420 INTRODUCTION TO CRTPTOGAMIC BOTANY. 



Cryptogams preserved in fluid ready for use ; and many Algae 

 suffer so much from drying, tliat fresh specimens preserved in 

 fluid are in many cases ahnost indispensable. Many media 

 have been proposed, as solutions of creasote, glycerine, and 

 other matters secured from evaporation by a quickly drying 

 cement, and one which will not penetrate the cell in which 

 the specimen is contained. The method adopted by Mr. 

 Thwaites, and described in the Annals of Nat. Hist., vol. xv., 

 p. 104, was not perfect in this respect, though I have a large 

 number of specimens prepared by him, which have stood the 

 test of some years. Whatever the fluid may be which is pre- 

 ferred, a cell must be prepared with a mixture of gold size 

 and japan- black, rubbed down with lamp-black and htharge, 

 lamp-black, or marine glue, in the centre of an oblong slip of 

 glass, taking care that it is perfectly dry before it is used. 

 This is then filled with the preserving fluid, and the specimen 

 nicely placed in it, perfectly free from air bubbles. A piece of 

 microscopic glass is then placed over the cells, and the edges 

 cemented securely with the same mixture. The preparation 

 of good specimens is a work of much nicety, and can only be 

 carried on successfully by those who are prepared to meet 

 difficulties from their own resources. A full account of the 

 necessary manipulation will be found in Queckett's Practical 

 Treatise on the Microscope, to which, for want of room, I must 

 refer for further information. Such methods, it may be ob- 

 served, are equally available for many objects belonging to 

 vascular Cryptogams. See also Ann. of Nat. Hist., vol. xxi., 

 p. 351. 



