Joly — On the Minerals of the Dublin and Wicklow Granite. 67 



the lumpy parts, washing and cleaning in boiling hydrochloric 

 acid, clear, glassy crystals, of extremely small size, appeared 

 in countless numbers through the residue. These, mounted in 

 Canada balsam, proved to be the identical crystals visible in the 

 sections — some beautifully sharp and clear, some partly decomposed 

 and overspread with a filiform, branching growth of olive-yellow 

 colour. Micrometric measurements gave 0*1 mm. as the length of 

 the larger specimens showing good angles. With such dimensions 

 it was difficult to deal with them singly. 



Again, by breaking up the hexagons and crushing the felspa- 

 thic matrix containing these crystals, treating carefully with 

 hydrofluoric acid, specimens were obtained fairly clean. But the 

 first source, where decomposition had removed the beryl and some 

 of the orthoclase, but had spared the small sharp crystals, was my 

 great source of supply. In these hollows slow-acting decomposi- 

 tion has effected a fairly perfect isolation, and I have opened cavi- 

 ties from which the tiny crystals could be poured in great numbers, 

 only requiring cleansing from their rust-coloured coating to be 

 ready for the microscope. 



The slides composed of these crystals present a spectaole of such 

 perfection of form, and, in the polariscope, such richness of colour, 

 as would far surpass any power of description. Feeling this, I will, 

 instead, refer the imagination of my reader to the soft crimsons, 

 purples, and tender blues of those cloud islands and vistas seen at 

 sunset, where the colour is not the dead brightness of opaque 

 reflection, but is living with transmitted light. And I would 

 remind him, that while in that case the imagination is affected by 

 the far-off peace of those regions to clothe them with an unreal 

 richness and tenderness of tint, these children of the rocks are not 

 so seen with the eyes of dreamland. I will ask him, then, to pic- 

 ture a precision of form and matchless depth of colour which, to 

 none but the scientific imagination, are as breathless objects of 

 adoration, as the infinite oceans of sunset. 



Having obtained the mineral thus isolated from its matrix, it 

 was resolved to treat some of the little crystals on the meldometer 

 along with orthoclase, and also compare their behaviour at high 

 temperatures with topaz, quartz, &c. I had only just begun to 

 use the apparatus, and was desirous of testing its value as a means 

 of differentiation ; for although no determinations of melting points 



