■o 



NATURE 



\_May 1 6, 1878 



necessarily causing the image formed to be seen ap- 

 parently situated at n, a point different from m. The 

 relative quantities of light passing along e P and b P (that 

 is, the relative brightnesses of the two images) depend on 

 the degree of obliquity of the incident light c; the greater 

 the angle abv {i.e., the more obliquely the light falls on 

 the mirror), the brighter is the image at n. The power of 

 glass thus to reflect light to a considerable extent without 

 any metallic film behind is utilised in the illusion known 

 popularly as " Pepper's ghost," which consists simply of 

 a large pane of glass sloping forwards from the stage at 

 an angle of about 45° (Fig. 2). Objects such as A B, placed 

 between the footlights E, and the pane of glass F in a 

 horizontal position, and strongly illuminated, will produce 

 to a spectator in front at P, a virtual image or " ghost,' ' 

 apparently situated at C D, the illusion being heightened 

 by hiding, by means of screens, all the apparatus in front 

 of the pane from the audience, and darkening that part 

 of the stage behind the pane, the real objects furnishing 

 the ghosts being placed on a dead black ground. When 

 the lights E are extinguished, and other lights illuminating 

 the stage behind the pane turned on, the ghosts disappear, 

 whilst the real actors at DC on the stage behind the pane 

 become visible through the transparent glass. 



Most of the metals used in the arts in the free state are 

 of considerable density, aluminium being by far the 

 lightest, a circumstance which, together with its consider- 

 able strength and power of resisting the tarnishing effects 

 of the air, renders it peculiarly suitable for numerous 

 purposes : the draw-tubes of telescopes, opera-glasses, 

 &c., and the graduated circles of surveying-instruments, 

 &c., are often made of this metal for these reasons. 

 According to the way in which a piece of metal has been 

 obtained, its density will vary somewhat, being increased 

 by hammering or any mechanical action which forces the 

 particles together, e.g., wire-drawing or sheet-rolling. 

 The following table gives the numerical values of the 

 average densities of most of the lAore important metals : — 



Specific Craiity of Metals {Water = l). 



Platinum 

 Gold ... 

 Mercury 

 Palladium 

 Lead ... 

 Silver ... 

 Bismuth 

 Copper 

 Nickel ... 

 Cadmium 



7-8 



7*3 



7*1 



67 



5-6 



2-6 



1-8 



0-97 



0-86 



0-59 



Although the property of being drawn into wire is 

 closely allied to that of being rolled or hammered into 

 foil and leaves, yet the two are not necessarily possessed 

 to equal extents by the same metal ; gold, silver, and 

 platinum are pre-eminently "ductile," whilst copper and 

 iron are but little inferior to them in this respect. 

 Aluminium and zinc can be obtained in tolerably thin 

 wire, whilst lead and tin have so little cohesion that they 

 cannot be drawn beyond a very limited degree of fineness. 

 On the small scale, wires are readily obtained by casting 

 the metals into thin pencils,^ slightly pointing the ends of 

 these and passing them into a funnel-shaped hole in a 

 steel plate {draw-plate) of suitable size, gripping with 

 pliers the protruding pointed part, and forcibly puUing 

 the Avhole bar through the hole, the process being then 

 repeated with a slightly smaller hole. 



In drawing wire on a manufacturing scale, the process 

 is just the same in principle, only, instead of drawing the 

 wire through the draw-plate by hand by means of a 

 wheel and axle, &c., the wire is pulled through by hand 



* For metals of moderately-low melting-points the fused substance may be 

 drawn up into a hot thin glass tube or pipe-stem by suction, and allowed to 

 solidify therein. By fusing the metal in the bcwl of a tobacco-pipe and 

 tilting this so that the stem is inclined downwards, the m )lten metal can often 

 be made to form a rough wire or thin rod in the stem readily obtainable by 

 breaking away the pipeclay after cooling. 



with pliers for a foot or two, and this portion then 

 fastened to a revolving drum which then pulls the rest of 

 the wire through, coiling up the drawn-out portion on the 

 drum ; the wire is then passed through the next smaller 

 hole, being uncoiled from the first drum, and coiled again 

 on a second in so doing, and so on until drawn to the 

 required degree of fineness. In this way great lengths 

 of wire are drawn at one operation. 



By forming metals into wires of equal dimensions, and 

 then determining the weight requisite to break these wires, 

 the differences in tenacity exhibited by metals and alloys 

 may be readily demonstrated. A convenient apparatus 

 for this piu-pose is made of an iron tripod six or seven 

 feet high, the legs of which are stayed together at the 

 bottom and in the middle; from the top of the tripod 

 is suspended by a stout hook a dynamometer or spring 

 balance furnished with a hook at the bottom, whilst 

 about half way up the tripod is affixed a horizontal 

 axle, supported by the stays in such a position that the 

 centre of the axle is perpendicularly beneath the hook of 

 the dynamometer. This axle is provided with a winch, 

 and round it is coiled a stout rope or leather band with a 

 hook at the end. The wire to be tested is formed into a 

 ring about three or four inches in diameter, the ends being 

 intertwisted and soldered together ; the hooks attached to 

 the bottom of the dynamometer and to the rope are then 

 inserted in this ring, and the handle turned so as to wind 

 up the rope and stretch the ring until its form becomes a 



Fig. 3. 



narrow oblong. The tension is then increased by winding 

 the rope until the wire breaks ; the reading of the dynamo- 

 meter is noted by an assistant at the moment of rupture. 

 In this kind of way the order of tenacity of the metals 

 is found to be as follows : 



25 Iron. 

 16 Copper. 

 14 Platinum. 

 12 Aluminium. 

 10 Silver. 



8 Gold. 

 7 Zinc. 

 1-5 Tin. 

 I Lead. 



Closely connected with the physical structure which 

 enables metals to exhibit the phenomena of crystallisa- 

 tion, malleability, and ductility, is the power which some 

 possess of returning to their original shape when deflected 

 therefrom by some external force not too great {elasticity) ; 

 a property possessed to an extreme degree by good steel. 

 The operations of wire-drawing, rolling, hammering, and 

 the like generally increase the elasticity of metals, whilst 

 annealing and fusing usually diminish it. Some metals 

 are almost wholly devoid of elasticity ; thus lead scarcely 

 exhibits a trace of this property, being so soft that it is 

 readily abraded by the nail. Some metals and alloys, 

 when worked into appropriate shapes and struck, con' 

 tinue vibrating for some time, and hence are powerfully 



