348 Mr Cadell on the Manufacture of Mosaic at Rome. 
the extraordinary medium is either tinged with a different num- 
ber of particles of the same colour, or with a colouring matter 
entirely different from that of the ordinary medium. In certain 
specimens of topaz, the colouring matter of the one medium is 
more easily discharged than that of the other ; and in two spe- 
cimens of emerald^ the colouring matter which tinges the ordi- 
nary medium in the one, tinges the extraordinary medium in the 
other, and vice versa. 
All those crystals in which the colouring matter of the one 
medium differs from the other, either in character or intensity, 
possess the property of absorbing the two tints according to the 
laws already mentioned, but there is also reason to believe, that 
polarised light suffers the same kind of absorption in those crys- 
tals whose two images have the same tine, and even in those 
which are perfectly colourless 
Art. XXVI. — Account of the Manufacture of Mosaic at Rome^ 
and History of the Antique Marbles, Alabasters, <^c. at Rome. 
By W. A. Cadell, F. B. S. L. & E., M. W. S. f 
X HE art of forming mosaic pictures of tesselated, that is to 
say, quadrangular pieces of enamel, Avas practised by the an- 
cient Romans. It was in high repute during the middle ages, 
and still continues to excite the admiration of the curious. The 
principal school of the art at the present day, is at Rome. The 
pictures in St Peter’s are of mosaic of the modern school, copied 
from the works of Raphael, Dominichino, and other celebrated 
masters. In Mr Cadelf's Avork, we find accounts of mosaic 
pictures of the different periods, which cannot fail to prove in- 
teresting to those who admire this species of imitation. But 
* The preceding experiments were made in January 1817, and signed by the 
President of the Physical Class of the Royal Society of Edinburgh on the Sl'th Ja- 
nuary 1817. A notice of them was read on the 21st April 1817 ; and the paper 
of which we have given an abstract, was read at the Royal Society of Edinburgh 
on the 30th April 1818, and at the Royal Society of London on the 12th Novem- 
ber 1818. The experiments themselves were frequently shewn in the years 1817 
and 1818 to several eminent foreigners. 
-|- Extracted, by permission, from the author’s learned and interesting book of 
Travels in Carniola and Italy, now in the press. 
