NATURAL PHILOSOPHY. 141 



transmitted light, the other a combination of all the pencils reflected 

 from the anterior surfaces of the films. He then pointed out the 

 difference between artificial glasses and naturally-formed cyrstals, 

 like rock crystal. In the glasses the atoms are forced, by melting 

 them at high temperatures, to unite by chemical affinity. In the 

 others the particles have united by peculiar polar actions while crys- 

 tallizing naturally. Hence, the atoms of crystals being simple and 

 similarly united throughout the entire crystal, have no tendency to 

 decompose or reunite in other forms at particular parts ; but the 

 forces by which the earths, alkalies, and metals are composed, net 

 being uniformly arranged as to the forces by which the different 

 parts are held together, tend to separate and reunite in new or 

 more natural crystalline relations in relation to particular points, 

 lines, or surfaces in their mass. 



Thus, the rock-crystal lens found by Mr. Layard at Nineveh was 

 as perfect in its structure now as it was many thousand j r ears ago, 

 when in the form of a crystal, while the glass was found altered as 

 in the specimens now shown ; and few bodies cease to exist with 

 such grace and beauty as glass, when it surrenders itself to time 

 and not to disease. In stables, where ammonia and other exhala- 

 tions prevail, and in damp localities, or where acids or alkalies pre- 

 vail in the soil, the process is more rapid, and it may frequently 

 be broken between the fingers of an infant, sometimes presenting in 

 the middle a plate of unaltered glass, to which the process has not ' 

 extended ; but it is in dry localities, where Roman, Greek, and 

 Assyrian glass has been found, that the process of decomposition 

 is exceedingly interesting, and its results singularly beautiful. At 

 one or more points in the surface of the glass the decomposition 

 begins. It extends round that point in spherical surfaces, so that 

 the first film is a minute hemispherical cup of exceeding thinness. 

 Film after film is formed in a similar manner, till perhaps 20 or 30 

 are crowded into the 50th of an inch. They there resemble the 

 section of a pearl (or of an onion), and as the films are still glass, 

 the colours of thin plates are seen when we look down through 

 their edges, which form the surface of the glass. These thin edges, 

 however, being exposed to the elements, sutler decomposition. The 

 particles of si lex and the other ingredients now readily separate, 

 and the decomposition proceeds downwards in films parallel to the 

 surface of the glass ; the crystals of silex forming a white ring and 

 the other ingredients rings of a different colour. Such is the pro- 

 cess round one point, but the decomposition commences at several 

 points ; generally these points lie in lines, so that the circles of de- 

 composition meet one another and form sinuous lines. When there 

 are only two points near, these circles of decomposition surround 

 the two points, like rings round two knots in wood ; but when 

 there are many points near, the curves unite and form sinuous 

 lines. When the decomposition is uniform, and the littlo hemispheres 

 have nearly the same depth, we can separate the upper film from 

 the one below it ; the convexities of the one falling into the con- 

 cavities of the others. The drawings of these were executed by 



