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equal to each other. But in quantity or extent they vary as the 
squares of their distances from the centre. Estimated, therefore, not 
as to the entire superficies, but along any line or radius, the attractive 
force must vary inversely as the square of the distance or radius ; 
which is the well-known law of gravitation. Thus the genesis at 
once of inertia, elasticity, and gravitation is given. They are shown 
to be homogeneous in their nature, and not occult properties inhering 
in matter, but merely uniform phenomena arising from the operation 
in different circumstances of one and the same law. 
The author then proceeds to show, that the law of assimilation 
also gives a definite conception as to the structure of least parts of 
matter in the last analysis which nature appears to permit or accom- 
plish. He is thus led to infer the existence of an universal ether 
or medium of light. And from this, to the molecules of bodies, he 
thinks that a transition may be made, by regarding the latter as 
products of the former ; the ethereal elements being definitely grouped 
into molecules ; which, under the law of assimilation, must be sym- 
metrical, and permanent or intransmutable (except in long epochs), 
as also ever bent on union with one another, wherever there is a 
difference between them ; and their union would not occasion too 
great a degradation of the symmetry which each possessed when 
existing separately. 
2. On the Structure and Functions of the Branchial Sac of 
the Simple Ascidiae. By Andrew Murray, Esq. 
The chief portion of this paper is occupied with an inquiry into 
the truth of Milne Edwards’ theory, that the branchial sac of the 
Ascidians is perforated throughout by apertures, or what he called 
branchial stigmata. 
Some experiments and observations made by Mr Murray last season 
had induced him to regard this view with doubt. In considering the 
question, he first examined the evidence that was recorded in support 
of it. Putting aside those writers who had taken M. Edwards’ theory 
on trust, he could only find four observers who seem to have critically 
tested it, viz., — Hr Lister, with whom the idea originated: who, while 
stating that the walls of the sac appeared to be traversed by open 
spaces, yet states that at times he thought he saw something like a 
veil stretching across them, and that the particles in suspension in 
