SECTIONAL TRANSACTIONS.— C. 341) 



cement, the thin section, polished on both surfaces, could be mounted on a suitable 

 piece of glass and examined under the microscope. 



Nicol's claim to consideration is that he substituted a glass plate for the wood 

 block, and Canada balsam for the lapidary's cement. Thus, he says, the specimen 

 could be reduced to any desired thickness and could be examined at any stage. Now 

 this is an important improvement of the older method, and Nicol states, in 1834, that 

 he had been using it for fifteen years. 



Yet he did not appreciate to the full the importance of the process, and in the 

 same article gives to Prof. Lindley the credit of having drawai attention to its possi- 

 bilities.* Yet Lindley in 183P had already given that credit to Witham. The situa- 

 tion is further obscured by an editor's remark (at the end of Nicol's paper) that he — 

 Prof. Jameson — had long known the method and had advocated its use to geologists.^ 

 The latter phrase indicates that Jameson had foreseen what Sorby put into practice 

 forty years afterwards. Truly both Brewster and Nicol had made thin sections of 

 discrete minerals, but Sorby was inspired to apply the method to rocks after a visit 

 to Edinburgh, where he examined the collection of slides and specimens belonging 

 to Alexander Bryson. This collection contained Nicol's sections, as well as many 

 prepared by Bryson himself, and its treasures were available for all those interested 

 in the study. It is sometimes stated that Williamson of Manchester showed Sorby 

 the method ; but such is not the case, though he may have helped Sorby in the 

 preparation of the sections. 



Modern practice differs little from Nicol's improved process. It is true that 

 polishing is no longer required, for the refractive index of Canada balsam is sufficiently 

 close to that of most rock-forming minerals to obviate the necessity of securing trans- 

 parency by polishing the surfaces. The use of glass coverslip protects the surface 

 from damage and renders the upper surface transparent, while the transference to 

 a clean slip of glass gives a more workmanlike finish than was formerly obtained. 

 Improved and new abrasives have ' speeded up ' the manufacture, and an average 

 section may easily be made in from twenty to thirty minutes. Dry grinding and wet 

 grinding are both used, the former in special cases where the material might break 

 down if wetted, while gum arabic, shellac, and other media are employed under certain 

 circumstances. Specimens illustrating various stages in the process were shown, 

 while some hints on ' cooking ' the Canada balsam and transference of the finished 

 sections were given. 



Prof. H. L. Hawkins. — Preparation of Fossil Echinoderms. 



The only special difficulties in this group are due to the crystalline cleavage of the 

 calcite, whether primary or secondary, of the stereom. Since this cleavage is always 

 at an angle to the surface of the test or ossicle, mechanical separation of the specimen 

 from its matrix has to be carried out with absence of jarring and the minimum of 

 pressure. Except in the case of chalk specimens, the test is normally softer (as well as 

 more friable) than the matrix, so that slow progress and delicate manipulation are 

 required. 



Calcite specimens that have been freed from an argillaceous matrix by caustic 

 potash need saturation with wax afterwards to prevent crumbling of the stereom. 



Sutures may be revealed (but not with certainty of success) by exposure of the test 

 to the action of the humic acids generated in rotting vegetation. 



Dr. A. Heard. — On a New Method of Treating Pyritised Plant 

 Remains. 



By this method, pyritised fossil plants of Devonian age, which originally exhibited 

 no trace of their internal morphology, have yielded every detail of the internal 

 anatomy of the vegetative organs. 



Fragments of the plants are embedded in a suitable mounting medium, e.g. 

 shellac, and sectioned in the desired direction. The section is very carefully polished 

 and treated with concentrated nitric acid, with, in some cases, potassium chlorate 

 added. The cell structures develop rapidly, and are observed in incident light. 



* Nicol, loc. cit.. p. 156. 



■'■ Lindley and Hutton. fossii JZora, vol. i, p. xxviii. 1831. 



' Nicol, loc. cit. Editor's remark at end of article. 



