BY MOLECULAR COALESCENCE. 25 
tions of these two parts vary according to the advance- 
ment of the process of coalescence, which in very large 
calculi is rarely so complete, as not to leave some remains 
of the amorphous matter in the form of a central opacity, 
presenting in many of these bodies the appearance of a 
nucleus; in some bodies it appears to be single, but in 
others to be in process of division; or, if the cause of the 
appearance were not known, it might be considered as in- 
dicative of fissiparous multiplication. I may observe 
that this latter idea has instantly occurred to those to 
whom the artificial products have been shown when 
ignorant of their real nature. This latter appearance of 
the nucleus will not, however, be found in calculi formed 
in the manner now described, but only in those which 
are formed by the coalescence of two large ones. The 
central portions of each, being the last to undergo disen- 
tegration and subsequent coalescence, will be recognisable 
by the aid of the polariscope after all the other parts have 
become thoroughly incorporated. Now, as respects the 
agency of gravity (that is, the mutual attraction of the 
particles of matter according to known laws), in the 
formation of these calculi, it will not be doubted but that 
the first stage of the formation—the collecting together 
of the floating particles into spherical masses—is due to 
its influence. 
Next, as might have been anticipated, the peripheral 
layer of these calculi is that in which the disintegration and 
subsequent coalescence commence and are firstcompleted, as 
indicated by the circumferential bright ring before noticed 
—a fact which is in perfect accordance with the relative 
force which gravity is proved to exert upon the particles 
of matter arranged im spherical masses. For this force 
exerting its maximum influence on the spherical particles 
