-MICROSCOPICAL EXAMINATION OF CHALK. 371 
found remark of the illustrious Galileo—“ La nature fait 
beaucoup avec peu, et ses opérations sont toutes également 
merveilleuses.” 
INSTRUCTIONS FOR THE MICROSCOPICAL EXAMINATION OF 
CHALK, FLINT, AND OTHER ROCKS. 
Cuatx.—The following method is that recommended by 
M. Ehrenberg. Place a drop of water upon a plate of thin 
glass, and put into it as much scraped chalk as will cover 
the fine point of a knife, spreading it out, and leaving it to 
rest a few seconds ; then withdraw the finest particles which 
are suspended in the water, together with most of the liquid, 
and let the remainder become perfectly dry. Cover this 
dried spot of chalk with Canada balsam (the turpentine of 
Abies balsamea), and hold the plate of glass over the flame 
of a lamp, until the balsam becomes slightly fluid, without 
froth or air-bubbles ; it should be maintained in this position 
(the glass being kept as hot as the finger will bear) for a few 
minutes, until the balsam is found to have thoroughly per- 
meated the substance to be examined. It is preferable to 
place a piece of very thin glass upon the balsam, and gently 
press it down, and allow it to remain. The best flatted 
crown-glass should be used for placing the chalk or other 
objects on. It is convenient to have the slips of glass of one 
size, or the specimens will require different boxes for their 
reception ; three inchés by one inch is that usually employed. 
These objects require to be viewed with a power magni- 
fying three hundred times linear, that is, in diameter ; and 
if the process has been properly conducted, it will be seen 
* that the chalk is chiefly composed of well-preserved organ- 
isms. In these preparations the cells of the foraminifera 
appear at first black, with a white central spot ; this is caused 
by the air contained in those cavities, for air-bubbles always 
appear as black annular bodies; by degrees, the balsam 
