Joly — On the Minerals of the Dublin and Wiclrfow Granite. 51 



very small, often only a couple of centimeters in diameter, and 

 frequently appear on the surface of the granite in great numbers 

 imparting a very extraordinary appearance to the rock. The 

 figure on plate iv. from a photograph, shows a very lovely speci- 

 men full size. It is of a delicate pale, bluish-green colour ; trans- 

 lucent. The radii pass through the block of granite removed with 

 it, appearing in coarse, crowded, hexagons on the other side, and 

 mixed with tourmaline. They are there of a rusty-brown colour. 

 Some of the crystals must scale over 15 cms. in length. This 

 specimen was found by Mr. Grerald Stoney, in company with 

 Mr. K. Doyle. 



The specific heat 1 of Grlencullen beryl, taken by the method of 

 condensation, was found to be O21401. The specimen used was a 

 green crystal taken from a group of radiating prisms. 



Experiments on loss of Colour. 



It has long been known that emeralds calcined at a low red 

 heat lose colour, becoming white and opaque, and parting with 

 water and organic matter. Such are the results of Lewy's experi- 

 ments, who ascribes the colour of emeralds to the presence of 

 organic matter. 



It appeared of interest, as throwing some light on the history 

 of these beryls, and on that of the granite containing them, to 

 repeat the experiment, and if possible fix an inferior limit to the 

 decolourizing temperature. 



Experiment 1. — A preliminary experiment on some fragments 

 of green beryl showed that a temperature far below that of red 

 heat sufficed to bleach and render opaque. The fragments were 

 heated on copper foil, over a fire for a few minutes, they could 

 almost be handled immediately on removal. 



Experiment 2. — Fragments of green and yellow beryl, dropped 

 into a test tube containing boiling mercury, lost nearly all colour 

 after about one hour's heating. 



Experiment 3. — Fragments of green and yellow beryl, sealed 



1 I hope shortly to puhlish an account of this method of investigation, and of the 

 means by which I hope to make it generally available. 



