OF WESTERN PENNSYLVANIA. 9 



Another thin mica square is now selected a little smaller 

 than the first one used and upon it is placed a small chunk of 

 glycerine-jelly, which is melted by holding the square in 

 the forceps over a suitable source of heat the writer holds 

 the square over the electric bulb of his desk light. The 

 glycerine when melted is smeared over the surface of the mica, 

 which is then inverted and quickly but carefully placed on 

 the square on which the dissections are disposed. To prevent 

 the dissected objects from changing their position too much, 

 and to obviate the inclusion of air-bubbles, it is best to lay the 

 square which is to serve as cover so that one edge only comes 

 into contact with the other square and then let the cover 

 settle down gradually, thus driving the air out in front of the 

 gradually advancing line of contact of the mica and mount- 

 ing medium. The slide is now ready for study under the 

 compound microscope and after this it may be placed in the 

 paper pocket along with the specimens from one of which the 

 dissections were made. In order to insure greater perma- 

 nency of the slide, as thus made, some w r orkers advocate seal- 

 ing the slide by running a little ring of Canada balsam around 

 the edg;e of the smaller mica square, thus keeping the air away 

 from the glycerine jelly and preventing any further drying 

 out. The object of placing the dissections in the ten per cent, 

 solution of glycerine is to gradually allow the dissections to 

 accommodate themselves to increasing density of solutions: if 

 the dissections were transferred immediately from pure water 

 to the melted glycerine jelly there would in most cases be 

 much shrinkage and curling, thus spoiling the slide for pur- 

 poses of study. In a few cases even the transferrance from 

 water to ten per cent, solution and thence to the jelly is too 

 great a change and in such cases it is necessary to pass the 

 dissections through a series of solutions of increasing glycerine 

 per cent., up to a strong solution, before using the glycerine- 

 jelly. Another way is to place the dissections in weak 

 glycerine solution and keep adding more solution as the water 

 evaporates from the first, thus gradaully increasing the 

 density. 



