[Illustration: Fig. 144. A A A. The glass plate, with the waxed side downwards, placed on the leaden tray containing the fluorspar and sulphuric acid. B. Spirit lamp.] The wax plate must not remain too long over the leaden tray, as the heat is apt to melt the wax, when the acid not only attacks those parts from which the wax has been removed by the etching needle, but also the surface of the glass generally, and thus the clearness of the design is spoilt. After exposure--and it is as well to prepare two or three glass plates for the experiment--the wax is quickly removed by rubbing and washing with oil of turpentine, and the design (beautifully etched into the glass) is then apparent. CHAPTER XII. CARBON, BORON, SILICON, SELENIUM, SULPHUR, PHOSPHORUS. This group of non-metallic elements has been frequently styled "Metalloids," meaning substances allied to, but not possessing, all the properties belonging to a metallic substance; and therefore perhaps the expression, non-metallic solids, is the best that can be adopted. They may be subdivided into two classes of three each, which have properties more or less allied to each other--viz., Carbon, Boron, Silicon; and Selenium, Sulphur, Phosphorus. [Page 152] CARBON. Symbol, C; Combining Proportion, 6. This element has almost the property of ubiquity, and is to be found not only in all animal and vegetable substances, in common air, sea, and fresh water, but also in various stones and minerals, and especially in chalk and limestone. There is, perhaps, no element which offers a greater variety of amusing experiments and elementary facts than carbon, whether it be considered either in its simple or combined state. A piece of carbon, in the shape of the Koh-i-Noor, was one of the chief attractions at the first Exhibition in Hyde Park. The diamond is the hardest and most beautiful form of charcoal; how it was made in the great laboratory of nature, or how its particles came together, seems to be a mystery which up to the present time has not yet been solved, at all events no artificial process has yet produced the diamond. Sir D. Brewster, speaking of the Koh-i-Noor, remarks that on placing it under a microscope, he observed several minute cavities surrounded with sectors of polarized light, which could only have been produced by the expansive action of a _compressed gas or fluid_, that had existed in the cavities when the diamond was in the _soft_ state.