ON FRESH WATEB A.LG I 05 



the solid coating which they form around the specimens constitutes 

 the best known protection. Except in ease of the Diatoms, how- 

 ever, these substances so shrivel and distort the fresh water i 

 immersed in them as to utterly ruin them. I lost so many 

 specimens by the old ways of mounting, that, becoming dis- 

 heartened, I gave up all idea of making a permanent cabinet, 

 until a new cement, invented by Dr. J. G. Hunt, of this city, wafl 

 brought to my notice. This is prepared as follows ■ — 



Take damar gum, any quantity, and dissolve it in benzole ; the 

 solution may be hastened by heat. After obtaining a solution just 

 thick enough to drop readily from the brush, add enough of the 

 finest dry oxide of zinc, previously triturated in a mortar with a 

 small quantity of benzole, until the solution becomes white when 

 thoroughly stirred. If not too much zinc has been added, the 

 solution will drop quickly from the brush, flow readily, and dry 

 quickly enough for convenient work. It will adhere, if worked 

 properly, when the cell-cover is pressed down, even when glycerine 

 is used for the preservative medium. Keep in an alcohol- lamp 

 bottle with a tight lid, and secure the brush for applying the cement 

 in the lid of the bottle. 



Its advantages lie in the circumstance that the glass cover can 

 be placed upon the ring of it whilst still fresh and soft, and that 

 in drying it adheres to both cover and slide, so as to form a joint 

 between them of the width of the ring of cement, and not, as with 

 asphaltum, gold size, &c, simply at the edge and upon the outside 

 of the cover. It is readily to be seen how much less liability to 

 leakage must result from this. The method of mounting with it 

 is as follows : — A ring of any desired size is made by means of an 

 ordinary Bhadbolt's turn-table, upon a slide, which is then placed 

 to one side to dry. When required for use, the specimen, cover, 

 &c, being all prepared and ready, the slide is again placed upon 

 the turn-table, and a new ring of cement put directly upon the old 

 one. The specimen is immediately placed within the cell thus 

 formed, and the requisite quantity of the carbolated water placed 

 upon it. The cover, which must be large enough to entirely or 

 nearly cover the cement ring, is now picked up with the forceps, 

 the under side being moistened by the breath to prevent adhesion 

 of air-bubbles, and placed carefully in position. It is now to be 

 carefully and equably pressed down with some force. By this any 

 superfluous water is squeezed out, and the cover is forced down into 

 the cement which rises as a little ring around its edge. The pres- 

 sure is best made with a stiff needle, at first on the centre, and 

 then upon the edges of the cover, which may finally be rnade slowly 

 to revolve underneath the needle point. The slide may then be put 

 aside to dry ; or better, an outside ring of the cement thrown over 

 its edge in the usual manner. Where a deep cell is required, 

 several coats of the cement should be placed one over the other, 

 each being allowed to dry in turn. If time be an object, and only 



