536 SYSTEMATIC DESCRIPTION OF TKECIOUS STUMES 



violent coughing. The non-volatile residue, amounting to about 70 per cent, of the weight 

 of the original material, is a shining black substance known as colophony of amber. It is 

 soluble in oil of turpentine and in linseed-oil, and its solution in either of these substances 

 is known as amber-lac or amber-varnish, and is much used for varnishing. This lac is 

 characterised by its great hardness on drying and only its dark colour stands in the way of a 

 more extensive application. When heated in linseed-oil amber becomes soft and pliable, a 

 property of which practical use is made, as we shall see later. 



When heated in the air amber ignites and burns with a bright sooty flame, hence the 

 German name Bernstein for amber. During combustion the same aromatic odour becomes 

 apparent, this being so characteristic of amber as to be sufficient to distinguish it from 

 other resins of similar appearance. It also leads to the use of a limited amount of amber 

 as frankincense. 



The physical characters of amber are in every respect those of a resin. It is 

 perfectly amorphous and shows no indication of crystalline structure. It occurs in irregular 

 rounded nodules having the form of rods, drops, plates, &c., and never in masses bounded 

 by plane surfaces. There is no cleavage and the fracture is conchoidal, but masses of 

 amber are often penetrated by irregular cracks. Sometimes a shelly, concentric separation 

 is observable, the mass being built up of thin layers one within another, and only loosely 

 held together. Specimens of this kind are known as shelly amber, while those which are as 

 compact as if they had been cast are known as massive amber. In extreme cases these two 

 kinds diff*er from each other very widely, but there are specimens to represent every possible 

 srradation between the two. The structure is closelv connected with the mode of formation 

 of the amber, and has an important bearing on the practical application of the stone. 



The specific gravity of amber ranges from 1*05 to ] 'lO ; the substance is thus only 

 slightly heavier than water, and approximates still more closely to the density of sea- water. 

 Its hardness, H = M, is a little greater than that of gypsum, and it can therefore be scarcely 

 scratched with the finger-nail. It is harder than most other resins, a difference which affords 

 another useful distinguishing feature. It is not very brittle and can therefore be carved, 

 worked on the lathe, or bored with little difficulty — important practical considerations. 

 When amber is cut with a knife parings are not obtained but only powder. 



When rubbed on cloth amber becomes strongly charged with negative electricity, and 

 attracts to itself scraps of paper and other light objects; it is, indeed, from the ancient 

 name for amber, electron, that the word electricity is derived. When rubbed very 

 vigorously the characteristic odour becomes perceptible, but the amber does not become 

 sticky as do other resins under similar circumstances, since the temperature of its melting- 

 point is far higher than that induced by the vigorous rubbing. Amber is a bad conductor 

 of heat, and in consequence feels warm when in contact with the hand. This character 

 alone serves to distinguish it from minerals and from glass of similar appearance, since these 

 feel cold to the touch. 



In most cases amber possesses a fine resinous lustre, which is considerably heightened 

 by polishing, but some specimens are dull and are not improved by polishing, these being 

 therefore unsuitable for ornamental purposes. 



Amber ranges from perfect transparency to complete opacity. In the same specimen 

 thei'e may be clear and cloudy portions side by side, these merging gradually into each 

 other and never sharply defined. This is a characteristic feature of genuine amber, and one 

 which distinguishes it from certain similar substances, to be mentioned below. In 

 transparent specimens it may be observed that amber is singly refracting, this being in 

 accordance with its amorphous character. A feeble double refraction is sometimes observed. 



