COLLOIDS 237 



violet or rose according to circumstances. A mixture of tin 

 salts added to a solution of gold also yields the purple of Cassius 

 which is long retained in suspension. The most beautiful ruby 

 glass also owes its colour to gold which is certainly in the metallic 

 state, not only on account of the fact that the temperature of 

 melting glass is far above that at which all known compounds of 

 gold are decomposed, but the particles have been examined by 

 the ultra-microscope and are known to be for the most part 

 spherical. Some of the coloured liquids just mentioned must 

 have been known to the alchemists, and it is not improbable 

 that one of them was an ingredient in the " elixir," the composi- 

 tion of which was their chief subject of study. " Soluble gold," 

 which was actually used in medicine down to the end of the 

 seventeenth or beginning of the eighteenth century, probably 

 consisted of one of them. 



To prepare a coloured solution or pseudo-solution of gold it is 

 only necessary to add to a weak solution of the chloride in water 

 any one of many easily oxidisable substances or as they are 

 called " reducing agents." Phosphorus has already been men- 

 tioned, but phosphorous and sulphurous acids, essential oils of 

 various kinds, formaldehyde, sugars, hydrazine, hydroxylamine 

 and many other substances have been used. The colour pro- 

 duced depends on the agent used. Thus when a weak solution of 

 pure chloride of gold is exposed to contact with carbonic oxide 

 gas bubbled through it, a red colour is produced which is pretty 

 stable, and the hydrochloric acid which is left in the liquid as a 

 consequence of the deposition of the gold can be removed by 

 dialysis. 



If the same gold chloride solution is neutralised very carefully 

 by means of a weak solution of sodium carbonate and a very 

 dilute solution of hydrazine hydrate is added drop by drop a 

 blue liquid is formed. And further, according to A. Gutbier 

 (Zeit. Anorgan. Chem., 1904, pp. 112-114), the same reducing < 

 agent added to the same gold chloride solution in successive 

 portions is capable of producing several colours successively. If a 

 very weak solution of gold chloride is mixed with a few drops of 

 a very weak phenylhydrazine hydrochloride solution a red 

 colour is first produced, a little more of the reagent produces a 

 violet colour, which with the addition of further quantities 

 becomes bluish and ultimately deep blue. 



These coloured liquids are producible in a very curious manner 



