io8 



NA TURE 



[Dec. 2, 1886 



colour of the gold was remarkable, but Boyle had only 

 produced one of the series of alloys most dreaded by 

 every jeweller — " brittle gold" — for the way in which an 

 alloy of gold and copper is affected by a small quantity 

 of impurity presents one of the most serious difficulties in 

 working gold. It has been known since the seventh 

 century, that minute quantities of certain metals render 

 gold brittle, and it may be well to demonstrate the fact. 



Here are two hundred sovereigns : I will melt them and 

 will add in the form of a tiny shot a minute portion of 

 lead amounting to only the 2000th part of the mass, first, 

 however, pouring a little of the gold into a small ingot, 

 which we can bend and flatten, thus proving to you that it 

 is perfectly soft, ductile, and workable. The rest of the 

 mass we will pour into a bar, and now that it is suffi- 

 ciently cold to handle, you s;e that I am able to break it 

 with my fingers, or at least with a light tap of a hammer. 

 The colour of the gold is quite altered, and has become 

 orange-brown, and experiments have shown that the 

 tenacity of the metal, that is, the resistance of the gold to 

 being pulled asunder, has been reduced from 18 tons per 

 square inch to only 5 tons. These essential changes in 

 the property of the metal have been produced by the 

 addition of a minute quantity of lead. I have cited these 

 facts mainly to show that the changes produced in the 

 colour and properties of metals by small variations of 

 composition were such as to lead the alchemists on in 

 their belief that it was possible to change lead or tin into 

 gold, and the hope in which they worked enabled them 

 to gather facts out of which chemical science was gradu- 

 ally constructed. We shall see presently that changes in 

 the colour of metals and alloys produced by the addition 

 of small quantities of foreign matter, are of great import- 

 ance in the application of metals to artistic purposes, but 

 we must first try to examine more closely some of the 

 prominent facts connected with the colour of metals, that 

 is, the effect metals have on light so as to produce the 

 effect of colour in our eyes. We are apt to think of gold 

 as being essentially and distinctively golden-yellow ; it 

 may, however, possess a wide range of colours without in 

 any way losing the condition of absolute metallic purity, 

 its relations to light depending entirely on the nature of 

 its surface, and especially on whether the metal is in 

 mass or in a more or less fine state of division. Interest- 

 ing as gold is to us in mass (and I may incidentally 

 mention that during my official connection with the Mint 

 I have been responsible for the quality of 462 tons of it) 

 it is perhaps still more interesting to us when beaten so 

 fine that a single grain, of the value of id., would cover a 

 space of 48 square inches, or when it is so finely divided 

 that the dimensions of a single particle may closely 

 approximate to those of the ultimate atom. 



This aspect of the question was investigated by Faraday, 

 and the experimental part of the subject remains practically 

 unadded to since his time. It is well known that a leaf 

 of gold when seen by transmitted light is either green or 

 blue, according to its thickness. Here is such a leaf of 

 green gold, as seen when light is actually sent through 

 it (Fig. i), so as to project a green disk on the 

 screen. A portion of the light will be reflected from its 

 surface, and this reflected ray may be caught in a mirror 

 and thrown on the screen so that you have, shown side 

 by side, the green disk of transmitted light and the 

 golden one of reflected light from the same leaf of gold. 



Gold may readily be converted into a soluble chloride 

 which produces a beautiful golden solution. If such a 

 solution contains very little gold, not more than 2 grains 

 in a gallon, and if certain chemical methods be adopted to 

 precipitate the gold, that is, to throw it out of solution in 

 a solid, though in a very fine state of division, the metal 

 may exhibit a wide range of tint, from ruby to black. 



[A few drops of phosphorus dissolved in bisulphide of 

 carbon had been added to about a gallon of a very dilute 

 solution of chloride of gold contained in a tall glass 



cylinder, as shown in the sketch (Fig. 2). The beam 

 from an electric light, thrown through the vessel, revealed 

 in the lower part the presence of finely-divided metal of 

 the natural golden colour, while the more finely-divided 

 gold in suspension imparted a brilliant ruby colour to the 

 liquid, and a glowing ruby disk was projected on a white 

 screen.] 



When gold is in the "ruby" state, it is so finely 

 divided that each particle probably approximates to the 

 dimensions of the gold atom. 



[The solar spectrum was then thrown upon the screen, 

 and the audience was invited to compare it with a dia- 

 gram which, while closely resembling the solar spectrum. 



really represented] the way in which pure metallic gold, 

 prepared by various methods, is capable of behaving in 

 relation to light so as to produce the sensation of a wide 

 range of colours.] 



It would be easy to show that light is similarly affected 

 by other metals ; but I have selected gold for the purpose 

 of illustration because it is easy to maintain it in a state 

 of purity, however finely divided it may be. We must there- 

 fore moLlify any views we may have formed as to a metal 

 having exclusively a special colour of its own, because it 



will be evident that a particular colour is only due to a 

 definite state of arrangement of its panicles. The inti- 

 mate relation between the state of the surface of a metal 

 and its colour is well shown by the beautiful buttons 

 devised by .Sir John Barton. He proved that if very fine 

 lines be drawn close together, so that about 2000 would be 

 ruled in the space of an inch, a beautiful iridescent effect 

 is produced, the tints being quite independent of the 

 metal itself due to an optical effect of the lines. 



[The image of such a button was then thrown upon the 

 screen.] 



