MOUNTING AND PREPARATION OF OBJECTS. 49 



slide upon which they are to be mounted; and this is 

 best accomphshed by the use of Mayer's albumin fixa- 

 tive.* A minute drop of this solution is placed on a clean 

 sUde and rubbed with the finger till only the thinnest 

 film remains. The section is then placed upon the shde 

 and gently heated till the paraffin just melts. The sec- 

 tion will adhere firmly to the slide and the paraflSn may 

 be dissolved in xylol or any other clearing agent. 



For objects over lo 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 hardened 

 by treatment with alcohol or chloroform. The process 

 is less simple than the parafiin 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. 



9. 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 I gram of sodium salicylate, shaken well and filtered. 



