USE OF CHEMICAL EEAGENTS. 211 



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, Tinder either of these, the section 

 may be rubbed down until it has been reduced to the requisite 

 degree of tenuity ; 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 de- 

 tails 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 per- 

 formed on both sides of tbe section, until all the marks of the 

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

 specimen will be fit for mounting, after having been carefully 

 cleansed from any adhering particles of putty powder. 



112. Chemical Actions. One important part of the preparation 

 of Microscopic objects, is often effected by the use of Chemical 

 Reagents. These may be employed, either for the sake of re- 

 moving substances of which it is desired to get rid, in order to 

 bring something else into view ; or for the sake of detecting the 

 presence of particular substances in the object under examina- 

 tion. Thus, in order to obtain the animal basis of Shell, Bone, 

 Tooth, &c., it is necessary to dissolve away the calcareous portion 

 of these tissues by the use of acids ; a mixture of nitric and 

 muriatic acids is preferable ; and this should be added, little by 

 little, to a considerable bulk of water, until a disengagement of 

 gas be perceived*to commence from the surface of the specimen. 

 Care should always be taken not to hurry the process by adding 

 too much acid, since, when the animal membrane is of very deli- 

 cate consistence, it is liable to be dissolved ; and in some cases 

 it is better to allow the action to go on for many weeks, adding 

 only a drop or two of acid at a time. When siliceous particles 

 are to be removed (such as those which form the loricce of the 

 Diatomacese), for the sake of leaving the organic membrane in a 

 state adapted to separate examination, hydrofluoric acid must be 

 employed as the menstruum. It is sometimes necessary, on the 

 other hand, to get rid of the organic matter, for the sake of ob- 

 taining the mineral particles in a separate state, as in the case of 

 the spicules of Sponges, Gorgonise, &c., this may be done either 

 by incineration, or (which is generally preferable) by boiling or 

 macerating for a long time in a solution of caustic potash. In 

 separating from Guano, again, the siliceous skeletons of Diato- 

 naaceee, &c., which it may contain, muriatic and nitric acids are 

 largely used, to dissolve away every part of the mass on which 

 they will act ; the microscopic organisms for which search is 

 made, being contained in a few grains of sediment which are 

 left when a pound of pure guano is thus treated. 



