ZOOLOCxY AND BOTANY, MICROSCOPY, ETC. 507 



them until wanted. The covers are put into a small vessel, such as a 

 porcelain crucible, that contains two or three drops of glacial acetic acid, 

 dropping in each cover separately to see that the acid wets it all over. 

 Water is added and poured off two or three times, and then the covers 

 are removed one at a time and dried and polished likethe slips 



" The covers that are to be used are placed on a suitable level surface, 

 such as a piece of plate glass ; if the prepared diatoms are in alcohol 

 this is poured off and replaced by distilled water and a drop of the liquid 

 is put upon each cover, seeing that it spreads suitably over the cover. 

 A bell-glass is put over them, and they are left to dry spontaneously. 

 Each cover is then examined all over its surface under a suitable low 

 power, such as an inch, and hairs and other obnoxious things are re- 

 moved, using either a, bristle or a needle. 



" They are now ready for the actual mounting. For this purpose I 

 prefer a tray made of thin sheet copper with two sides and one end turned 

 up about £ in., and of a size to just take a 3 x 1 slip easily. This is 

 supported across one corner of the triangular top of the usual iron tripod 

 as used in chemical laboratories, and by its side is placed a sheet of thick 

 sheet copper about 4 in. x 3 in. A stout glass slip is put in the tray, 

 and a Bunsen burner arranged so that a flame about f in. high may be 

 brought under the tray, the top of the burner being about 1| in. below 

 the tray. The flame must be small so that the heating may be very 

 gradual. 



" One of the covers with the diatom on it is now finally examined 

 under the Microscope, and any hairs, etc., that may have settled on it 

 are removed ; it is put on aflat piece of platinum foil that has two edges 

 turned up and heated over a very small flame to just below redness. 

 This gets rid of volatile matter, and any minute threads of organic 

 matter are carbonized. The cover is placed, diatoms uppermost, on the 

 slip in the tray, a piece of the medium of suitable size is put on the 

 cover, the cleaned slip that is to be used is put on the plate by the tray, 

 and the small flame as already described is brought under the centre of 

 the tray. I place a rather large funnel over the tray and resting on the 

 plate to prevent dust falling on the cover. In from 20 to 30 minutes 

 the little lump of medium will have assumed a cushion shape, or it might 

 be called spherical, except for the flat side upon which it rests on the 

 cover. The heating must not be hurried or the medium will get too 

 hot, and a good crop of small bubbles is likely to result. The assumption 

 of this shape indicates that it is thoroughly softened. The cleaned slip 

 is now removed from the plate, where it has got hot, dusted lightly with 

 a soft brush in case anything may have fallen upon it, and gradually 

 brought with its clean side downwards upon the softened drop of medium, 

 and by means of the forceps it is pressed, at first gently and afterwards 

 more strongly, down upon the medium, which is thus gradually flattened 

 out. As it becomes thinner the process becomes slower, so that it is 

 likely to be ten minutes or even more before it is thin enough. During 

 the operation the reflections of the window frame or of the lamp, or 

 some other convenient object, are watched from time to time to see that 

 the slip that is being pressed down is parallel to the slip beneath that 

 supports the cover glass, and finally the reflection from the cover-glass 



