[133] _On Thermography, or the Art of copying Engravings or any printed characters from paper or plates of metal, and on the recent discovery of Moser, relative to the formation of images in the dark_, by Robert Hunt: Reports of the Royal Cornwall Polytechnic Society for 1842, and Philosophical Magazine, vol. xxi. p. 462.--_On the Spectral Images of M. Moser_, by Robert Hunt: Philosophical Magazine, vol. xxiii. p. 415. [134] _Catalytic force, or attraction of surface concerned in the diffusive power of gases: an occult energy or power in saturated saline solutions_; Prater.--Mechanic’s Magazine, vol. xlv. p. 106. _Ueber elektrische Abbildungen_; by G. Karsten.--Poggendorff’s Annalen, vol. lvii. p. 402.--Melloni and Brewster may be consulted for much that is most remarkable connected with radiation from coloured surfaces. [135] Cornelius Agrippa is said to have possessed such a mirror. The Chinese make mirrors which, when placed in a particular light, show upon their polished faces the pattern on the back of the metal, although it is invisible in every other position. This is effected by giving different degrees of hardness to the various parts of the metal. In _Natural Magic_, by Sir David Brewster, several curious experiments belonging to this class are named. CHAPTER IX. ELECTRICITY. Discovery of Electrical Force--Diffused through all Matter--What is Electricity?--Theories--Frictional Electricity--Conducting Power of Bodies--Hypothesis of two Fluids--Electrical Images--Galvanic Electricity--Effects on Animals--Chemistry of Galvanic Battery--Electricity of a Drop of Water--Electro-chemical Action--Electrical Currents--Thermo-Electricity--Animal Electricity--Gymnotus--Torpedo--Atmospheric Electricity--Lightning Conductors--Earth’s Magnetism due to Electrical Currents--Influence on Vitality--Animal and Vegetable Development--Terrestrial Currents--Electricity of Mineral Veins--Electrotype--Influence of Heat, Light, and Actinism on Electrical Phenomena. If a piece of amber, _electrum_, is briskly rubbed, it acquires the property of attracting light bodies. This curious power excited the attention of Thales of Miletus; and from the investigations of this Grecian philosopher we must date our knowledge of one of the most important of the natural forces--Electricity.