ELECTRICITY. 



287 



porous cell was new and not thoroughly soaked with 

 the acids, and it was some time before I could get 

 any curreat at all through it. Thus, magnesium and 

 copper was equal to a Bunsen's and superior to a 

 Grove's cell. The magnesium, with carbon and ni- 

 tric acid or bichromate for a negative, is, I think, the 

 most powerful practical arrangement on record, and 

 is very constant for a long time. Sodium or potas- 

 sium is, no doubt, stronger, but is difficult in the 

 use. Probably lithium would be very powerful. 



Electro- Deposition of Aluminium. Mr. J. 

 B. Thompson, writing in the London Chemi- 

 cal News refutes the assertion that aluminium 

 has never been deposited on other metals by the 

 battery. He says that for more than two years 

 he has been depositing aluminium daily on 

 iron, steel, and other metals, and driving it 

 into their surfaces at a heat of about 500 F., 

 in the same way that he operates with silver 

 and nickel. He has also been doing the same 

 with the alloy called aluminium bronze, of vari- 

 ous tints, from the palest lemon to the richest 

 gold-color, Mr. Thompson does not describe 

 his process. 



The, Electrical Shadow. Prof. N". "W. Wright, 

 in a communication to the American Journal 

 of Science, suggests the phenomenon of the 

 electrical shadow to explain occurrences that 

 are well authenticated, where men struck by 

 lightning have had the images of certain ob- 

 jects (as trees) imprinted on their persons. 

 Several cases have been reported by sea-cap- 

 tains, of the images of brass numbers attached 

 to the rigging of a ship being printed by the 

 lightning on the bodies of persons killed by it, 

 and it was supposed, in those instances, that 

 the brass numbers acted as a negative pole in 

 respect to the person struck. Prof. Wright 

 says : 



But it is unnecessary to suppose that the discharge 

 in such cases always proceeds from the object de- 

 lineated, and many of the instances recorded forbid 

 such a supposition. The experiments in the pro- 

 duction of the electrical shadows show that it is 

 merely necessary that the object should interrupt 

 the lines of action of the electricity, and that it may 

 be at a considerable distance from the electrified 

 cloud, the chief and indispensable condition being 

 that the latter should be negatively electrified. We 

 should then have the body exposed to the lightning 

 positively electrified by induction, and, as the ten- 

 sion became sufficient, the dark discharge accom- 

 panied by the glow would take place, followed by 

 the lightning-stroke. If, then, any object should be 

 in the path of the discharge, its image would be 

 formed in the glow, and this might, in rare cases 

 like those recorded, be sufficiently intense to leave a 

 permanently visible impression. The fact that the 

 image in many cases is very much reduced in size, 

 shows, indeed, that the cloud, or other body servin^ 

 as the negative pole, is much more distant from the 

 object represented than is the body struck, for in 

 the experiments described in my former paper it 

 was found that the image of the paper grating grew 



this effect would be likely to be still more marked 

 where the negative body had a very extended surface 

 as would be the case were it a cloud. 



In some experiments recently made, the negative 

 pole of the machine was covered with several folds 

 of woollen flannel, which prevented the formation of 

 ajet. Instead of this, several square inches of the 

 cioth were covered with a bright glow, somewhat 



resembling that on the positive ball, but, unlike that, 

 seemingly made up of innumerable minute points or 

 patches of light, having a peculiar swarming motion, 

 like that ^described by Neef in his researches upon 

 the negative discharge as seen under the microscope. 

 When the^ paper grating was placed between the 

 poles, the image on the positive was not readily ob- 

 tained with distinctness, but was generally much 

 smaller than when the negative pole was not thus 

 covered. 



A similar effect, but much more brilliant, was pro- 

 duced when, the poles being separated nine or ten 

 inches, the hand was placed upon the negative, and 

 the arm was approached to the positive pole. The 

 woollen sleeve exhibited a bright glow covering a 

 large area, and appearing like a strongly phospho- 

 rescent powder sifted profusely upon it. When this 

 was approached so near that the interval was only 

 an inch and one half or an inch, the positive glow- 

 became much more intense, and took a delicate pur- 

 plish tinge, and the whole space between the two 

 was filled with a very faint auroral light, which ap- 

 peared, unlike other forms of the discharge, to be 

 perfectly continuous and steady. The dark discharge 

 under _ these circumstances was evidently so much 

 intensified as to become luminous and visible. 



Subterranean Electrical Disturbances. A 

 few minutes before and after the earthquakes 

 of March 17th, in. England (according to a 

 statement in Nature), powerful positive elec- 

 trical currents were rushing toward England 

 through the two Anglo-American cables, which 

 were broken near Trinity Bay, Newfoundland. 

 Mr. C. F. Varley, reflecting upon this fact, 

 makes the suggestion that some earthquakes 

 may be due to subterranean lightning. He 

 imagines that, as the hot centre of the earth is 

 approached, a layer of hot dried rock may be 

 found which is an insulator, while the red-hot 

 mass lower down is a conductor. If this con- 

 jecture be true and there is plausibility in it 

 then the world itself is an enormous Leyden 

 jar, which only requires charging to a very 

 moderate degree, to be equal to the production 

 of terrific explosive discharges. The French- 

 Atlantic cable was disturbed at the same time, 

 and so were many of the English land-lines, 

 but the only observations as to the direction 

 of the current were made by means of the 

 Anglo-American telegraph cables. A number 

 of Mr. Varley's charts about earth-currents 

 were published in the Government Blue Book 

 of 1859-'60, showing that the direction of 

 these currents across England Avas in a very 

 notable degree determined by the contour of 

 the coast, and that the same auroral discharges 

 would often produce currents at right angles 

 to each other in direction, in different parts 

 of Britain. 



Coloring the Electric Spark. The electric 

 spark, as M. Becquerel shows, may be beau- 

 tifully colored, of different tints, by passing it 

 through saline solutions. If this spark from 

 an inductive apparatus be made to pass into 

 the extremity of a platinum wire suspended 

 over the surface of the solution of a salt, the 

 spark will acquire special coloration according 

 to the chemical composition of the solution 

 traversed. The saline solutions are best con- 

 centrated, and the platinum wire positive. 



