INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 699 



Having selected a piece of coal with as few cracks as possible, 

 cut off a piece with a saw about three-quarters of an inch square and 

 perhaps one quarter of an inch thick. One of the larger surfaces is 

 then rubbed flat on the pumice-stone, keeping it well wetted with water, 

 and then polished upon the hone, also moistened with water. Some- 

 times it is found to be advantageous to soak the piece of coal in a very 

 thin solution of Canada balsam in chloroform or benzole, as directed 

 for softer rocks, or in a solution of shellac in spirits of wine ; in either 

 case allowing the specimen to dry thoroughly in a warm place. The 

 polished surface is next cemented to an ordinary microscopical glass 

 slip (3 inches by 1 inch) with the best marine glue ; and this process 

 requires care, for it is not easy to exclude all the air-bubbles, and if 

 they are not excluded the section is very apt in the last stages to 

 break away wherever they occur. The piece of coal is next reduced 

 to about one-sixteenth of an inch by means of a grindstone ; some of 

 the softer kinds may be cut down with a penknife. Care should be 

 taken not to scratch the glass in the process of grinding, for most 

 sections of coal, when once ground thin, are too fragile to allow of their 

 being removed from the glass, but have to be covered and finished off 

 upon the same slide. The pumice-stone or corundum stick is next 

 brought into use. The section being turned downwards, hold the 

 glass slide between the middle finger and thumb, whilst the forefinger 

 is placed upon the centre of the slide. In this manner the section may 

 be rubbed round and round over every part of the pumice, using plenty 

 of water, until it is sufficiently reduced in thickness ; experience alone 

 showing how far this process may be carried. The section is finally 

 rubbed in a similar manner upon the hone (or Water-of-Ayr stone). It 

 is sometimes found necessary to use the hone even while the section is 

 absolutely opaque, for many coals are so brittle that they crumble to 

 pieces upon the pumice long before they show any indications of 

 transparency. "When sufiiciently transparent the section may bo 

 trimmed with a penknife and the superfluous marine glue cleaned off. 



The section is now to be moistened with turpentine, a drop of 

 ordinary Canada balsam (not too hard) placed upon it, and covered in 

 the usual way. Whatever heat is necessary should be carefully applied 

 to the cover-glass by reversing the slide for a moment or so over a 

 spirit lamp, otherwise the marine glue may be loosened and the section 

 spoiled. Balsam dissolved in benzoic must not be used for mounting, 

 as the benzole softens the marine glue, and a good section may in this 

 way be destroyed." 



Cutting Rock Sections. — Mr. Hanks considers it a mistake to cut 

 a rock section so thin as to be wholly transparent. In some cases 

 this is necessary; but, as- a general ruh', the section should be hft as 

 thick as possible, and strongly lighted by the aid of a parabolic 

 reflector. The beauty of many specimens is destroyed in tho eflbrt 

 made to fit them for observation by simple transmitted light. 

 Mr. Attwood's plan * to cement tlio section to a glass slide, and to 

 examine it from time to time under the Microscope as tho work 

 progresses, is very important, as it will enable the student to stop at 



• Ante, i>. 325. 



