GRINDING AND POLISHING THIN SECTIONS. 193 



•with water on a ' Water-of-Ayr '-stone, on a hone or ' Turkey '- 

 stone, or on a new stone recently introduced under the name of the 

 1 Arkansas '-stone ; the first of the three is the best for all ordinary 

 purposes, but the two latter being much harder may be employed 

 for substances which resist it.* When this has been sufficiently 

 accomplished, the section is to be attached with Canada balsam to 

 a slip of thick well-annealed glass ; and as the success of the final 

 result will often depend upon the completeness of its adhesion to 

 this, the means of most effectually securing that adhesion will now 

 be described in detail. Some Canada balsam, previously rendered 

 somewhat stiff by the evaporation of part of its turpentine, is to 

 be melted on the glass slip, so as to form a thick drop, covering a 

 space somewhat larger than the area of the section ; and it should 

 then be set aside to cool, during which process the bubbles that 

 may have formed in it will usually burst. When cold, its hard- 

 ness should be tested, w T hich is best done by the edge of the 

 thumb-nail ; for it should be with difficulty indented by its 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 it is found to 

 be 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, special care 

 being taken to avoid the formation of bubbles ; and the section is 

 then to be gently pressed down upon the liquefied balsam, the 

 pressure being at first applied rather on one side than over its 

 whole area, so as to drive the superfluous balsam in a sort of wave 

 towards the other side, and an equable pressure being finally made 

 over the whole. If this be carefully done, even a very large section 

 may be attached to glass without the intervention of any air- 

 bubbles ; if, however, they should present themselves, and they 

 cannot be expelled by increasing the pressure over the part beneath 

 which they are, or by slightly shifting the section from side to 

 side, it is better to take the section entirely off, to melt a little 

 fresh balsam upon the glass, and then to lay the section upon it as 

 before. 



140. When the Section has been thus secured to the glass, and 

 the attached parts thoroughly saturated (if it be porous) with hard 

 Canada balsam, it may be readily reduced in thickness, either by 

 grinding or filing as before, or, if the thickness be excessive, by 

 taking off the chief part of it at once by the slitting-wheel. So 



* As the flatness of the polished surface is a matter of the first import- 

 ance, that of the Stones themselves should be tested from time to time ; 

 and whenever they are found to have been rubbed down on any one part 

 more than on another, they should be flattened on a paving-stone with 

 fine sand, or on the lead-plate with emery. 







