NATURAL PHILOSOPHY. 150 



subjected to nitrate of silver. A sheet of paper impregnated with chloride of 

 mercury, and exposed to the sun, gives an image with chloride of tin, chloride 

 of sodium, soda, potash, and sulphuret of sodium. In the same manner a 

 sheet impregnated with chloride of tin, and exposed to the sun, gives an im- 

 age with sulphuret of sodium, chloride of mercury, chloride of gold (Cl'iAiu) 

 and nitrate of silver. Correspondence of M. JVzcfcfes, Sittiman's Journal. 



In addition to the above, the following results, attained by M. Xicpce, 

 were announced by the Abbe Moigno, in the " Cosmos," for July, ISo'J. 

 " If a solution of starch or dextrine be subjected to the action of solar light 

 for a short time (say for a quarter of an hour, if there be but a very small 

 quantity of matter), it will be found to be completely changed into glucose 

 (grape sugar), whose presence is easily recognized by the ordinary reactions, 

 and even by its sweet taste. M. Niepce thinks that he has determined, that, 

 by surrounding the bunches of grapes in the early part of autumn by bags 

 of white paper dipped in tartaric acid, not only is their ripening hastened, 

 but the quantity of sugar which they contain is greatly increased. Tartaric 

 acid is now well known to have the power of storing up the light in the con- 

 dition of chemical efficacy." 



PHOTOGEAPHS IX NATURAL COLOPcS. 



The following process for obtaining a photograph of the prismatic spec- 

 trum, in its natural colors, has been devised by M. Becquerel. He takes a 

 well-polished silver plate, and, after covering the back of it with varnish, so 

 as to leave the front surface alone exposed, he attaches it by copper hooks 

 to the positive conductor of a voltaic battery of one or two cells ; to the* neg- 

 ative conductor of the battery is attached a piece of platinum. The plate of 

 silver and the platinum are then plunged into a mixture of eight parts of 

 water and one of hydrochloric acid. The electric current decomposes the 

 acid, and causes a deposit of chlorine on the surface of the silver, while 

 hydrogen is liberated at the negative pole. The chlorine gas unites with 

 the silver, and forms a violet tinted coating, which would become quite 

 black if the operation were continued a sufficient length of time. This 

 coating is tolerably sensitive to light when very thin, and in that condi- 

 tion produces the natural tints, although they are very weak. By increas- 

 ing the thickness of the layer, the tints become much brighter, but the 

 sensitiveness diminishes. In order to ascertain exactly the amount of chlo- 

 rine deposited on the silver plate, M. Becquerel introduces into the voltaic 

 circuit an apparatus for the decomposition of water, and since chemical 

 decomposition is similar in quantity for each cell of a battery, by measuring 

 the amount of hydrogen produced by this decomposition, the quantity of 

 chlorine liberated on the surface of the silver-plate, is easily arrived at. 



An idea of the extreme tenuity of this film may be obtained when we 

 learn that, with six or seven centimetres of chlorine to the square decime- 

 tres, the layer of chloride of silver is only one thousandth of a millemctrc in 

 thickness, equal to about 0-00004 of an inch. With a film of this thickness the 

 best results are obtained. Before exposure to the spectrum, the surface has a 

 plain wood color, but if it be heated to between 1.10 or 200 centigrade (100* 

 to 390 "" Fahrenheit), it becomes rose-colored on cooling. If, however, instead 

 of raising the plate to a hi":h temperature, it be inclosed within a copper 

 box, and gently warmed, say from 90 3 to 93 Fahrenheit, and maintained at 

 this heat five or six days; or, better still, placed in a frame covered with a 



