MOUNTING AND PREPARATION OF OBJECTS. 49 
slide upon which they are to be mounted; and this is 
best accomplished by the use of Mayer’s albumin fixa- 
tive.* A minute drop of this solution is placed on a clean 
slide and rubbed with the finger till only the thinnest 
film remains. The section is then placed upon the slide 
and gently heated till the paraffin just melts. The sec- 
tion will adhere firmly to the slide and the paraffin may 
be dissolved in xylol or any other clearing agent. 
For objects over 10 mm. in diameter paraffin is un- 
suitable, since large blocks split under the knife; for 
such specimens celloidin may be used as an imbed- 
ding medium. This substance is used dissolved in 
ether, and after the evaporation of the latter is hardéned 
by treatment with alcohol or chloroform. The process 
is less simple than the paraffin procedure, and it is not 
possible to cut such thin sections of the objects imbedded. 
The freezing microtome, in which the water in the 
specimen acts at a low temperature, as its imbedding 
substance, is rapid and yields very thin sections. Cell 
structures are, however, somewhat distorted in this proc- 
ess, and its use is confined mainly to the preparation of 
pathological material. 
g. Staining.—One more process in the preparation of 
objects for the microscope still demands reference— 
the process of differential staining. Since the elements 
of a tissue or of a cell differ in chemical composition, it is 
possible to apply certain dyes which shall enter into 
combination with some of them and not with others, 
* 50 cc. of the albumin of hen’s eggs is mixed with 50 cc. of glyc- 
erin and 1 gram of sodium ‘salicylate, shaken well and filtered. 
