220 Precious Stones. 



they would be valuable for the purposes of the optician ; but as 

 they do not comply with these conditions, the glassmakers have 

 been applied to, and they have made glass of very high specific 

 gravity, and nmch more refractive than in ordinary kinds. 



The diamond is usually spoken of as infusible, but this is 

 open to doubt. Professor Miller says, " When the diamond is 

 introduced into the name of the voltaic arc it undergoes a 

 remarkable change ; as soon as it becomes white hot it begins 

 to swell up, loses its transparency, suddenly acquires the power 

 of conducting electricity, becomes specifically lighter, and is 

 converted into a black opaque mass resembling coke." The 

 swelling up looks very much like incipient fusion, and in an 

 experiment made by Mr. Gassiot with a very powerful Grove's 

 battery, the appearance of fusion was greater than had been 

 previously noticed ; and when the action of the battery was 

 arrested, the change into ' c a substance resembling coke " had 

 not taken place. 



Mr. Emanuel follows the common practice of speaking of 

 the diamond as pure carbon. Professor Miller, however, states 

 that it is not so, and that when burnt " it always leaves a 

 minute yellowish ash, which has been found to contain silica 

 and oxide of iron."* 



The practical use of the diamond in cutting glass is well 

 known. For this purpose a particular angle is required, and 

 diamonds possessing it are called " glaziers." It is astonishing 

 what wonderful fine writing points can be obtained. Mr. 

 Farrants tells us that with such a diamond point and Mr. 

 Peters' machine the Lord's Prayer has been legibly written in 

 the three hundred and fifty-sixth thousandth part of an inch. 

 In this specimen the writing is so small, that in similar cha- 

 racters the Bible and Testament, said to contain 3,566,480 

 letters, could be written twenty-two times in the space of an 

 English square inch. With a diamond Mr. Norbert rules his 

 amazing lines on glass, putting them at the rate of seventy 

 in one thousandth of an inch in the closest of his bands. 

 Those who have not an opportunity of seeing one of Mr. 

 Norbert's ruled glasses, should look at Mr. Eichard Beck's f 

 admirable plate, representing their appearance when magnified 

 thirteen hundred times. Under this power the closest lines 

 were almost in contact. 



Another practical use of the diamond is the employment of 

 it in a powdered state, to cut stones too hard for any other 

 material to operate upon. The diamond dust forms teeth in 

 the rim of a steel wheel used in these processes. 



The art of cutting the diamond in regular facets was not 



* Elements of Chemistry, toI. ii., p. 61, second edition, 

 t Beck on the Microscope. Van Voorst. 



