44 
Rainey^ on Artificial Calculi. 
result. Tliese, however, will only exist towards the upper 
part of the slide ; and examined from above downwards, they 
furnish an opportunity of seeing all the changes which the 
form of the crystals of *carbonate undergo as they become 
combined with a successively increasing proportion of glu- 
tinous material. The particles, as thus examined, will be seen 
passing through all forms intermediate between the perfectly 
rectilinear figures and true spheres. (See Plate IV, fig. 1.) If, 
on the contrary, the density of the alkaline solution be not 
sufficient, the globules will fall to the bottom of the bottle too 
rapidly to coalesce in sufficient numbers to produce large 
calculi. When the densities of the two solutions are pro- 
perly proportioned, as in the formula here given, the globular 
carbonate, without triple phosphate, will be found chiefly at 
the upper part of the slide, and the combination of the two 
at the lower. And still lower down are the crystals of triple 
phosphate unmixed with carbonate. Sometimes these crys- 
tals are very minute and imperfect, whilst at others they 
are larger, and exist in a perfectly crystalline state, increasing 
in size as they are deposited higher up on the slide until they 
come into the vicinity of the globules of carbonate, when 
they gradually loose their rectilinear form, and become so 
thoroughly incoporated with one another, or with the glo- 
bules with which they are brought into contact, as to lose all 
traces of their previous crystalline arrangement (fig. 1). 
It may seem^ singular that gum should have the power of 
opposing the rectilinear arrangement of the molecules of car- 
bonate of lime, and not of triple phosphate or of phosphate 
of lime, as mentioned in my first paper, to which I must here 
refer, although each is eqiially in contact with the gum when 
it assumes its solid form. This probably arises from the 
carbonate of lime having a chemical attraction or affinity for 
the gum, and therefore combining with it, whilst the triple 
phosphate has not ; and therefore, this substance not entering 
into intimate molecular union with it, the cohesive attraction 
between the molecules of the gum cannot influence efifectively 
the repulsive force acting upon the ultimate molecules of the 
triple phosphate ; but when the triple phosphate is brought 
into contact with carbonate of lime combined with gum, 
which has been shown to have a strong attraction towards 
solid bodies, as is seen, for instance, by its action on glass, 
then the molecules of triple phosphate becoming, through 
the chemical agency of the carbonate of lime, in more inti- 
mate union with the molecules of the gum, and now having 
the repulsive force separating them overcome or neutralized, 
are brought under the effective influence of the attraction of 
gravitation; so that the globular compound of carbonate of 
