202 THE MICROSCOPE. [October, 



mount will be expedient. Attention is first directed to the mate- 

 rial and apparatus used. 



The glycerine should be pure and free from dust and air bub- 

 bles. To keep it free from these contaminations, devices such as 

 are recommended by Carpenter and Prof. James are excellent. 

 These are bottles, containing the glycerine, and provided with 

 glass tubes, whereby the glycerine is forced out by air-pressure. 



The cements may be of a balsamic nature, but preferably zinc 

 oxide or asphalt. Any cement not affected by the medium may 

 be employed, but experience has proven that the two above named 

 are the best. 



The other essential parts of the completed mount are the slip 

 and cover-glass. No special mention is required concerning these 

 except that they should be pei-fectly clean. To insure this, the 

 practice of leaving them until ready for use in a bath of ordinary 

 battery fluid is recommended. 



In preparing a mount the operations naturally divide them- 

 selves into four divisions, coming under the heads, i. Preparing 

 the cell ; 2. Preparing the section ; 3. Placing section in the cell, 

 and 4. Securing the cover-glass to the cell. 



Under the first head attention is called to three points, viz., 

 thickness of the cement, depth of cell, and age of the cell. 



Upon the consistency of the cement depends in a great measure 

 the formation of a good cell. It should not be thin enough to 

 spread, yet should flow readily and smoothly from the brush. 

 The depth of the cell should be such that a complete support 

 shall be provided for the cover-glass without causing it to bear 

 upon the object when cemented down, and yet should not be of 

 such a depth as to interpose an unnecessary stratum of glycerine 

 between the section and cover-glass. 



Of more importance, perhaps, than any other point, is the direc- 

 tion regarding the age of the cell. It is a common practice to 

 ring a cell and use it w^hile fresh, the manipulator arguing that a 

 more perfect union of cell-wall and cover-glass is secured in this 

 manner. Perhaps this is true : but it is at the expense of the 

 slide's usefulness. An author already quoted is authority for the 

 statement that an ordinary balsam cell will, in drying, shrink 



Under these conditions and in view of the fact that glycerine is 

 non-compressible, something must give way when the cell con- 

 tracts ; and this is either the cover-glass or cell-wall. Whichever 

 it is, the final result is the destruction of the mount and loss of all 

 the work involved in its preparation. This leads us, then, to make 

 the following statement : Never use a '-'- green ''^ cell. The older 

 the cell the better and, at ordinary temperatures, two weeks is the 

 shortest space of time in which a cell of medium depth will 

 become seasoned- 



