200 THE MICROSCOPE AND ITS REVELATIONS. 



195. By a slight variation in the foregoing process, sections may be 

 made of structures, in which (as in Corals) hard and soft parts are com- 

 bined, so as to show both to advantage. Small pieces of the substance 

 are first to be stained thoroughly ( 202), and are then to be t dehydrated ' 

 by alcohol ( 190). A thin solution of copal in chloroform is to be pre- 

 pared, in which the pieces are to be immersed; and this solution is to be 

 concentrated by slow evaporation, until it can be drawn out in threads 

 which become brittle on cooling. The pieces are then to be taken out, 

 and laid aside to harden; and when the copal has become so firm that 

 the edge of the finger-nail makes no impression, they are to be cut into 

 slices, and ground down attached to glass, in the manner already 

 described, the sections being finally mounted in Canada balsam. The 

 sections (attached to glass) may* be partially or completely decalcified, the 

 soft parts remaining in situ, by first dissolving out the copal with chloro- 

 form; when, after being well washed in water, they should be again 

 stained, and mounted either in weak spirit, or (after having been dehy- 

 drated) in Canada balsam. 1 



196. A different mode of procedure, however, must be adopted when 

 it is desired to obtain sections of Bone, Tooth or other finely tubular 

 structures, ^penetrated by Canada balsam. If tolerably thin sections 

 of- them can be cut in the first instance, or if they are of a size and shape 

 to be held in the hand whilst they are being roughly ground down, there 

 will be no occasion to attach them to glass at all: it is frequently conveni- 

 ent to do this at first, however, for the purpose of obtaining a ' hold ' upon 

 the specimen; but the surface which has been thus attached must after- 

 wards be completely rubbed away, in order to bring into view a stratum 

 which the Canada balsam shall not have penetrated. As none but sub- 

 stances possessing considerable toughness, such as Bones and Teeth, can 

 be treated in this manner, and as these are the substances which are most 

 quickly reduced by a coarse file, and are least liable to be injured by its 

 action, it will be generally found possible to reduce the sections nearly to 

 the required thinness, by laying them upon a piece of soft cork or wood 

 held in a vice, and operating upon them first with a coarser and then with a 

 finer file. When this cannot safely be carried farther, the section must be 

 rubbed down upon that one of the fine stones already mentioned ( 193) 

 which is found best to suit it: as long as the section is tolerably thick, 

 the finger may be used to press and move it; but as soon as the finger 

 itself begins to come into contact with the stone, it must be guarded 

 by a flat slice of cork, or by a piece of gutta-percha, a little larger than 

 the object. Under either of these, the section may be rubbed down to 

 the desired thinness; but even the most careful working on the finest- 

 grained stone will leave its surface covered with scratches, which not 

 only detract from its appearance, but prevent the details of its internal 

 structure from being as readily made out as they can be in a polished 

 section. This polish may be imparted by rubbing the section with 

 putty-powder (peroxide of tin) and water upon a leather strap, made by 

 covering the surface of a board with buff-leather, having three or four 

 thicknesses of cloth, flannel, or soft leather beneath it: this operation 

 must be performed on both sides of the section, until all the marks of 

 the scratches left by the stone shall have been rubbed out; when the 



1 See Koch in " Zoologischer Anzeig.," Bd. i., p. 36. The Author, having seen 

 (by the kindness of Mr. H. N. Mosely) some sections of Corals prepared by this 

 process, can testify to its complete success. 



