Prof. Haughton's Notes on Mineralogy. 509 



The equation of the tangent circle is therefore 



or reducing, 



.r2 + ?/^-l-2^(Aa? + Br/-V) = 0; 

 and recollecting that Ao^ + B?/ — v = is the equation of the axis 

 of similitude, the equation shows that the axis of similitude is a 

 common chord or radical axis of the orthotomic circle and the 

 two tangent circles. 



2 Stone Buildings, W. C. 

 May 15, 1857. 



LXVIII. Notes on Mineralogy. — No. V. On Hypostilbite and 

 Stilbite. By the Rev. Samujel Haughton, M.A., Felloiv of 

 Trinity College, and Professor of Geology in the University of 

 Dublin^. 



IN the twelfth volume of the Philosophical Magazine, p. 406, 

 an analysis is published of a mineral allied to stilbite, by 

 Mr. J. W. Mallet, and labelled hypostilbite by the person from 

 whom he received it. 



In the same volumCj p. 552, this mineral is pronounced to be 

 Laumonite by Dr. Heddle, who disposes of the label hypostilbite 

 by means of the hypothesis, that "the christening may be ex- 

 plained by the ignorance of some unprincipled dealer resorting 

 to a rare title in order to get rid of a superabundance of an in- 

 significant-looking substance." 



From the description given by Mr. Mallet and Dr. Heddle of 

 the mineral, I have no doubt of its being the same as a mineral 

 which has been some time in ray possession, from the Isle of 

 Skye ; and as I find in my note-book the details of an accurate 

 analysis made of it some years ago, I believe it is worth while to 

 publish this analysis, as I entertain no doubt myself but that 

 this mineral is identical with the hypostilbite of Beudant. 



Analysis of Isle of Skye Mineral. 



99-97 

 I place in comparison with the foregoing analysis of the mi- 

 * Communicated by the Author. 



