LABORATORY AND TEACHING METHODS 655 



as fast, as they are cut, should be placed in 50 per cent, alcohol. Where free-hand 

 sections are used they should be placed immediately in 50 per cent, alcohol. 



Crlloidin Method. — It is customary to use two solutions of celloidin, a "thick" 

 and a "thin." The thick solution (about 10 or 12 per cent.) should have the con- 

 sistency of thick syrup. The thin may be made by mixing equal parts of thick 

 and ether alcohol. The material inoculated as described in the preceding lessons is 

 fixed in chrom-acetic acid solution prepared as follows. 



Chrom-acetic Acid Fixative. 



Chromic acid, i gram 

 Glacial acetic acid, i c.c. 

 Water, 98 c.c. 

 Fle.mming's Fluid (Weaker solution). 



[ r per cent, chromic acid, 25 c.c. 



A. { 1 per cent, acetic acid, 10 c.c. 

 [ Water, 55 c.c. 



B. I per cent, osmic acid, 10 c.c. 



Keep the mixture A made up, and add B as the reagent is needed for use, since 

 it does not keep well. 



Wash the fixed material carefully in running water for several hours and put into 

 30 per cent, alcohol, then by successive steps into 50 per cent. 75 per cent., 95 per 

 cent, and absolute alcohol. After dehydrating in absolute alcohol, the succeeding 

 steps are taken. 



1. Ether alcohol, i to 2 days. 



2. Thin celloidin, 2 to 6 days. 



3. Thick celloidin, 3 to 10 days. 



Use of Alcohols and Celloidin. — The celloidin is dissolved in equal parts of ether 

 and absolute alcohol about i part by weight of celloidin to 15 parts of the solvent. 

 After the material is thoroughly penetrated by this solution, it is passed to a stronger 

 solution, containing i part of celloidin to 11 parts of the solvent and finally to a 

 solution containing i part of celloidin to 8 parts of the solvent. After remaining a 

 suitable time in the last solution, the object is ready for imbedding. For this 

 purpose, a paper strip may be wound tightly about the end of a small block of suit- 

 able size and material, so as to form the sides of a box open above, with a bottom 

 the end of the block of wood. This box is now filled with the thickest celloidin 

 solution, and in it the object is placed and oriented carefully by needles wet with the 

 ether-alcohol mixture. As soon as a strong film has developed over the surface of 

 the celloidin, the whole block of material is plunged into 80 per cent. After the 

 celloidin has hardened in the alcohol, the paper ring is removed and the mass is 

 trimmed to the desired size. 



In cutting, the block is clamped in the sliding microtome, where the knife is set 

 obliquely, so that the celloidin sections may be cut with a long drawing stroke. 

 The knife and top of the block should be kept wet with 80 per cent, alcohol, and as 

 rapidly as the sections are cut, they should be placed in the alcohol (Fig. 230). 



The sections are attached to the slide by placing the slide in a closed chamber 



