46 
Rainey, on Artificial Calculi. 
the globules deposited on their upper surface may be rubbed 
off with the finger, and this surface, if necessary, washed 
with hydrochloric acid, care being taken that it does not 
touch the edge of the slide, and so reach the opposite surface. 
Afterwards, the lower surface, this having the clearest glo- 
bules upon it, must be well washed for several minutes by a 
stream of water running from a tap, so that all the gum may 
be removed. It will then be necessary to wash it in distilled 
water, in order that no deposit from the impure water may 
be left on the glass. The specimen should now be dried on 
a plate over boiling water, especially if it is to be put up in 
Canada balsam, and washed with oil of turpentine. The 
Canada balsam must not be boiled on the same slide, as the 
globules containing the triple phosphate would, in that case, 
become filled with rhomboidal crystals, as before observed, 
but the inspissated balsam may be poured hot upon the cal- 
culi from another slide. Lastly, a thin glass cover, of the 
width of the slide, may be put upon it, resting at each end 
upon a ledge of thin glass. The calculi which remain in the 
bottle may next be examined. Those of the largest size are 
not quite so clear as those on the lower surface of the slide ; 
but the rest, which are of all sizes, especially the very small ones, 
are much more accurately elliptical than those adherent to the 
surface of the glass — the mechanical conditions under which 
they are formed being less disturbed by the attraction of 
adjacent objects. The internal ellipses, also, and all other 
points indicative of the manner in which they were formed, 
are more recognisable in those which have subsided from the 
fluid in the bottle. There are, among these, calculi which 
have remained in the solution of gum for about a year, very 
large, generally dumb-bell shaped ones, of a transparency 
almost equal to that of glass. They especially resemble the 
very early deposits of the shell of the oyster.^ Of these 
elliptical particles which are adherent to the slide, one of the 
poles, being blended with the substance of the glass, seems to 
be gradually shaded off, and is thus made to appear imper- 
fect j this molecular union with the glass furnishing an 
example of the remarkable tendency which the carbonate of 
* They exhibit somewhat of a nacreous appearance, being irridescent when 
seen by transmitted light ; but by polarized light, though showing a very dis- 
tinct cross, they do not exhibit prismatic colours. This I believe is to be 
accounted for by their cohesion being so perfect, that the molecules, though 
attracted in straight lines by the force of gravity, are still capable of resist- 
ing a force of repulsion which in a lower state of cohesion would bring 
them into a crystalline condition ; which condition is, I believe, the cause of 
the exhibition of the prismatic colours in the other, larger calculi as seen by 
polarized light. (See plate, &c.) These calculi undergo no change by being 
boiled in distilled water. 
