360 



A ruby glass, coloured by gold, is well known. This is considered 

 by the author as analogous to the ruby fluid just spoken of, being a 

 diffusion of gold particles through vitreous matter. The ruby fluid 

 by association with jelly is rendered much more permanent than 

 before ; and then it may by a little warmth be had in the fluid state, 

 or by cooling as a tremulous jelly, or by desiccation as a hard ruby 

 solid, presenting all the transitions between the gold fluid and the 

 ruby glass. By soaking the dried jelly and then warming it with 

 water, these transitions may be passed through in the reverse di- 

 rection, and so on, any number of times. 



The relations of gold (and other metals) to polarized light are of 

 the following nature. A leaf of gold inclined at a certain angle across 

 a ray of polarized light (the inclination not being in the plane of 

 polarization or at right angles to it), affects it as a thin plate of any 

 uncrvstallized transparent substance would do, i. e. the light appears 

 in the analyser, and the plane of polarization is rotated ; or if a leaf 

 of gold be held in an inclined position across a ray of unpolarized 

 light, the beam is polarized as it would have been in passing through 

 a like inclined plate of uncrystallized transparent matter. The gold 

 greened by heating or pressure, when thus examined, does not appear 

 to have acquired any particular tension or structure. Sulphide of 

 carbon and crown-glass are optically so near each other, that a plate 

 of the latter immersed in the former is neutralized ; and though 

 placed in an inclined position to a ray of light, either polarized or 

 not, does not then affect it ; but gold (and all metals) is still far 

 above either of these. Hence the gold films obtained by phosphorus, 

 when attached to glass, could be examined, and were found to have 

 the optical properties of leaf-gold ; the effect having no reference to 

 the thickness of the film, but being most perfect in the thinner films 

 because they were in a more regular and perfect condition. It should 

 be remembered that these films are not continuous layers like coats 

 of varnish or fluid, but easily pervious to vapours. In like manner 

 the deposits of gold (and other metals) obtained by electric deflagra- 

 tions, were examined and found to have the same marked qualities 

 in a high degree ; places where the film was scarcely visible on the 

 glass, instantly showing the presence of the gold by their action on 

 the polarized ray. In the same manner the very thin and almost 

 invisible films, deposited occasionally on the sides of the vessels con- 



