ISi 



scorches and elevates the cuticle in blisters ; this effect is prevented 

 by covering the hand or face with thin black kerseymere, and the 

 same purpose is attained by the black rete mucosum of the negro. 

 In these cases perspiration comes on, which it does not in the former, 

 and the surface, though actually hotter, is uninjured. 



Nature has provided a similar defence from the injury of light, by 

 placing a black pigment at the bottom of the eye of those animals 

 who are exposed to the intensity of the sun's rays ; whereas in the 

 inhabitants of shady and dark situations the pigment is either pale, 

 or altogether wanting. 



The author concludes this paper by ascribing the beneficial effects 

 which he has shown to result from the black matter in the negro's 

 skin, and at the bottom of the eyes in certain animals, to the power 

 which black surfaces possess of converting the radiant matter of the 

 sun into sensible heat. 



On the Magnetic Phenomena produced by Electricity. In a Letter from 

 Sir Humphry Davy, Bart. F.R.S. toW. H. Wollaston, M.D.P.R.S. 

 Read November 16, 1820. [Phil. Trans. 1821, p. 7.] 



In repeating the experiments of Oersted, Sir Humphry found that 

 with a voltaic battery of 100 pair of 4-inch plates, the south pole of 

 a magnetic needle placed under the communicating wire of platinum 

 (the positive end of the apparatus being on the right) was strongly 

 attracted by the wire, which was shown to be itself magnetic, by its 

 power of attracting steel filings, and of communicating permanent 

 magnetism to steel bars attached to it transversely, while similar bars 

 placed parallel to the wire were only magnetic during its connection 

 with the voltaic apparatus. The actual contact, however, of the steel 

 wire with that of platinum, or other metal forming the voltaic con- 

 ductor, is not necessary, for magnetism was communicated to a 

 needle placed transversely to it, but at some distance. 



SSir Humphry Davy next details some experiments, showing that 

 the magnetic power is proportionate to the quantity of electricity 

 passing through a given space, without any relation to the trans- 

 mitting metal ; and that the finer the wires, the stronger their mag- 

 netism. He found an analogous effect produced by the discharge of 

 a Leyden phial through a wire ; and by passing the discharge of a 

 Leyden battery of 17 square feet through a silver wire, with a steel 

 bar transversely attached to it of two inches in length, the latter be- 

 came powerfully and permanently magnetic. The same effect was 

 produced at a distance of five inches through air, water, and even 

 through thick plates of glass. 



When several wires parallel to each other form part of the same 

 circuit, each becomes similarly magnetic to the single wire ; and the 

 opposite sides of such wires are in different magnetic states, and con- 

 sequently attract each other. By arranging two voltaic batteries 

 parallel to each other, the positive end of the one being opposite to 

 the negative end of the other, and transmitting their electricitie? 



