450 MATHEMATICS AND NATURAL PHILOSOPHY. 



cally, proves to be pure metallic gold. This deposit transmits various coloured 

 rays ; some parts are grey, others green, or amethystine, or even a bright ruby. 

 In order to remove any possibility of a compound of gold, as an oxide, being pre- 

 sent, the deflagrations were made upon topaz, mica, and rock crystal, as well as 

 glass, and also in atmospheres of carbonic acid and of hydrogen. Still the results 

 were the same, and ruby gold appeared in one case as much as in another. BeiDg 

 heated, all parts of the deposit became of an amethystine or ruby colour; and by 

 pressure these parts could be changed so as to transmit the green ray. The pro- 

 duction of fluids, consisting of very finely divided particles of gold diffused 

 through water, was spoken of before. These fluids may be of various colours, by 

 transmitted light, from ruby to blue ; the effects being produced only by diffused 

 particles of metallic gold. If a drop of solution of phosphorous in bisulphide of 

 carbon be put into a bottle containing a quart or more of very weak solution of 

 gold, and the whole be agitated, the change is brought about sooner than by the 

 process formerly described ; or if a solution of phosphorous in ether be employed, 

 very quickly indeed; so that a few hours' standing completes the action, All the 

 preparations have the same qualities as those before described. The differently 

 coloured fluids may have the coloured particles partially removed by nitration i 

 and so long as the particles are kept by the filter from aggregation, they preserve 

 their ruby or other colour unchanged, even though salt be present. If fine isin- 

 glass be soaked in water, then warmed to melt it, and one of these rich fluids be 

 added, with agitation, a ruby jelly fluid will be obtained, -which, when sufficiently 

 concentrated and cold, supplies a tremulous jelly ; and this, when dried, yields a 

 hard ruby gelatine, which being soaked in water becomes tremulous again, and by 

 heat and more water yields a ruby fluid. The dry hard ruby jelly is perfectly anala- 

 gous to the well known ruby glass, though often finer in colour, and both owe the 

 colour to particles of metallic gold. Animal membranes may in like manner have 

 ruby particles diffused through them, and then are perfectly analogous in their 

 action on light to the gold ruby glass, and from the same cause. When a leaf of 

 beaten gold is held obliquely across a ray of common light, it polarizes a portion 

 of it, and the light transmitted is polarized in the same direction as that trans- 

 mitted by a bundle of thin plates of glass ; the effect is produced by the heated 

 leaf as well as by the green leaf, and does not appear to be due to any condition 

 brought on by the heating, or to internal structure. When a polarized ray is 

 employed, and the inclined leaf held across it, the ray is affected, and a part passes 

 the analyzer, provided the gold film is inclined in a plane forming an angle of 45° 

 with the plane of polarization. Like effects are produced by the films of gold 

 produced from solution and phosphorous, and also by the deposited dust of gold 

 due to the electric discharge. The same effects are produced by the other defla- 

 grated metals so long as the dusty films are in the metallic state. As these finer 

 preparations could be held in place only on glass or some such substance, and as 

 glass itself bad an effect, it was necessary to find a medium in which the power 

 of the glass was nothing ; and this was obtained in the bisulphide of carbon. 

 Here the effect of gold upon a ray of light which was unaffected by the glass 

 supporting it, was rendered manifest, not only to a single observer, but also to a 

 large audience. The object of these investigations was to ascertain the varied 

 powers of a substauce acting upon light, when its particles were extremely 

 divided, to the exclusion of every other change of constitution. It was hoped 

 that some of the very important differences in the action upon the rays might in 



