May 1 6, 1878] 



NATURE 



69 



PHYSICAL PROPERTIES OF METALS \ 



ONE of the most characteristic properties of metals is 

 the power possessed by them when in more or less 

 compact masses of acquiring (by polishing, pressure, or 

 other mechanical treatment) such a condition of surface 

 that light incident thereon is for the most part again 

 reflected, whereby a peculiar glistening appearance is pre- 

 sented, known as the metallic Itcstre. 



Owing to the influence of the air, moisture, vapours 

 arising from putrefaction, &c., metallic surfaces, even 

 ■when highly polished and brilliant, become more or less 



Fig. 1. 



rapidly tarnished, so that the power of reflecting light is 

 to a considerable extent lost. Before the invention of 

 glass polished metallic surfaces were employed as ftiir- 

 rors ; and for reflecting telescopes such surfaces are still 

 in use. Now, however, it is usual to employ as mirrors 

 glass surfaces behind which a thin coating of some 

 lustrous metallic mass is placed, so that the smooth 

 surface of the glass at once determines the peculiar 

 reflective power of the metal applied to it, and preserves 

 the metal from mechanical injury and from the corrosion 

 of the air. For this reason these household appliances 



Fig a. 



are ordinarily termed " looking-glasses," although strictly 

 speaking it is not the glass that is the essential part. 



Three principal methods of applying these metallic sub- 

 stances to glass are in use ; the best plate-glass mirrors 

 (perfectly plain surfaces) are prepared by spreading out 

 on a table surrounded with a deep groove or gutter, and 

 capable of being raised on hinges so as to be placed at 

 any angle with the horizon, a sheet of tinfoil, and 



' From a forthcoming volume of the Nature Series — " Metals and their 

 Chief Industrial Applications. Being, with some Considerable Additions, the 

 Substance of a Course of Lectures Delivered at the Royal Institution of Great 

 Britam in 1877." By Charles R. Alder Wright, D.Sc, &c., Lecturer on 

 Chemistry in St. Mary's Hospital Medical School. (London: Macmillan 

 and Co., 1878.) 



smoothing it with a soft brush ; mercury is then poured 

 on and gently rubbed over the tinfoil with a hare' s foot 

 or a roll of flannel so as to penetrate and brighten the 

 tin ; more mercury is then poured on, and the surface 

 cleansed from dross, &c. ; finally, the perfectly clean sheet 

 of glass is dexterously slid over the brilliant mercurial 

 surface in such a way as to avoid inclosing any particles 

 of dust or air-bubbles between the metal and glass. 

 The table is then slightly raised at one end, so that the 

 surplus mercury may gradually run off and be caught in 

 the gutter ; and the slope is increased daily, a piece of 

 flannel being placed on the glass with weights on it to 

 facilitate the draining off of the mercury. 

 After two to four weeks, according to the 

 size of the plate, the mirror is complete, the 

 tin amalgam having then completely set, and 

 being tolerably firmly adherent to the glass, 

 although easily rubbed off and scratched on 

 account of its slight tenacity. To preserve 

 the back of the mirror from injury a suitable 

 wooden frame is provided, in which the 

 whole is fixed, when a finished mirror is the 

 result. 



For curved surfaces, such as the insides 

 of globes, flasks, &c., for ornamental pur- 

 poses, a somewhat different plan is em- 

 ployed : a fluid or semi-fluid amalgam 

 capable of adhering to glass is poured into 

 '"-. the vessel to be "silvered," and shaken 



"^ about therein until the inner surface is 

 ~". ~"^ covered with a film of the composition; 



^^^ the surplus amalgam is then poured out and 



used for other similar objects. A mixture 

 of one part each of lead, tin, and bismuth, with two 

 parts of mercury, answers well, the mixture being made 

 perfectly fluid by slightly warming it before pouring into 

 the vessel to be silvered. 



A method which has of late years come largely into 

 use for silvering mirrors of various kinds, and notably 

 the reflectors of telescopes and lighthouses, is based on 

 the power of certain chemical reagents to throw down 

 silver in the metallic state from certain of its solutions, 

 &c., the reduced silver in many cases adhering firmly to 

 the surface of the vessel in which the action takes place, or 

 to objects immersed in the liquid. Thus, 

 if calcium tartrate in a moist state be 

 placed in a glass vessel with a crystal of 

 silver nitrate and a drop of ammonia 

 solution, and the mixture cautiously 

 heated, and made to flow successively 

 over the whole inner surface of the glass, 

 a fine mirror may be developed. Alde- 

 hyde, oil of cloves, and other essential 

 oils, grape-sugar, and some other organic 

 substances, may also be employed as 

 reducing agents, especially the first sub- 

 stance. 



If a "mirror" {i.e., a glass surface 

 with a brilliant metallic film behind) be 

 carefully examined, it will be found that 

 in most positions it will give a double image of any 

 object reflected, one image being usually more brilliant 

 than the other. Fig. i illustrates how this is brought 

 about ; a ray of light from an object at a, strikes the 

 glass surface at b, and is reflected to the eye of the 

 observer at P, so that an image is seen situated at in. 

 Another ray of light incident on the glass at a point c, 

 is partly reflected along cf, this portion of the ray con- 

 sequently never reaching the eye at P at all ; the rest of 

 the ray enters the glass, being refracted along cdj at 

 the junction of the glass and metallic siyfaces reflection 

 takes place along de, and at e the ray is refracted along 

 ev, thus also reaching the eye of the observer, but 



