THE CHEMICAL RADIATIONS. 19 



Prof. Wheatstone's experiments were immediately connected with two 

 large insulated metallic surfaces exposed to the air, so that the pri- 

 mary act of induction after making the contact for discharge might 

 be in part removed from the internal portion of the wire at the first 

 instant, and disposed for the moment on its surface jointly with the 

 air and surrounding conductors, then I venture to anticipate that the 

 middle spark would be more retarded than before. And if those two 

 plates were the inner and outer coatings of a large jar or Leyden bat- 

 tery, then the retardation of the spark would be much greater." The 

 experiment was not made for sixteen years. It was then shown as the 

 explanation of the retardation of the current in our subterraneous and 

 submarine wires. 



Sir Francis Ronalds, with wonderful prescience, had in 1823 fif- 

 teen years before Faraday suggested "the probability that the elec- 

 trical induction which would take place in a wire inclosed in glass 

 tubes of many miles in length (the wire acting like the interior coat- 

 ing of a battery) might amount to the retention of a charge, or at 

 least might destroy the suddenness of the discharge." Faraday's pro- 

 phetic vision and Konalds's far-sighted knowledge are Terified in every 

 working cable. The accuracy with which our cable-repairers are 

 directed by our electricians to the spot where the wire is broken, the 

 exactitude with which the working speed of a cable is predicted, the 

 unfelt and invisible supervision which is exercised over the care and 

 maintenance of our telegraphs even though they pass through dis- 

 tant countries and different climes are evidences that electricity, in 

 this particular field, is approaching the last and prophetic stage of its 

 growth. This field is resistance, and Ohm is its prophet. Telegraphic 

 Journal. 



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THE CHEMICAL RADIATIONS. 



By W. J. YOUMANS, M. D. 



WITH that proneness to go wrong, which we notice in most things 

 human, and which crops out in science as well as elsewhere, 

 the art of making pictures by the chemical action of I'adiant forces 

 has got a false name. This is all the worse, as it was at first correctly 

 designated, and that too by him who had the clearest right to give the 

 process a title. Davy and Wedgwood, early in the century, had la- 

 bored to produce sun-pictures by means of the camera-ohscura^ but 

 had met with little success. In 1814 M. Neipce, of Chalons, in France, 

 took up the subject, and, in the course of ten years' assiduous work, he 

 succeeded in a method of forming sun-pictures on chemically-pre- 

 pared copper, pewter, and glass plates, by which the lights, semi-tints, 

 and shadows, were represented as in Nature, and he also succeeded in 



