I SO HOW TO WORK 



the same level. If not dressed smooth by slo\v grinding, the hard 

 portions will stand out in relief; and when the section is finished, the 

 soft parts may be all ground away before the hard are sufficiently 

 thin, and the structure of the rock may be quite misunderstood. 

 Having duly prepared one flat surface, it should be fastened down 

 on a piece of glass with Canada balsam. This should be kept hot 

 until it is so hard as to be just brittle when cold. I find it best to 

 remove, time after time, a small piece, until it has become so hard 

 that when cold, it can :be rubbed to powder between the thumb and 

 finger. The piece of stone should be made hot, but no hotter than 

 needful, so that liquid may not be expelled from the fluid-cavities, 

 and balsam should be spread over the flat surface, and kept hot for 

 a while ; which penetrates into the softer parts and hardens them. 

 Before fixing the specimen on the glass, it is -well to remove this 

 balsam, and fasten it down by that on the glass. I find it much the 

 best to use square pieces of glass. The usual 3-inch by i glasses are 

 very unsuitable for the purpose ; since they are much too long in one 

 direction, and too short in -the other. I use glasses i -finch square, 

 and generally make sections about i inch square, which is a very- 

 suitable size. Since the section ought not to be removed from the 

 glass, care should be taken in grinding down not to scratch the 

 glass. This may be avoided by fastening small bits of sheet zinc at 

 each corner with balsam, and grinding the stone with emery until 

 they all come flat down on the plate. The stone is then equally thin 

 all over; and having removed the bits of zinc it must be further 

 ground down on the stones until of the proper thickness, and the upper 

 surface finished off in the manner already described. The thickness 

 must depend very much on the nature of the rock. If coarse-grained 

 and composed of comparatively transparent minerals, y^-gth of an 

 inch may not be too thick, whereas some very fine grained and 

 opaque rocks should be not above T o-V o^ 1 of an inch. Of course it 

 is requisite so to grind them down as not to break up or disturb the 

 different constituents ; and, since some parts may be very hard and 

 some very soft, it is impossible to prepare perfect sections unless they 

 are slowly ground down on a fine-grained stone, which may gradually 

 wear away the hardest parts without injuring the softest. After 

 having finished the section I find it often better to keep it some time 

 before I mount over it a thin glass cover, in order that the balsam 

 may become quite hard. I then melt some balsam at a gentle heat 

 on a thin glass cover of proper size, and just before I place it on, I 

 wet the surface of the section with a drop of turpentine, which soaks 

 into the pores so as to make it more transparent, and renders it much 

 easier to fasten down the glass without any bubbles. This must be 



