180 THE MICROSCOPE 



about a quarter to half-an-inch square, and a large quantity of 

 fluid used ; a large, wide-mouthed, stoppered bottle, holding from 

 six to ten ounces, according to the quantity of material, is best. 

 Change the fluid at the end of twenty-four hours, and again every 

 third day, and the material will be hardened in from eight to 

 twelve days, as can be easily proved by taking out a piece and 

 feeling it. If not hard enough, they feel elastic like India rubber 

 when slightly pressed between the thumb and finger. If allowed 

 to remain too long, it gets brittle. When it is found to be mode- 

 rately hard, usually after about eight or ten days, pour off the 

 chromic acid mixture, and wash well ; replace it by dilute spirit, 

 made thus : — Methylated spirit, 2 parts ; and water, i part. Let 

 the material remain in this for from twenty-four to thirty-six hours, 

 never longer than three days, and then replace it by pure methy- 

 lated spirit. It may remain in this for an indefinite time, but it 

 will often be found that the spirit becomes cloudy and full of 

 deposits in a few days. In this case, it is only necessary to 

 change the spirit until it remains clear. In some cases a i — 6th 

 solution of chromic acid may be used without the spirit with 

 advantage. In other cases, it may be necessary to use a solution 

 much weaker, as a i — loth per cent. 



Chromic and Bichromate Solution.— Dissolve i gramme 

 chromic acid and 2 grammes potassium bichromate in 1,200 cc. 

 water. 



Chromic and Nitric Fluid. — Chromic acid, i gramme; water, 

 200 cc. ; then add slowly 2 cc. nitric acid. 



Chromic and Osmic Acids. — A mixture of chromic acid with a 

 few drops of osmic acid is often very useful, as it combines the 

 advantages of both reagents. Since I used the above, Dr. Max 

 Flesch has brought out a modification, which is as follows : — 

 Osmic acid, o-io ; chromic acid, 0*25; distilled water, 100 

 parts. Mix. It answers particularly well for the auditory organs 

 of smaller animals, many details of structure of the cochlea 

 coming out with quite diagrammatic clearness. The hairs of the 

 air-cells are, however, mostly lost. It answers well for examina- 

 tion of the growth of bone in the epiphyses of small animals, and 

 for general views of the retina, conjunctiva, cornea, and eyelids. In 



