NATURAL PHILOSOPHY. 209 



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M. Secclii adds, " The question whether volcanic action in the moon is 

 actually extinct, can be answered only after there shall have been made a 

 map of the moon's surface for a given period with the utmost accuracy and on 

 a large scale." It is to help onward this project, that he has undertaken the 

 work above described. 



IMPROVED METHODS OF ENGRAVING. 



The following are the details of a discovery by M. Devincenzi, of Paris, as 

 communicated by him to the French Institute, which it is thought may to a con- 

 siderable extent supersede wood, lithographic, and copper-plate engraving : 



Electrography has for its principal object to convert into 'an engraving 

 in relief all drawings made with a greasy, a bituminous, or a resinous 

 body upon a metallic plate. Amongst all metals, zinc is the most proper 

 for this kir.d of engraving, and its low price renders it still more desirable. 

 The kind employed is the laminated zinc of commerce, the surface of 

 which is grained with sifted sand, hi the same manner as the stones are 

 grained in lithography. You may draw on these plates, either with a 

 crayon, with lithographic ink, or with any other substance employed id 

 lithography. The plate, once drawn upon, is then prepared hi the same 

 way as if it were intended to be used for a lithographic impression ; indeed, 

 zinc plates have often been employed instead of stones, and it is to Senefelder 

 himself, the inventor of the lithographic art, that this application is due. The 

 effect of the preparation is: First, to render the crayon or drawing ink 

 insoluble in water, and to fix them on the plate ; and next, to change the 

 affinity of the metal. Zinc, in its natural state, has a great affinity for greasy 

 substances, and for this very reason can be easily drawn upon. But once 

 prepared, this affinity is altered : after the preparation the zinc has a greater 

 affinity for gum and water than for greasy substances. The slightest humidity 

 on its surface suffices to repel the latter. I give this preparation to the zinc 

 plate by plunging it for a minute into a simple decoction of gall nut, and after- 

 wards wash the plate with clear water, and then cover it again with a solu- 

 tion of gum arable. The decoction of gall nut is made with the gall broken 

 into good sized lumps, in the proportion of 125 grammes (something less than 

 lib.) in a litre (about If pint), reduced by boiling to half the quantity. The 

 zinc plates, which are used hi lithographic fashion, are generally prepared with 

 the same decoction of gall nut ; but, in imitating the preparation of the stones, 

 nitric acid is added, and often hydrochloric acid. These acids I entirely do 

 away with. It is known how delicate the operation is of preparing stones 

 for lithography, on account of these acids, for the preparation very often 

 injures the half tints by the action which the acids exercise both on the ink 

 and the stone. On the other hand, the simple decoction of the gall nut, 

 while it makes an excellent preparation, exercises no ulterior action either 

 upon the drawing or the plate. This experiment may be safely repeated. 

 After the drawing is made with lithographic chalk or ink upon a zinc plate, 

 the latter may be left for hours or even days hi the gall nut decoction with- 

 out any alteration being produced either in the lines or the surface of the 

 plate. In lithography, on the contrary, by prolonging the acidulation, both 



