

 will be pretty evenly distributed and not overlie each other. The water 

 is then driven off by heat, a drop of thin Canada balsam placed upon the 

 dry diatoms, and a cover placed on them in the usual manner. In many 

 cases, especially when dealing with the smaller forms, it will be found 

 desirable to mount them upon the cover in this same way, instead of upon 

 the slide, as they will then be brought as near as possible to the objective 

 of the microscope. Single or remarkable specimens of diatoms may be 

 picked out and mounted by themselves ; but the manner of accomplishing 

 this would occupy more space than it has been thought desirable to 

 devote to this portion of our subject, and the reader is referred to 

 the books on mounting microscopic objects for the particulars of the 

 process. 



The main principles of preparing and mounting diatomaceae for preser- 

 vation and study have been given, and the intending student will be able 

 to devise modifications and improvements for himself, so that he will be 

 able to put up specimens in as finished a manner as any to be procured 

 from the dealers. 



