324 



SCIENTIFIC RECREATIONS. 



consists in the substitution of one metal for another. The copper is dis- 

 solved in the liquid, and is replaced by the lead, but no metamorphosis is 

 brought about. We may vary at will the form of the 

 Platinum Density 21.50 V ase or the arrangement of the wire ; thus it is easy 

 to form letters, numbers, or figures, by the crystalliza- 

 tion of brilliant spangles. 



Mercury D. 13.56 Copper, when it is pure, has a characteristic red 



colour, which prevents it being confounded with any 

 other metal ; it dissolves easily in nitric acid, and 

 with considerable effervescence, giving off vapour very 

 abundantly. This property has been put to good use 

 in engraving with aqua fortis. A copper plate is 

 covered with a layer of varnish, and when it is dry 

 some strokes are made on it by means of a graver ; 

 if nitric acid is poured on the plate when thus pre- 

 pared, the copper is only acted on in the parts that 

 have been exposed by the point of steel. By after- 

 wards removing the varnish, we have an engraved 

 plate, which will serve for printing purposes. 



Among experiments that 

 Aluminium D. 2.55 m ay be attempted with com- 



mon metals, 

 Magnesium D. 1.43 



<?old D. 19.25 



Lead D. 11.35 



Silver D. 10.47 



Bismuth D. 9.82 



Copper D. 8.78 



Nickel D. 8.27 



Tin D. 7.29 



Iron D. 7.20 



Zinc D. 6.86 



Sodium D. 0.97 



Fig. 313. Representation of bars of metal, all of the same weight. 



mention 

 that in 

 which 



salts of tin are employed. Tin has a great tendency to assume a crystalline 

 form, and it will be easy to show this property by an interesting ex- 

 periment. A concentrated solution of proto-chloride of tin, prepared by 

 dissolving some metallic tin in hydrochloric acid, is placed in a test 

 glass ; then a rod of tin is introduced, as shown in fig. 3 I 5. Some water 

 is next slowly poured on the rod, so that it gradually trickles down, and 



prevents the mingling of the proto-chloride 

 of tin. The vessel is then left to stand, 

 and we soon see brilliant crystals s-tarting 

 out from the rod. This crystallization is 

 not effected in the water ; it is explained 

 by an electric influence, into the details of 

 which we cannot enter without over- 

 stepping our limits ; it is known as 

 "Jupiter's Tree." It is well known that 

 alchemists, with their strange system of 

 nomenclature, believed there was a certain 

 mysterious relation between the seven metals then known and the seven 

 planets; each metal was dedicated to a planet ; tin was called Jupiter ; silver, 



Fig. 314. Tree of Saturn. 



