April 4, 1895] 



NATURE 



547 



some of the colours contained in ordinary daylight are more 

 absorbed than others, and the light emerges more or less 

 coloured ; in consequence of differences of absorption, some 

 gem-stones appear differently coloured according to the direction 

 in which one looks through them. I need not dwell upon this 

 curious property because the instrument used to observe it is the 

 one piece of scientific apparatus sometimes, but by no means 

 generally, used by gem experts — I mean the instrument known as 

 thedichroscope. (A diagram, kindly lent by Prof. Judd, illustrated 

 the appearance seen with this instrument.) Far less familiar is the 

 method of studying the absorption by means of the spectroscope, 

 although the value of this extremely simple method was pointed 

 out many years ago by Prof. Church. Every one knows the 

 colours of the spectrum seen by daylight through a glass prism, 

 and it is also well known that if light transmitted through 

 various vapours be appropriately observed through such a prism 

 by means of the spectroscope, certain black lines are seen in 

 the spectrum, indicating that the vapour has absorbed light of 

 a certain colour ; in this way astronomers are able, by merely 

 looking at the sun and stars, to ascertain many of the elements 

 which they contain. 



But it is not commonly known that a precisely similar effect is 

 produced by many transparent minerals. It is only necessary 

 to look through a pocket spectroscope in a bright light at any 

 transparent mineral containing the rare element didymium, and 

 certain black bands characteristic of that element are at once 

 seen in the spectrum. 



(A diagram of the spectrum of the phosphorescent light 

 emitted by ruby when made to glow in the electric discharge in 

 a vacuum tube, lent by Prof, (irookes, though not a picture 

 exactly of what is here described, served to illustrate the appear- 

 ance of the black bands in the spectrum of a red mineral.) 



Now, there are two gem-stones which give very characteristic 

 black bands when looked at through a spectroscope, namely, the 

 jargoon or jacynth, and the variety of garnet known as alman- 

 dine, commonly called carbuncle. When a stone, say one set 

 in a ring, is looked at in this way, and gives the characteristic 

 spectrum of zircon, it is at once known to be a jargoon without 

 further trouble. 



When one remembers how many pocket spectroscopes are 

 bought by people who wish to see the rain-band and predict the 

 weather, it is surprising that it has not also come into use for 

 the examination of gems. 



To pass from optical to other characters, there is a very re- 

 markable property possessed pre-eminently by one mineral wtiich 

 has not, so far as I know, been previously recommended as a 

 practical test. 



A crystal of tourmaline while being warmed or cooled be- 

 comes electrified ; one end becomes charged with positive, the 

 other end with negative electiicity. The fact has long been 

 known. But a few years ago an extremely pretty and ingenious 

 way of showing the electrification was devised by Prof. Kundt. 

 If a mixture of powdered red lead and sulphur be shaken or 

 blown through a sieve, the particles become electrified by mutual 

 friction, and if it then be dusted upon a crystal of tourmaline 

 which is being warmed or cooled, the positively electrified end 

 of the crysial attracts the negatively electrified yellow sulphur, 

 and the other end attracts the positively electrified red lead ; 

 one end of the crystal becomes red, and the other end yellow ; 

 and so the difference of electrification is made visible. Now 

 every crystal of tourmaline behaves in this way, and I find it 

 perfectly easy to show the properly in an ordinary small jewel, 

 even when mounted in a setting. .\\\ that is necessary is to 

 warm the stone, and then, while it is cooling, to dust it with ihe 

 mixture ; at once one part of the stone becomes red, and another 

 part yellow. 



(.\ facetted stone treated in this way was shown upon the 

 screen by reflected light.) 



The last character which I have to mention is Ihe one to which 

 I alluded at the beginning, namely, the heaviness or specific 

 gravity. The use of the balance is, as I said, too laborious ; but 

 within the last few years an entirely different method has been 

 introduced. 



Cork and wood float in water because, bulk for bulk, they 

 are lighter; stone and iron sink because, bulk for bulk, they are 

 heavier than water. But find some substance whose density is 

 exactly that of water, and it will neither rise nor sink, but will 

 remain poised in the water like a balloon in mid air. 



Several liquids have been discovered which are more than 

 three and a half times as dense as water, in which, therefore. 



amethyst, beryl, and other light stones will actually float. Prof. 

 Church strongly recommended mercuric and potassium iodide ; 

 but a still more convenient liquid is now available, namely, 

 methylene iodide. This liquid has a specific gravity of 33, so 

 that tourmaline readily floats in it ; further, it is not corrosive 

 or in any way dangerous, which is more than can be said for 

 several of the other liquids which have been recommended. 



Now it is scarcely possible to prepare a number of liquid^, 

 each having the specific gravity of one gem-stone, in order to 

 identify each stone, but methylene iodide is easily diluted by 

 adding benzene to it ; each drop of benzene added makes the 

 liquid less dense, and so it may be used to separate tourmaline 

 and all the lighter gem-stones from each other. Nothing can be 

 easier or more satisfactory than this method ; no matter how 

 minute the stone may be, it can be identified by its density in a 

 few moments. Suppose it be doubtful whether a certain gem 

 is aquamarine or chrysoberyl, all that is necessary is to place it 

 in a tube of the liquid, together with a small fragment of true 

 aquamarine to serve as an index ; if it be a chrysoberyl, which 

 has a specific gravity of 3'6, it will sink like lead; if it be 

 an aquamarine, which has a specific gravityof2'7, it will float ; 

 and if the liquid be then stirred and diluted until the index 

 fragment is exactly suspended, the gem also will neither float 

 nor sink, but will remain poised beside it. 



The delicacy and simplicity of the method is marvellous ; the 

 only reason why it has not been more generally adopted is that, 

 unfortunately, the greater number of gem-stones are heavier 

 than methylene iodide. What is the use of employing such 

 liquids whenjthey cannot float jargoon, carbuncle, sapphire, 

 ruby, chrysoberyl, spinel, topaz, peridote, and diamond, to 

 mention only those stones whose names are familiar? 



But this objection is now entirely removed, thanks to a 

 discovery made quite recently by the distinguished Dutch 

 mineralogist, Retgers. He has found a colourless solid com- 

 pound which melts, at a temperature far below that of boiling 

 water, to a clear liquidfive times as dense as water ; and there- 

 fore sufticiently dense to float any known precious stone. 



This compound is the double nitrate of silver and thallium, 

 and it further possesses a most remarkable property ; it will 

 mix in any desired proportion with warm water, so that by 

 dilution the specific gravity may be easily reduced. The fused 

 mass may be reduced in density by adding water drop by drop 

 so as to suspend in succession jargoon, carbuncle, sapphire and 

 ruby, chrysoberyl, and spinel. 



This wonderful compound should certainly be employed by 

 all who wish to distinguish gems with ease and certainty. 



Let me now remind you how one could apply the methods 

 which I have been describing, to identify with absolute certainty 

 some gem-stone. Take, for example, a cut tourmaline. 

 Dropped into methylene iodide it would just float, and, when the 

 liquid is diluted, it would remain suspended beside an inde.K 

 fragment of tourmaline, and no other gem-stone. Examined 

 with the dichroscope it would show two coloured images, in- 

 dicating remark.ible diiTerences of absorption characteristic of 

 tourmaline, and no other mineral ; the absence of absorption 

 bands, when it is viewed through the spectroscope, would show 

 that it is neither garnet nor jargoon ; in the polarising 

 microscope it would show the interference figure of tourmaline. 



Even if the stone were mounted in a sttling so that these 

 tests could not be applied, it could be examined with the 

 reflectometer, the boundary of the shade would cross the scale 

 at a point exactly corresponding to the refractive yower of 

 tourmaline ; and lastly, it could be warmed and dusted with 

 red lead and sulphur, when the two coloured patches would 

 betray the electrical properties of tourmaline. There is 

 enough evidence here to satisfy any one but an English jury 

 hearing expert witnesses, and everything can be done with- 

 out inflicting even a scratch upon the stone. 



Another mineral character of great value in distinguishing 

 gem-stones in the rough I have not alluded to, because it can 

 only be made use of when they are more or less well crystallised ; 

 I mean the shape of the crystals. (This feature was illustrated 

 by some very beautiful photographs of gem-stones and other 

 minerals in their natural state, which were taken from specimens 

 in the British Museum by the distinguished photographic 

 expert, Mr. Ilepworth.) 



It might be asked, with some show of reason, why should we 

 require all these scientific tests which I have described, when 

 the varieties of precious stones are so few in number ? In reality, 

 however, gem-stones are far more numerous than is commonly 



NO. 



1327, VOL. 51] 



