66 bastin: colloidal gold and silver 



reaction proceeded similarly until the yellow color of the solu- 

 tion gave way to pale green-blue ; upon further heating, however, 

 the solution became deep blue by transmitted and bright brick red 

 by reflected light, due, as later proven, to the presence of gold in 

 colloidal solution. This solution remained unchanged for days. 

 To exclude the possibility of this very intense coloration being 

 due to the action of the copper salt formed, on the gelatine, 

 gelatine solution was added to the solution resulting from the 

 action of AuCls solution alone on chalcocite; even in prolonged 

 boiling there was no intensification of the pale coloration of this 

 solution To determine whether the brown gold once precipi- 

 tated can be '' dissolved" in an emulsoid, the piece of gold-coated 

 chalcocite obtained from the action of AuCls solution alone was 

 boiled in a gelatine solution. There was no noticeable change. 

 Although some metallic minerals such as smaltite precipitate 

 lustrous bronze-colored gold from auric chloride solution under 

 the conditions of these experiments, none known to the writer 

 precipitate dull reddish-brown gold, except chalcocite. It is 

 well known, however, that many metals precipitate dull brown 

 to black gold from AuCls solutions, and it was desirable to deter- 

 mine whether colloidal gold solutions could be obtained by the 

 aid of these metals. One part of ^ AuCls solution was mixed 

 with one part of dilute gelatine solution and to equal parts of 

 this solution in four test-tubes were added small amounts of 

 metallic zinc, copper, cadmimn and tin. The solutions were 

 then boiled. In each case dark-brown to nearly black gold was 

 precipitated on the metal. With zinc and copper colloidal gold 

 solutions were obtained which were identical in appearance with 

 that obtained with chalcocite. Cadmium yielded a colloidal 

 gold solution that was dark brick-red by reflected and purple 

 by transmitted light. ^ Tin yielded the well-known ''purple 

 of Cassius" solution. 



' The appearance of colloidal gold solutions is so characteristic that there 

 could be no reasonable doubt upon this ground alone of the nature of the solutions 

 obtained. Several of them were, however, through the courtesy of Mr. George 

 Vinal of the Bureau of Standards, examined under the ultra-microscope and 

 found to show all the characteristics of colloidal suspensoids. Moreover they 

 deposit films of gold upon standing for several days. 



