252 THE AMEEICAN MONTHLY [November, 



croscopists and is useful for the majority of preparations, is made from 

 a mixture of glycerine one part, water two parts, and alcohol three 

 parts, commonly known as the one-two-three mixture. If it is desired 

 to render an object transparent, it may be soaked in this mixture and 

 lightly covered up to protect it from dust, when the alcohol will evap- 

 orate, leaving the glycerine and water in the tissues. 



There is one disagreeable drawback in glycerine as a preservative, 

 and that is, the difficulty of cementing it in securely. It needs great 

 care and scrupulous cleanliness in its use ; but this may, to a great ex- 

 tent, be acquired by employing as frequently as possible the same size 

 of cover-glass, w^hen, by practice, the amount of glycerine which will 

 be sufficient to flow to its edges without extending beyond, may be ac- 

 curately judged. A cement may then be applied which will keep the 

 glycerine in. If any flows beyond the edge it may be taken up with 

 blotting paper, and cleaned away ; but it is difficult to make any 

 cement stick to the slide securely when it has been once wet with 

 glycerine. Several plans have been suggested and adopted to over- 

 come this difficulty ; one of which seems to answer fairly well. It 

 consists in drying up the glycerine as thoroughly as possible, and 

 painting around the edge of the cover a thin layer of glycerine jelly. 

 When this has set, any of the usual cements may be laid over it. A 

 cement to be used for glycerine mounting, and which is said to be very 

 easily applied, and to stand very permanently, is made as follows : Car- 

 bonate of lead, two drachms ; red oxide of lead, two drachms ; litharge, 

 three dradims, to be well mixed and powdered. When wanted for 

 use, mix only enough for the mounting with a little gold size and apply 

 at once. This cement is also spirit proof. There are many plans sug- 

 gested for overcoming this difficulty. The employment of these two 

 cements, and the mixture of gold size and india-rubber solution al- 

 ready given, will be found amply sufficient to meet all demands. 



There are other preservative media, which, next to glycerine, are 

 comparatively easy of employment, such as alcohol and distilled water, 

 camphor water, syrup, etc. 



Alcohol is well known as a preservative, but it has so great a tendency 

 to render delicate tissues opaque by coagulation, that for microscopical 

 mounting purposes it is never used in its full strength. A preservative 

 fluid should interfere as little as possible with the normal characters of 

 a preparation, and should be what histologists designate as an indifler- 

 ent fluid. Some beginners might think that distilled water ought to 

 answer to this character, but it does not, and should only be used for 

 the most transient examination. One may prove this statement by 

 the examination of blood corpuscles in water, and he will soon see 

 them changed into every shape but their normal one. That water 

 may be used for even a casual examination it is necessary that it 

 should be combined with some other substance that will give it 

 a specific gravity equal to the liquids contained in the tissue under ex- 

 amination. For this purpose a weak solution of common salt in water 

 is employed, and answers fairly well. It gives its best eftect if a little 

 gum arabic is dissolved with it. A stock solution of it may be made 

 by dissolving one drachm of common salt in a pint of distilled water, 

 and adding four grains of gum arabic ; this is then always ready for use, 

 should immediate examination be necessary. This forms a very fair 



