The third volume, which is now to be commenced, refers to a class of operations entirely dissimilar to those which have been described in the foregoing pages; as the former descriptions and instructions have referred alone to the treatment of such materials as admit of being cut with steel tools. In page 12 of the Introduction to the first volume of this work, it is stated that -

"The third volume will be devoted to the explanation of abrasive processes; namely, those for restoring or sharpening the edges of cutting tools; those for working upon substances to which, from their hardness or crystalline structure, the cutting tools, (made of hardened and tempered steel,) are quite inapplicable; and also to the modes of polishing, which may be viewed as a delicate and extreme application of the abrasive process, and the final operation after the cutting tools, and lastly, to the ordinary modes of staining, lackering, varnishing, and other miscellaneous subjects."

In addition to the broad distinction between the processes which have been hitherto described, and that are performed with cutting tools of steel, there is another conspicuous difference, namely, that in works executed by cutting, the material is mostly removed in chips and fragments, which in the case of woods may be burned as fuel, or in metals usually admit of being reunited by fusion, and again converted into ingots, bars, or sheets, for subsequent use in the arts, - whereas in the second class of effects now to be considered, or those of abrasion by various frictional processes, the removed materials are ground to powder, and are mostly unsuited to further use.

On examination of the various abrasive processes, and of which grinding for the production of form, and polishing for the production of surface, may be considered as the extremes, it will be seen there are in every case of abrasion three distinct points to be considered.

First, the substances that are to be ground or polished. Secondly, the materials or abrasive powders by the successive employment of which different substances are polished.

Thirdly, the tools or apparatus by the agency of which abrasive substances are applied to the objects to be ground or polished.

Much variety necessarily exists under all three of these heads, and sometimes the very same substance may be referred to all of them; for example - glass is frequently polished, as in plate glass, cut glass, and lenses. - Glass is frequently used as a polishing material when pulverized and glued upon paper. - And glass is also frequently used by watchmakers and some other artizans, as a tool or rubber through the medium of which, some of the polishing powders are applied to metal works in the act of polishing them.

The same thing may be observed of iron. - This metal in its various metallic forms is continually ground and polished. - An iron disk is used with diamond powder under the name of a skive, as the lap whereby diamonds for jewellery are polished, - and iron when reduced to the form of the peroxide or crocus, is used for very many purposes in the arts, and amongst others for polishing the specula of reflecting telescopes.

By way of condensing the numerous particulars that are to be offered under these several heads and rendering them easy of access and comparison, they will be arranged in alphabetical form in a "Descriptive Catalogue of the Apparatus Materials and Processes for Grinding and Polishing, commonly employed in the Mechanical and Useful arts."

The catalogue will be found to contain much general information upon, and many practical examples of abrasive processes, and will be followed by one chapter on the grinding and sharpening of tools of various kinds; - one chapter on the figuration of materials by abrasion, - in which will be described under distinct sections, some of the modes of producing plane surfaces, cylindrical and conical surfaces, spheres and spherical surfaces, and various mixed and arbitrary forms. After which, two other chapters will relate respectively to the art of the lapidary, and those of the engravers on glass and gems, - which several chapters will be materially assisted by the matter contained in the alphabetical catalogue, - before proceeding to which it is proposed to add a few explanatory remarks on the three classes of information contained in the catalogue.