208 THE MICROSCOPE. 



hardness should be tested with the edge of the thumb 

 nail, for it should be with difficulty indented by pressure, 

 and yet should not be so resinous as to be brittle. If it 

 be too soft, as indicated by its too ready yielding to the 

 thumb-nail, it should be boiled a little more ; if too hard, 

 which will be shown by its chipping, it should be re-melted 

 and diluted with more fluid balsam, and then set aside to 

 cool as before. When of the right consistence, the section 

 should be laid upon its surface, with the polished side 

 downwards ; the slip of glass is next to be gradually 

 warmed until the balsam is softened, care being taken to 

 avoid the formation of bubbles, and the section is then to 

 be gently pressed down upon the liquefied balsam in a sort 

 of wave towards the side, and an equable pressure being 

 finally made over the whole. When the section has been 

 thus secured to the glass, it may be readily reduced in 

 thickness by grinding. When the thinness of the section 

 is such as to cause the water to spread around it between 

 the glass and the stone, an excess of thickness on either 

 side may often be detected by noticing the smaller distance 

 to which the liquid extends. In proportion as the section 

 attached to the glass is ground away, the superfluous 

 balsam which may have exuded around it will be brought 

 into contact with the stone ; and this should be removed 

 with a knife, care being taken that a margin be still left 

 round the edge of the section. As the section approaches 

 the degree of thinness which is most suitable for the 

 display of its organization, great care must be taken that 

 the grinding process is not carried too far ; and frequent 

 recourse should be had to the microscope to examine it. 

 The final polish must be given upon a leathern strap, or 

 upon the surface of a board covered with buff-leather, 

 sprinkled with putty-powder and water, until all marks 

 and scratches have been rubbed out of the section. 



In mounting sections of bone, or teeth, it is important 

 to avoid the penetration of the Canada balsam into the 

 interior of the lacunce and canaliculi; since, when these 

 are filled by it, they become almost invisible. The benefit 

 which is derived from covering the surfaces of the 

 specimen with Canada balsam, may be obtained, without 

 the injury resulting from the penetration of the balsam 



