38 FORMATION OF SHELLS OF ANIMALS, ETC., 
four alkaline earths, where it is stated that the carbonate 
of magnesia is the only one which does not become globu- 
lar in a solution of gum. (See p. 7.) In this respect, 
there is a resemblance between the ammoniaco-magnesian 
phosphate and the simple carbonate of magnesia. There 
is also another fact connected with the same property of 
the carbonate of lime that may be mentioned, which is its 
property of combining with hard substances. This is best 
shown by its action on the glass of the slide upon which 
it is deposited. The particles of this substance which are 
attracted by a slide and detained for some weeks in con- 
tact with it, become so intimately blended with its sub- 
stance that, after the surface is washed with hydrochloric 
acid, impressions are left on the slide of the form of the 
part of the globule attached sufficiently deep to admit of 
being received on a film of collodion, on which they can 
be seen by the microscope. Hence the slides which have 
‘been employed in making the artificial calculi never re- 
cover their transparency, but remain more or less dull 
according to the time they had been in the solution of 
gum. The form of these calculi becomes also affected. 
The dumb-bell-shaped ones are made longer than those 
which had been formed whilst floating in a fluid medium. 
The elliptical particles, in the place of having a regular 
elliptical contour, as when formed whilst suspended in fluid, 
are lengthened, by the attraction of the glass opposing re- 
sistance to the apposition of their constituent spherules. 
They are also shaded off at one of their poles, that which 
had been most forcibly attracted by and blended with the 
surface of the slide ; and the calculi of the largest size are 
flattened where they were in contact with the glass. Also, 
if the surface of the slide had been scratched, the caleuli 
becoming attached to these parts im greatest quantities 
