136 ANNUAL OF SCIENTIFIC DISCOVERY. 



COLORS OF SOAP-BUBBLES. 



Sir David Brewster, at the meeting of the British Association in 

 1867, stated,- as the result of his experiments, that the colors of 

 soap-bubbles are not produced by different thicknesses of the film 

 itself, but by the secretion from it of anew substance flowing over 

 the film, expanding under the influence of gravity and molecular 

 forces into colored groups of various shapes, and returning spon- 

 taneously, when not returned forcibly, into the parent films. All 

 the phenomena are emitted by ordinary soap-bubbles, though a 

 mixture of glycerine made the films last much longer. 



PERSISTENT ACTION OF LIGHT. 



M. Niepce St. Victor, some years ago, made the discovery that 

 the actinic properties of light could be stored up, and made, after 

 a long interval, to produce their photographic effect. The accu- 

 racy of his results was called in question at the time, but his most 

 recent experiments completely confirm them. In these he ex- 

 posed to sunlight strips of ordinary paper covered with plates of 

 colored glass, arranged as the colors of the solar spectrum ; after 

 the exposure, the strips were placed in perfect darkness upon 

 other strips of paper sensitized with chloride of silver, which, it 

 was found, were blackened by that part of the exposed strip 

 which had been covered by the blue, indigo, and violet glass. 

 Here would seem to be a clear proof of the persistent action of 

 light. 



The same physicist also announces the extraordinary fact that 

 porous or rugose surfaces which have been exposed to light have 

 a definite decomposing action on salts of silver, when placed in 

 contact with them in the dark. It has been considered probable 

 by many natural philosophers that phenomena like phosphores- 

 cence are due to the emission of light previously absorbed. Till 

 M. St. Victor's discovery, this h}'potbesis was supported only by 

 vague speculation. He has shown that pieces of pasteboard, 

 which have been exposed to the light, give out actinic force in the 

 dark, and may be employed in producing decomposition of silver 

 salts. 



According to the " Laboratory," in the optical room of the 

 *' Conservatoire des Arts et Metiers" at Paris, near a window, is a 

 frame, containing half a dozen test tubes filled with powders, 

 bearing a descriptive label by M. Becquerel. On closing the win- 

 dow shutter, for darkness is required to reveal the beauties of the 

 apparatus, the powders exhibit in a most striking manner the 

 phenomenon of phosphorescence, each shining with a different 

 colored light. It is called by its French makers the " Phosphoro- 

 scope," though this name has been applied to a very different in- 

 strument ; but as a scientific toy it is likely to become known in 

 England as " A Trap to Catch Sunbeams." Most of the powders 

 are sulphides, and the brightest emanation is probably from the 

 tube containing sulphide of barium. The phosphorescence may 



