444 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS 



it on the slide to which the object is to be attached, yet it may be 

 preferably hardened en masse by exposing it in a shallow vessel to 

 the prolonged but moderate heat of an oven, until so much of its 

 volatile oil has been driven off that it becomes almost (but not quite) 

 resinous on cooling. If, when a drop is spread out on a glass and 

 allowed to become quite cold, it is found to be so hard as not to be 

 readily indented by the thumb-nail, and yet not so hard as to ' chip/ 

 it is in the best condition to be used for cementing. If too soft, it 

 will require a little more hardening on the slide, to which it should 

 be transferred in the liquid state, being brought to it by the heat of 

 a water-bath ; if too hard it may be dissolved in chloroform or ben- 

 zole for use as a mounting ' medium ; ' we do not recommend its use 

 for mounts with glycerin. 



Brunswick black is a very useful cement, obtainable at the op- 

 tician's as prepared for the use of microscopists. It is one of the best 

 cements for the purpose of ringing mounts, and it may be recom- 

 mended for turning cells. x We have already stated that we do 

 not, as a rule, recommend opaque or black-ground mounting ; but if 

 this is desired or needful no better ' ground ' can be obtained than 

 by putting on the centre of the slide a disc of Brunswick black the 

 size of the outside of the cell or cover-glass, and while it is wet 

 putting a thin cover-glass upon it. The cover-glass becomes quickly 

 fixed, and a pleasant surface is formed to receive the object which it 

 is intended to mount. Should it be desirable to have the floor of the 

 opaque cell dead instead of bright, this can be quickly accomplished 

 with a little emery-powder and water applied to the surface by a 

 flattened block of tin fixed in boxwood. 



Brunswick black is soluble in oil of turpentine, and it dries 

 quickly. 



Glue and honey mixed in equal parts is very valuable for special 

 purposes, and softens with heat. 



Shellac cement is made by keeping small pieces of picked shel- 

 lac in a bottle of rectified spirit, and shaking it from time to time. 

 It cannot be recommended as a substitute for any of the preceding, 

 but it may be employed to put a thin film upon the edge of all 

 mounts however closed and finished that are to be used with homo- 

 geneous lenses. It is a sure protection against the otherwise in- 

 jurious action of the cedar oil. Hollis's liquid glue may also be 

 employed with confidence for this purpose. 



Sealing-wax varnish, which is made by digesting powdered 

 sealing-wax at a gentle heat in alcohol, should never be used as a 

 cement ; it is serviceable only as a varnish, and resists cedar oil. 



Venice turpentine is the liquid resinous exudation of Abies larix. 

 It must be dissolved in enough alcohol to filter readily, and after 

 filtering must be placed in an evaporating dish, and by means of a 

 sand-bath must be reduced by evaporation one-fourth. 



This cement is used for closing glycerin mounts. Square covers 

 are used, and we find it best to edge the cover with glycerin jelly. 

 A piece of copper wire of No. 10 to No. 12 gauge is taken, and one 

 end of it is bent just the length of one of the sides of the cover at 

 right angles to the length of the wire. This end is now heated in a 



