INTRODUCTORY CHAPTER. 11 



are mounted in the dry way. A solution may also be made by dissolving the pow- 

 dered gum in diluted acetic acid. 



French cement consists of lime and India-rubber, and is very valuable for mount- 

 ing large microscopical preparations. It is made in the following manner : A 

 quantity of India-rubber scraps is carefully melted over a clear fire in a covered iron 

 pot, and not suffered to catch fire. When the mass is quite fluid, lime in a per- 

 fectly fine powder, having been slaked by exposure to the air, is added in small 

 quantities at a time, the mixture being kept well stirred. When moderately thick 

 it is removed from the fire, and is then well beaten in a mortar and moulded in the 

 hands until it has the consistence of putty. It may be colored with vermilion or 

 any other coloring matter. The principal advantages of this cement are that it 

 never becomes perfectly hard, thus permitting considerable alterations to take place 

 in the fluid contained in a cell without permitting the entrance of air, and it also 

 adheres very firmly to the glass, though the surface of the latter should be smooth. 



The cements which have been described are employed for forming cells, fixing 

 glass cells upon the glass slide, cementing the cover upon the prepared object 

 after it has been placed in the cell, and for other microscopical purposes. 



Preserving Fluids. Objects which would lose their peculiarities by drying, can 

 only be preserved in anything like their original condition by moistening them in 

 fluid and the choice of the fluid in each case will depend not only upon the 

 character of the object, but also on the purpose sought in its preservation. 



For the preservation of minute vegetable forms, and also of a great number of 

 animal substances, Dr. Beale recommends the following preparation : Mix 3 

 drachms of creasote with 6 ounces of wood naphtha, and add in a mortar as much 

 prepared chalk as may be necessary to form a thick smooth paste. Water must 

 be gradually added to the extent of 64 ounces, a few lumps of camphor are then 

 thrown in, and the mixture allowed to stand in a lightly covered receptacle for two 

 or three weeks, with occasional stirring ; after which it should be filtered, and pre- 

 served in well-stopped vessels for use. 



Of late years glycerine has been much used as a preservative fluid, as it allows 

 the colors of vegetable objects to be restored. The best preparation of the kind 

 is made by mixing one part of glycerine to two parts of camphor-water. Deane's 

 Gelatine is one of the most convenient fluids for preserving the ' larger forms of 

 confervas and other microscopic algae. This is prepared by soaking 1 ounce of 

 gelatine in 4 ounces of water, until the gelatine is quite soft, and then adding 5 

 ounces of honey, previously raised to boiling heat in another vessel ; the whole 

 is then to be made boiling hot, and when it is somewhat cooled, but is still per- 

 fectly fluid, 6 drops of creasote and | an ounce of alcohol, previously mixed to- 

 gether, are to be added, and the whole is then to be filtered through fine flannel. 

 When required for use the mixture must be slightly warmed, and a drop placed 

 upon the preparation on the glass slide, which should also be warmed a little. 

 Next, the glass cover, having been breathed upon, should be carefully laid on, and 

 the edges covered with a coating of asphaltum cement. 



A mixture of gelatine and glycerine, in equal parts, has been used for the 

 same purpose. A solution of chloride of calcium in three parts of water, is also 

 employed by vegetable microscopists. This last solution has the disadvantage of 

 not preserving color, but the advantage that it does not dry up in the cell. 



For the preservation of animal tissues, a mixture of one part of alcohol to five 

 parts of water answers tolerably well. 



