141 
Sir Charles Gieseeke on Cryolite, 
planation of this partial insensibility of the tympanum, and I con- 
ceive, that the insensibility of some eyes to weak impressions of 
light, requires no other explanation^ than that either from origi- 
nal organisation, or some accidental cause, the retina of one per^^ 
son may be less delicate and less susceptible of luminous impres- 
sions than the retina of another, without being accompanied 
with any diminution of the powers of vision. If a sound ear, 
therefore, may be deaf to sounds of a certain pitch, without our 
looking for the cause of this in the form of any part of the brain, 
why should we appeal to such an uncertain guide for an expla- 
nation of ^the analogous phenomenon of the insensibility of the 
eye to certain colours ? 
Art. XXV. — On Cryolite *; a Fragment of a Journal hy 
Sir Charles Giesecke', F. R. S. E., M.R.I.A., M. G. S., 
&c. and Professor of Mineralogy to the Royal Dublin Society. 
T?owards the end of September 1806, on returning from my 
mineralogical excursions around Cape Farewell, and part of the 
eastern coast of Greenland, 1 was informed by one of the Green- 
landers who accompanied me, that they sometimes found loose 
pieces of lead {Ahertloh of the natives) in a frith to the north- 
ward of Cape Desolation {Nunarsoit of the Greenlanders), but he 
could not tell me the exact spot. Though the unfavourable sea- 
son was already advanced so far, and the equinoctial gales had 
begun blowing so violently as to make it unadvisable to venture 
upon such a doubtful excursion, yet I resolved to go in search of 
the place, as we were near to the mouth of the frith in question. 
The name of the frith is Arlcsut, (Engl, the Leeward) : it was 
divided into two arms ; that on the right of the entrance had a 
south-easterly, that on the left an easterly direction. I steered 
up the eastern arm about sixteen miles, and put on shore at dif- 
ferent places. I already began to despair of finding lead, when I 
observed, at some distance, but near the shore, a snow-white spot. 
At first sight, I suspected it might be a small glacier ; but con- 
* I know no name in the system of mineralogy more expressive of the exter- 
nal character and the fusibility of this substance, than that adopted by my de- 
ceased friend Dr Abilgard, late Professor in the University of Copenhagen, who 
■was the first who noticed and analysed this substance. 
