NATURAL PHILOSOPHY. 143 



which the eye apparently saw formed by the superposition of the two 

 stereoscopic corpulse presented the remarkable appearance of not being 

 on the same flat surface ; nearly every alternate word started up or 

 retreated to a different plane from its neighbor ; and the whole effect 

 was most strange and disjointed, as if the observer were looking at 

 three or four superposed sheets of glass, with the words forming the 

 page dotted at random on different sides of the glass plates. 



Pursuing the train of investigation thus suggested, Prof. Dove has 

 been led to further discoveries. It is known that wires of different met- 

 als, drawn through the same plate, are not all of the same thickness, 

 for they are of different degrees of elasticity, and after being drawn 

 through the plate they expand to different amounts. This expansion 

 is proved by the fact that, with the exception of gold, no wire can be 

 drawn through the same aperture through which it has been pressed. 

 Silver requires the least force, but the expansion caused by elasticity 

 continues for several weeks. It appeared probable to Prof. Dove that 

 in stamping metals something similar would take place, and that med- 

 als of different metals, stamped in the same die, would be of different 

 sizes. This would be most readily seen in those medals in which the 

 impression is symmetrically arranged in reference to the edge, as is the 

 case with the medals of the French Exhibition, in which the coat of 

 arms encircle the French eagle in the middle. One of these in silver 

 and one in bronze were placed in the stereoscope, the eagle being 

 fixed in the middle. After some time the stereoscopic combined med- 

 als were seen in the form of a hollow escutcheon, and of the color of 

 an alloy of the two metals. Evidently the reason of this lies in the 

 nonius-like shifting of the individual lines of the impression. This 

 same result was also obtained by the Professor with large gold and 

 silver medals, which were kindly entrusted to him by the royal mint in 

 Berlin. It was probable that medals obtained by casting would show 

 the same thing, and this was found to be the case with tin, bismuth, 

 and lead. Hiero's crown led to the discovery of specific gravity to de- 

 tect an adulteration. The stereoscope may now be used for the same 

 purpose. 



PEESISTEXT ACTIVITY OF LIGHT. 



M. Niepce St. Victor, the celebrated French photographist and 

 chemist, has recently communicated to the Academy some additional 

 researches respecting what he calls " the persistent activity of light." 

 He exposed to the influence of bright sunlight for three hours a piece 

 of porcelain plate, then he removed it and laid it upon a piece of paper 

 which had been prepared with chloride of silver. Some parts of the 

 paper were intentionally not laid under the porcelain, for the purpose 

 of discovering what would be the difference, if any, between the cov- 

 ered and uncovered parts. After twenty-four hours had elapsed, the 

 porcelain was removed, and the paper examined, when it was found 

 that the silver salts were reduced in that part of the paper which had 

 been placed under the porcelain, but no effect was produced in the 

 paper which had not been covered. This led him to conclude that 

 solar light communicated activity to some bodies, which they retained 

 after exposure to the sun's rays. He then tried experiments with a 

 steel plate, one part of which was polished and another part made 



