202 Transactions of the Society. 



drying, so that this may act as a support whilst one side is being ground 

 down. Short corks may be attached by means of marine glue to each 

 end of the slide, so as to afford a firm hold. 



We are now in possession of a hard mass, the future treatment of 

 which resembles that by which a piece of pumice is sectionized by the 

 first author's method. The fragment if large must, as already has been 

 mentioned, be attached during drying to a piece of glass, but if of fair 

 thickness a section two centimetres or three-quarters of an inch may be 

 held in the fingers. The flatter or more complete side is held against 

 the side of a grindstone, or rubbed in a circular manner on a flat slab of 

 the same material until a fairly level surface has been obtained. A 

 small quantity of carbonate of soda or, better, soap solution may be added 

 to the water, to prevent the stone clogging with the balsam and tissue 

 ground ofi". The quantity requires careful watching to prevent dis- 

 solving more balsam than is ground ofi", in which case the section will 

 appear cloudy when finished. 



After washing, the ground face is now applied to the hone and a 

 drop of the solution of soap in alcohol and water is to be put upon the 

 nearly level surface of the stone upon which water is slowly dropping. 

 The grinding is now continued until a fine level surface has been 

 obtained. From time to time the object will begin to catch on the 

 stone and little rolls of balsam form, which is an indication for more 

 soap solution. When a perfectly level smooth surface has been obtained, 

 the object is washed in clean water with a soft tooth-brush or a camel's 

 hair pencil, dried and warmed just sufficiently to drive out any moisture 

 in cracks or cavities, hut not to soften the balsam. 



The permanent glass slip is now cleaned with alcohol and varnished 

 with oil of cloves (slightly grinding the surface is sometimes useful in 

 order to make the object stick to the glass), then heated and rubbed with 

 perfectly hard balsam, so as to leave a thin layer of this substance 

 covering the slip. The ground face of the specimen is also very thinly 

 varnished with oil of cloves, and pressed into the warm balsam so as 

 to come in complete contact with the glass, avoiding air-bubbles, &c. 

 Large sections can rarely be fixed at once in this way, therefore they 

 are put on a piece of paper which lies on an elastic mass, such as some 

 sheets of paper, a piece of indiarubber, &c., and the glass touched by the 

 flame opposite points where the object is not yet fixed, pressing it into 

 contact at every point. The preparation is now allowed to cool very 

 gradually to anneal the balsam. 



The object is now brought against the circumference of the grind- 

 stone, and the thickness reduced all over. When we have removed as 

 much as convenient, the flat side of the stone may be used and the pre- 

 paration levelled down to almost the requisite thinness for microscopical 

 examination. It is now transferred, after washing, to the hone for the 

 final grinding, which requires great care and gentleness to prevent the 

 edges of the section cracking away. If such is persistently the case 

 in some examples the mass may after the stoving be enveloped in 

 plaster strengthened by threads, a method suggested by Prof. Von Koch. 

 When the desired thinnesss is acquired the section is again thoroughly 

 washed in water with the aid of a brush, and spontaneously dried. When 



