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Scieniijic Intelligence, 
of an inch broad. I now placed it in a fluid of the same re- 
fractive power, so that I could transmit polarized light through it 
in every direction, and, upon a careful examination of its struc- 
ture, I found that it was entirely destitute both of the ordinary po- 
larizing structure,^ and of the circularly polarizing structure^ ha- 
ving no other action upon light than a piece of well annealed 
glass. In one corner, a small bubble of air had created, by its 
expansion, a slight depolarizing structure, such as I have descri- 
bed in this Journal, vol. hi. p. 98, D. B. 
11. Singular Properties qf‘ Chlorophaite^ found in Scotland 
and Iceland. — In the year 1817 I received from Major Peter- 
son, on his return from Iceland, a mass of amygdaloidal rock, 
containing what he considered a new substance, and which he 
had observed possessed the singular property of being perfectly 
transparent, and of a bottle-green colour, when taken out of the 
rock, but which became quite opaque when removed from its 
place, or exposed to the air. I was anxious to ascertain whe- 
ther this remarkable change was an optical or a mechanical one ; 
and the observations I made put it beyond a doubt that it was 
of a mechanical nature. The cause of this change may be con- 
ceived, by supposing a number of prisms assembled in a parti- 
cular manner, and kept together by screws, so as to bring their 
touching surfaces into that close contact which prevents total re- 
flection at the junctions. The mass of aggregated prisms will be 
now quite transparent ; but if we either diminish the compres- 
sing forces by loosening the screws, or suppose some force similar 
to the disintegrating force of the atmosphere, to act in opposi- 
tion to the cohesive force represented by the action of the screws, 
the touching surfaces will be separated, and the whole mass be- 
come opaque. We were not aware till lately, that Dr Mac- 
Culloch, in his very interesting work on the Western Islands, 
had discovered, many years ago, in Fife and in Rum the same 
substance, to which he has given the name of Chlorophmte. He 
found its specific gravity to be 2.020. It is easily scratched by 
a quill ; is unchangeable before the blowpipe, and is as refrac- 
tory as quartz. He found it to consist principally of silica^ and 
to give indications of a considerable proportion of iron, and a 
little alumkia. See MacCulloch'’s Description of the Western 
Islands., voL L p. 504, D„ B. 
