5IO PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS 



appearance of a section in this stage under the microscope, it is to 

 be remembered that its transparence will subsequently be consider- 

 ably increased by mounting in Canada balsam : this is particularly 

 the case with fossils to which a deep hue has been given by the 

 infiltration of some colouring matter, and with any substances 

 whose particles have a molecular aggregation that is rather amor- 

 phous than crystalline. When a sufficient thinness has been attained 

 the section may generally be mounted in Canada balsam ; and the 

 mode in which this must be managed will be detailed hereafter. 



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

 made of structures in which (as in corals) hard and soft parts are 

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

 substance are first to be stained thoroughly and are then to be 

 ' dehydrated ' by alcohol. A thin solution of copal in chloroform is 

 to be prepared, 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 chloroform ; when, after 

 being well washed in water, they should be again stained, and 

 mounted either in weak spirit or (after having been dehydrated) in 

 Canada balsam. 1 



A different mode of procedure, however, mu *; be adopted when 

 it is desired to obtain sections of bone, tooth, or other finely tubular 

 structures, impenetrated by Canada balsam. If tolerably thin sec- 

 tions 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 convenient 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 afterwards be completely rubbed away in 

 order to bring into view a stratum which the Canada balsam shall 

 not have penetrated. As none but substances 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 secticms nearly to the 

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



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

 the kindness of Mr. H. N. Moseley) some sections of corals prepared by this process, 

 can testify to its complete success. 



