PROCEEDINGS 



ROYAL SOCIETY OF EDINBURGH. 



VOL. II. 1844-5. No. 26. 



Sixty-Second Session. 



Monday, 11 th February 1845. 



Right Reverend BISHOP TERROT in the Chair. 



The following Communications were read :— 



1. On the Existence of peculiar Crystals in the Cavities of 

 the Topaz. Part I. By Sir D. Brewster. 



"2. On the Use of Colourless Ink in Writing. By Sir George 

 Mackenzie. 



Many years ago, the author had attempted to separate the com- 

 ponent parts of common ink, with the view to get rid of its incon- 

 venience in soihng everything with which it came in contact, by 

 committing some of the parts to paper, and some to the pen. Work- 

 ing with solutions, he found that, in all his trials, the paper was, 

 sooner or later, discoloured more or less, so as to unfit it for the 

 market, and he abandoned the attempt. He afterwards tried salts of 

 iodine, but failed to fix the colour which they yielded. After an- 

 other interval, the subject again recurring, he was led, by an acci- 

 dental circumstance, to think he might attain the long sought-for 

 object by using dry powders for the paper, instead of solutions. The 

 first trial satisfied him that he was, at length, to succeed ; and, 

 after persevering a considerable time, he brought certain processes 

 so far as to yield good paper. With an almost colourless ink, 

 prepared with permuriate of iron, traces were instantaneously pro- 

 duced, dark enough for ordinary purposes. The powder introduced 



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