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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



monium -chloride deposit can be removed by simply blowing the 

 breath upon the object so coated. The hydrochloric acid and am- 

 monia used should be of great strength to secure the best results. 

 Small quantities only should be employed so that the bottles can be 

 emptied and dried frequently, as the reagents not only absorb mois- 

 ture but lose their strength in a day or two of use. 



Thin sections. — The preparation of satisfactory sections is not 

 difficult but some experience and care are required to produce uni- 

 formly good results. Without a machine for cutting rock sections 

 the following method gives excellent results. The materials required 

 are, first, a piece of sandstone 8 or 10 inches wide, several inches thick, 



Fig. 2. — Apparatus for preparation of ammonium chloride sublimate. 



and 18 or 20 inches long ; second, a water hone an inch thick and 4 or 

 5 inches long ; and third, a block of wood 2 inches wide, 4 or 5 inches 

 long, and an inch thick. In place of the sandstone a carborundum 

 slab about an inch thick, 8 inches wide and 18 inches long, to be ob- 

 tained from the Carborundum Co., at Niagara Falls, is very durable 

 and more efficient. The wooden block should have its upper edges 

 rounded to fit the hand, while on the lower side an excavation should 

 be made of a size to fit the ordinary glass slip. A carborundum 

 hone of considerable fineness is also quite useful. 



The procedure for sectioning specimens large enough to be handled 

 without difficulty is as follows : With a pair of wire nippers a frag- 



