4 22 



of the modification introduced by Gray, who uses a weak gelatin 

 solution in place of the gum arabic, and, after the sections have ex- 

 panded and are fastened in their proper positions, soaks the slide in 

 a very weak solution of potassium bichromate, which, in the presence 

 of light, renders the gelatin film insoluble in water, and hence capable 

 of resisting aqueous stains. To those desiring accurate preparations, 

 these methods are strongly recommended as preferable to the more 

 rapidly applied and generally used 



Collodion and Clove-Oil Mixture. This is made by adding i 

 part of collodion to 3 parts of clove oil ; the mixture should be made 

 up in small quantities, as it becomes less reliable with age. The slide 

 is lightly painted over with the mixture and the paraffin sections 

 placed in position ; the sections cannot be moved after touching the 

 mixture, hence care must be exercised in their placing. When the 

 slide is full, it is gently warmed until the fumes of the clove oil ap- 

 pear ; meanwhile the paraffin melts and the section sinks down into 

 the film of the mixture, from which the clove oil is driven off, leaving 

 the tissue attached to the slide by the film of collodion alone ; this 

 union is not attacked by any of the aqueous, alcoholic, or other solu- 

 tions ordinarily used. The ability of resisting many fluids, together 

 with its simplicity and rapidity, has long rendered the method a 

 favorite, and, for very many cases, deservedly so, due care being 

 exercised in heating the slide to avoid injury to the tissue. In spite, 

 however, of these considerable advantages, the inability of securing 

 perfect extension of the sections is a shortcoming which for accurate 

 investigations is fatal ; when, therefore, accurate preparations are de- 

 sired, it should be discarded for the gum or the gelatin method. The 

 sections being securely fixed to the slide by one of the foregoing meth- 

 ods, the paraffin on the slide must be removed, as preliminary to 



7. Mounting the sections for preservation. The paraffin is best 

 removed by immersing the slide in benzole or xylol for a few mo- 

 ments and then transferring to turpentine for a short time. The 

 sections having cleared up in these fluids are ready for the applica- 

 tion of the mounting medium, the balsam. The slide is removed 

 from the turpentine, drained, and hastily wiped on the back and 

 edges, care being taken not to touch the sections ; a drop of pure 

 balsam is then placed on the centre of the slide and the latter held 

 for a few moments over a spirit flame to liquefy more thoroughly the 

 balsam, when the cleaned cover-glass, previously caught by the for- 

 ceps and passed for a moment over the spirit flame, is lowered into 

 position ; this manipulation should be executed with steadiness and 

 evenness, avoiding as far as possible the imprisonment of air-bubbles. 

 Should these, however, appear after the cover is in place, they need 



