OF MICROSCOPIC OBJECTS. 41 



case by experiment and let the covers to be mounted be cleaned 

 and laid upon the brass plate. (See Chapter I.) By means of a 

 glass tube, about one-twelfth of an inch in diameter, stopped by 

 the wetted finger at the upper end, take up as much of the diluted 

 material as will form a moderately convex drop extending over 

 the whole cover. When all the covers required are thus prepared, 

 apply a lamp below the brass plate, and raise the temperature to 

 a point just short of boiling. By this means the covers will be 

 dried in a few minutes, and the specimens equally distributed 

 over the whole area. The spread of the fluid upon the covers is 

 facilitated by breathing upon them ; and, to insure uniformity, 

 care must be taken to avoid shaking them whilst drying. The 

 best plan is to mount at least half a dozen at once. 



Before mounting, Mr. Rylands always burns the diatoms upon 

 the glass at a dull red heat, whether they are used with balsam or 

 dry. This burning, he says, is not only an additional cleaning 

 process, but it effectually fixes the diatoms, and prevents them 

 floating out if mounted with balsam. The thinnest covers may be 

 burnt without damage if they are placed upon a small piece of 

 platinum foil of the size required, which should be about one- 

 hundredth of an inch thick, perfectly flat, and having three of its 

 edges slightly bent over, so as to prevent its warping with the 

 heat. The small flame of a spirit-lamp, or, where there is gas, a 

 Bunsen's burner, may be employed. The cover should be shaded 

 from direct daylight, that the action of the flame may be observed 

 more perfectly. Care must then be taken to raise the tempera- 

 ture only to the dull red heat before mentioned. The cover will 

 then be in a fit state for mounting as required. 



It has been stated in another place that it is assumed the opera- 

 tor is not mounting diatoms simply as microscopic objects, but as 

 instructive specimens. It is not, therefore, sufficient to take a 

 single slide as all that is required, but to have the same diatom 

 prepared in as many ways as possible. The following are the 

 principal : 



1. Mounted crude in fluid (see Chapter IV.). 



2. Burnt crude upon the cover, and mounted dry or in bal- 



sam (as before mentioned). 



