On Crystals in the Cavities oj Minerals. 4-97 



ported in by a comparison of a specimen kindly furnished me 

 by Dr. Hofmann * ; further investigations will clear up this 

 point : in the meantime I refrain from proposing a name, as 

 Liebig f has lately proposed the name Tyrosine for the sub- 

 stance prepared from casein. As the latter body arises evi- 

 dently from a process of oxidation, and as I had obtained the 

 first crop of crystals from a liquid from which the colouring 

 matter had been precipitated by the basic nitrate of lead, I 

 thought that this body might owe its formation to the action 

 of the nitric acid liberated by the sulphuretted hydrogen ; but 

 this supposition proved to be erroneous, for in later experi- 

 ments in which acetate of lead had been used, the same body, 

 and in exactly the same quantity, was obtained. From this 

 we may assume that this substance is contained ready-formed 

 in the cochineal insect. 



My engagements for the present preventing me from con- 

 tinuing these researches, I must defer for a future period their 

 completion, but hope to be enabled to communicate to the 

 Society a second paper. In conclusion I may be allowed to 

 express my thanks to my friend Dr. Hofmann for his valuable 

 instruction in the methods of organic research, and his kind 

 advice during the progress of this investigation. 



LXXIII. On the Existence of Crystals tvith different jn-imitive 



forms and j)hysical properties i7i the Cavities of Minerals ; 



with additional Observations on the Nexv Fluids in which 



they occur. By Sir David Brewster, K.H., LL.D., 



F.'R.S., and V.P.R.S. Edin.X 



[With a Plate.] 



IN 1823 and 1826 I communicated to the Society two 

 papers on the nature and properties of two immiscible 

 fluids, which I discovered, in contact with each other, in the 

 cavities of topaz and other minerals §. Although the facts 

 contained in these papers were of so extraordinary a nature 

 as to be received with scepticism by some, and with ridicule 

 by others, yet I am not aware that, during the twenty years 

 which have elapsed since their publication, any person has 

 either repeated my observations, or advanced a single step in 

 the same path of inquiry. In showing to strangers some of 

 the leading phaenomena of the two new fluids, my attention 

 has been frequendy recalled to the subject ; but it was not till 



* This specimen had been prepared by Baron Liebig himself. — A.W.H. 



f Researches on the Chemistry of Food, p. 16. 



X Read before the Royal Society of Edinburgh on the 17th of February 

 1845, and published in their Transactions, vol. xvi. part 1. p. U. 



§ Edinburgh Transactions, vol. x. p. 1 and 407. 

 Phil. Mag. S. 3. No.211. Stippl. Vol. 31. 2 K 



