Rainey, on Artificial Calculi, 
43 
mation of the largest and most beautiful calculi, and which 
are connected with some very singular facts tending to throw 
light upon the subject of crystallization. This compound is 
the ammoniaca-magnesian or triple phosphate. I am not 
aware that this salt has ever been noticed before in gum 
arable; but the existence of all the elements entering into 
its composition are mentioned in the analysis of gum by 
several chemists. (See Turner's ^ Chemistry/ p. 855.) It is 
thrown down by the excess of subcarbonate of potass ordered 
in the formula, for if no more of that salt were added to the 
gum than just sufficient to neutralize the vegetable acid in 
combination with the lime, the triple phosphate would be 
retained in solution. Hence this substance, not beginning 
to be deposited until after the carbonate is formed, occupies 
a place on the glass slides, just beneath the lowest particles 
of the globular carbonate with which it combines, forming a 
compound of carbonate of lime, gum, and triple phosphate 
all molecularly, and now, I believe, chemically, combined. 
But this latter fact would require a more accurate analysis 
of the compound than I have yet been able to make. The 
examination of these slides shows what is taking place in 
different heights in the solutions during the progress of their 
diflPusion ; and from the downward direction in the surface ol 
the slide upon which the globules to be examined are depo- 
sited, they become attached to it as the result of their motion 
upwards — the necessary consequence of the diffusion of fluids 
of unequal densities so placed one with respect to the other. 
And the success of the process for forming the largest and 
most perfect globules will require that the adjustment of 
these densities be such that the two compounds — the globu- 
lar carbonate and the triple phosphate — should be formed as 
nearly at the same time as possible, and at the same height 
in the fluid, and that they should remain suspended until 
all the smaller globules in the same vicinity have become 
attracted by, and incorporated with, one another. After 
which some of the larger ones thus formed will fall to the 
bottom of the bottle, whilst others, being attracted by the 
surface of the glass, placed in an inclined plane above them, 
will become adherent to, and blended with it, so that after 
their separation a mark will remain permanently on the 
glass, having the form of the part of the globule which had 
thus been connected with it. If the alkaline solution be too 
thick, and the simple solution of gum not sufficiently so, the 
alkali will ascend more rapidly in the bottle than the gum, 
which not being sufficient in proportion to the quantity of 
carbonate formed to prevent the crystalline arrangement of 
its molecules, regular crystals of carbonate of lime will 
VOL. VI. / 
