110 GOLD. —Fulminating Gold. Purple of Cassius.

reference to the changes in these colours. Thus,when the bottle containing the solution was chemi-cally clean, the particles remained suspended formonths, giving a ruby red colour to the liquid bytransmitted light, but it appeared to be turbid andbrown by reflected light. On adding a little com-mon salt this coloration entirely disappears, andthe gold is in the course of a few hours depositedas a purple powder. The perchloride forms severaldouble chlorides, the chief being those with potas-sium, sodium, and ammonia. Gold combines withiodine, forming two iodides (Aul and Aul 3 ),which correspond in composition with the chlorides;these however are unimportant, as is also the onlycompound of gold and bromine, namely, the ter-bromide (AuBr 3 ).

The cyanides , two in number, also correspond incomposition with the chlorides. They have a greattendency to combine with other cyanides, especiallythose of the alkalies and alkaline earths.

The protocyanide (AuCy) may be prepared inthe following manner. Fulminating gold is pre-pared as before described by the addition of ammoniato perchloride of gold. It is then dissolved incyanide of potassium with the application of heat.On cooling, crystals of the double cyanide of goldand potassium are deposited. If these be nowdissolved in hydrochloric acid, evaporated to dryness,and mixed with water, a yellow insoluble powderremains behind, which is the protocyanide of gold,and should be washed and dried in the dark. Itexists both amorphous and crystalline, and is easilydecomposed by heat. The most important of thedouble cyanides is that of gold and potassium(AuKCy,), largely used in electro-gilding, for whichpurpose it may be prepared by dissolving in warmwater 77 parts of the protocyanide of gold, and 23parts of cyanide of potassium, and evaporating theliquid; the double cyanide remaining behind in theform of colourless prismatic crystals.

The percyanide (AuCy 3 ) is an unimportant saltprepared by treating the percyanide of gold and silver(AgCy,AuCy 8 ) with hydrochloric acid; evaporatingthe solution in a vacuum, and redissolving in-water,out of which the salt is crystallized by concentration.

Fulminating Gold .—As mentioned in the accountsof the oxide and chloride of gold, this may be pre-pared from both of these salts, but although detona-ting power exists in both cases, the compounds arenot identical. In the former (prepared from theperoxide) there is no chlorine present, whereasthat prepared from perchloride of gold does containchlorine. In the one case the colour is dark greyish,whereas the presence of chlorine imparts a yellowcolour to the fulminate. It is possible to decom-pose it without detonation by heating with from 20to 30 times its weight of sulphate of potassium.To the first form Berzelius gives the formulaAu(NH 4 )0. 2 NH s , or AuII 0 2 ,2NII 3 ; whereas Dumasconsiders it to be more correctly expressed byAu 2 N,NII 3 ,3II 2 0. This latter chemist gives thefollowing as the result of an analysis of the formcontaining chlorine:—

Gold,. 73 0

Chlorine,... 4'5

Nitrogen. 9*8

Hydrogen,...,. 2*2

Oxygen,. . 10*5

100*0

The composition therefore of fulminating gold, asdeduced from these percentages, is expressed by theformula—

Au 3 ClN„H 8 ,5H a O.

Both preparations are extremely dangerous, deton-ating by a slight blow, by friction, by a slight heat,or often even spontaneously. They should there-fore only be prepared in very small quantities ata time.

Purple of Cassius .—When a mixture of proto-chloride and bichloride of tin, very much diluted, isadded drop by drop to a dilute neutral solution of ter-chloride of gold, a flocculent purple deposit is formed,which has been termed purple of Cassius, from thecircumstance that the method of preparing it wasfirst described by Andreas Cassius of Leyden andhis son, in 1685, and they called it gold purple. Itmay likewise be prepared by fusing together 150parts of silver, 20 of gold, and 35*1 of tin, and actingon the alloy with nitric acid, which dissolves out thesilver, and leaves a purple residue containing thetin and gold. To prevent the oxidation of the tinduring the fusion the three metals should be pro-jected into a red-hot black-lead crucible, which con-tains a little melted borax. Fuchs states that thefinest purple is produced by mixing a solution ofsesquichloride of iron with aqueous protochlorideof tin, till the yellow colour is converted into palegreen, and precipitating the gold solution with themixture thus formed; he adds that the protochlorideof iron in the liquid does not affect the product.

If while this powder is moist on the filter it bewashed with ammonia, it is dissolved, and a splendidpurple liquid passes through. From this liquid thecolouring matter separates only after standing for-several weeks. To obtain a gold purple of constantcomposition the following method is recommendedby Pelletier :—20 grammes of gold are dissolvedin 100 of aqua regia, made by the addition of 20parts of nitric acid to 80 of commercial hydro-chloric acid, the solution being evaporated to drynessin a water bath. The residue, after being dissolvedin water, is filtered and diluted with 7 or 8 decilitresof water, and placed in contact with tin filings;a vigorous action ensues, and in a few minutes theliquid becomes brown, and deposits a purple pre-cipitate, which should be washed and dried at agentle heat.

This purple possesses the following composition:—■

Stannic acid. 32*716

Protoxide of tin. 14*618

Protoxide of gold,. 44*772

Water. 7*864

100*000

Other modes of preparation have also been recom-mended, which however need not be described.