CHEMICAL SCIENCE. 185 



second, that the fissures filled by brown carbonate of lime, giving rise, 

 when waterworn, to the tessellated appearance of the tortoise shell, 

 have been formed by expansion along radical lines, producing openings 

 where there was least cohesion. 



Prof. Henry gave an illustration of experiments which he had made 

 himself with gold and other metals. He had placed a lump of silver on 

 a piece of iron, and then put it in a furnace ; but he did not find that 

 it permeated. He subsequently inquired of a jeweller, what results he 

 had perceived from a similar process, in which copper was used instead 

 of iron, and he said he had often seen the silver disappear. The 

 experiment was tried in his (Prof. Henry's) presence, and, sure enough, 

 the silver was apparently gone, and the copper surface alone appeared. 

 He immediately put it in a galvanic battery, and the silver reappeared, 

 which showed that it did not dissolve, but merely went down below 

 the surface of the other metal. 



NEW METHOD OF ENGRAVING. 



PoiTEYrN" has described a method by which we may obtain, on 

 plates, raised or sunk impressions from drawings or engravings. These 

 plates, in their turn, may be used for multiplying the impressions. 

 The engraving is exposed to iodine vapors, which only adhere to the 

 black parts ; the sheet is then attached to a silver plate, polished ac- 

 cording to Daguerre's method, by means of slight pressure ; the iodine 

 is transferred to the silver, so that layers of iodide of silver are formed, 

 corresponding to the shadows of the engraving. The plate is then im- 

 mersed in a concentrated solution of sulphate of copper, and used as 

 the negative pole of a weak battery ; it is removed before the iodized 

 portions are coated with copper. The plate is at once washed, and the 

 iodide removed by hyposulphate of soda ; the copper surfaces are then 

 oxidized by heat until they become dark brown ; the exposed silver 

 surfaces are amalgamated after cooling, and the plate being covered 

 with two or three layers of gold-leaf, the mercury is volatilized by heat. 

 The gold is brushed off from those parts which are covered with oxide 

 of copper, and to which it does not adhere. The oxide of copper is 

 then dissolved by a solution of nitrate of silver, and the silver, as well 

 as the adjacent copper, exposed to the action of dilute nitric acid. The 

 parts covered with gold are not affected, so that the etching may be 

 carried to any depth. The plate which is thus obtained may be em- 

 ployed for taking impressions in the manner in. which wood blocks are 

 used. In order to obtain plates engraved as deeply as the plates used 

 for ordinary copper-plate engravings, a plate of gilt copper is employed. 

 By proceeding as above, the light parts are covered with copper ; and 

 the shaded parts being deprived of the iodine, the gold amalgamated 

 is removed from the shaded, and the oxide of copper from the light 

 parts, by acid. The latter will then be protected against the further 

 action of the acid by the gold, and we obtain a deep engraving. 



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