101 



glass until it adheres completely, and this it must do all around 

 before we attempt to add the outer coat of varnish. I have often 

 dispensed entirely with this extra coat, though, if the varnish be 

 at all brittle, it will be necessary. 



E. The process of mounting in fluid is very similar to the last 

 described, except that the Diatoms are placed in the cell, and the 

 fluid dropped on them until it rises slightly above the edge of the 

 cell, to which it will adhere by capillary attraction. The cover 

 has then a line of fluid drawn across it with a glass rod, and one 

 side, on which the line terminates, placed down first. It is then 

 allowed to fall slowly, driving a wave of fluid before it, as in the 

 case of balsam mounting. The line of fluid attracts the mass of 

 it, and no bubbles are allowed to enter. If any air should get 

 into the cell, either from evaporation or carelessness, the cover 

 will have to be removed, and the operation commenced over 

 again. After the cover is placed on the ring, the exuded fluid is 

 wiped off, and a ring of varnish placed over the edge of the glass. 

 This part of the operation is explained in the handbooks, so I 

 will not repeat it here. Mr. Farrant's medium, which I have 

 mentioned above, does not require any cell, and in many cases no 

 exterior ring of varnish, as it becomes almost solid, and retains 

 the cover in its place. Many species will require all of these 

 modes of mounting to show all of their characters, but some will 

 require to be mounted only in balsam and in the dry way. Some 

 species are ruined by balsam, while others are improved, there- 

 fore difierent modes should be tried, and that which seems to give 

 the best results adopted. 



Diatoms may be kept in quantity in alcohol, dried on mica 

 (which is the best), or dried on mica after boiling in acid. These 

 last are sometimes difficult to remove from the mica. Carbonate 

 of soda and water, or even acid, will often have to be used. Dia- 

 toms kept on mica have the advantage of being always ready to 

 be mounted by any of the above methods, and they may also be 

 sent through the post, as they are not bulky, nor do they weigh 

 anything considerable. 



When the gatherings are small, they may be kept, after they 

 are cleaned, in small bottles similar to those mentioned above, but 

 when in any quantity, they will require to be kept in two or four 

 ounce vials ; and, so that we may remove any quantity for mount- 



