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serves, that, supposing the functiony to be known, “ we 
can immediately apply the general method given in the Mé- 
canique Analytique, and which appears to be more especially 
applicable to problems that relate to the motions of systems 
composed of an immense number of particles mutually acting 
upon each other.” Such is certainly the great advantage of 
starting with that general principle; but the chief difficulty 
attending it, namely, the determination of the function vy, on 
which the success of the investigation essentially depends, has 
not been surmounted by Mr. Green, who has consequently 
been led to very erroneous results, even in the simple case 
of uncrystallized media, to which his researches are exclu- 
sively confined. In this case Mr. Mac Cullagh’s theory 
confirms the well-known formule of Fresnel, one of which 
Mr. Green conceives to be inaccurate, and proposes to 
replace by a result of his own, which, however, will not 
bear to be tested numerically. The present theory applies 
with equal facility to all media, whether crystallized or not, 
and is distinguished throughout by the singular elegance 
and simplicity of its analytical details ; a circumstance which 
the author regards as a strong indication of its truth. 
Mr. Lloyd exhibited to the meeting a specimen of a re- 
markable substance recently found in the principality of 
Carolath, in Silesia. It formed part of a cloth of 200 square 
feet in surface now in the possession of the King of Prussia. 
No description of this substance has yet been published; but 
Major Sabine and Mr. Lloyd were informed by Baron Hum- 
boldt (by whom the present specimen was kindly given) that 
: M. Ehrenberg had examined it microscopically, and had 
r found it to be an organic substance, consisting partly of ve- 
__ getable and partly of animal matter ;—the vegetable compo- 
nent being the conferva rivularis, the animal different species 
of Infusoria, of the family known by the name of Bacillaria. 
To illustrate the origin of this substance, Mr. Lloyd. 
