1884.] NATURAL SCIENCES OF PHILADELPHIA. 25 



time hardens them somewhat. Any other of the hardening fluids 

 may be used, and for these I can refer the reader to Dr. C. O. 

 Whitman's paper on this subject, which appeared in the Amer- 

 ican NaturalUt^ (vol. xvi, 1882, pp. 697, 772). Chromic acid, 

 however, is the reagent that Prof. Semper always uses, and it 

 seems to answer ever3'^ purpose. 



After the object has been left a sufficient length of time in the 

 fluid, this is poured oflJ" and the vessel filled with water, which 

 should be constantly' changed until there is no yellow color either 

 in tlie object or in the water. In other words, as much of the acid 

 must be withdrawn as possible. This part of the process is 

 considerably shortened by allowing a current of water to flow 

 through the vessel. This stage takes from ten to twenty hours, 

 or even more. 



After this is completed the object is placed in weak alcohol, 

 from 30 to 40 per cent., for at least a day ; when the specimen is 

 quite small, ten or fifteen hours are sufficient. Then the alcohol 

 may be strengthened to 60 or 70 per cent., and the object remain 

 in this for two or thi'ee days (with larger objects, a week). 



The object may now be placed in strong alcohol, from 90 to 

 95 per cent., for about the same length of time as with the 70 

 per cent. It may, indeed, remain here for weeks or months. 

 I have often taken specimens that had been well preserved, 

 after having been for a year in 90 per cent, alcohol, with as 

 good a result as if freshly prepared. 



In cases of dissections where parts have been pinned apart, 

 after passing through the 70 per cent alcohol stage, they may be 

 taken carefully out of the traj's, and the rest of the process gone 

 through with in closely stopped bottles, for they are at this point 

 quite stitf. 



When objects have remained a sufficient length of time in the 

 strong alcohol, they are placed in absolute alcohol. If the strong 

 alcohol be changed once or twice, it will necessaril}'^ save the 

 absolute alcohol to some extent. 



This stage of absolute alcohol is the most critical part of the 

 whole process. Absolutely every particle of the water must be 

 removed, and the secret of the whole success depends on this one 

 point. If an}^ water be left in the tissue, it will become spotted 

 and eventually spoil. I feel positive that those who have tried 

 this method and have failed to produce satisfactory results, have 



