^14 THl AMERiCAI^ MONTHLY [July 



hardens but makes the mass quite transpareut ; and some 

 workers go to the extent of freezing the congealed mass 

 during cutting. I have tried all methods, and recom- 

 mend the student to do the same, selecting the process 

 he finds in his hands the most successful. , If any diffi- 

 culty be experienced in obtaining colloidin, common col 

 lodion will give the same results, and the method of prepa- 

 ration does not differ from that given above. The two ma- 

 terials are almost identical, but the former is more con- 

 venient for use. Let me caution the student against the 

 adoption of methylated spirits. As ordinarily sold, it 

 contains mineral naphtha, which renders it quite unfit for 

 histological work. Kectified spirits is the safest, or alco- 

 hol of 70 per cent. It the imbedded objects are not re- 

 quired for immediate use, the paper can be left on and 

 particulars written thereon with a lead pencil. 



Cutting. — Very good work can be obtained with the 

 Cathcart microtome and a good razor. It is not within 

 the province of this paper to deal with the machine em 

 ployed — it is taken for granted that the student has had 

 experience in this direction. But a microtome must be 

 used that will permit the flooding of the mass with alco- 

 hol, or enable it to be cut under the fluid. The instru- 

 ment mentioned is cheap, costing some $4 without the 

 ether spray, and I have cut many thousands of sections 

 with it. The celloidin block with its imbedded object 

 must be clamped in the vice (which is where the wooden 

 base comes in handy), and trimmed to a square form, but 

 not approaching too close to the specimen itself. A 

 truncated pyramid offers the greatest resistance to the 

 knife, for celloidin is elastic, and is liable to give under 

 the impact of the cutter. It is to overcome this that the 

 mass is sometimes frozen. It is an absolute essential 

 that both the knife audthe mass be kept well wetted with 

 alcohol, and the sections as cut must be floated into saucer 

 of the same. The squared block should be set in the 



