CARBON—LITTLE 243 
or enough for a tennis court. The accumulation of carbon dioxide 
in the blood provides the normal stimulus to respiration. 
Pure air contains about 0.03 per cent of carbon dioxide. in 
crowded halls the proportion may rise to 0.5 per cent. Since under 
ordinary atmospheric conditions the gas dissolves in water about 
volume for volume, it is constantly being washed down and slowly 
attacks the silicated rocks with formation of calcium and magnesium 
carbonates, by which soft waters are rendered hard and much trouble 
is caused by boiler scale. Ultimately such waters find their way 
to the sea, where marine animals, the chambered nautilus, the coral 
polyp, the oyster, the humble clam, and, most important of all, the 
minute foraminifera fix the calcium carbonate in their shells. If 
you rub down to a thin paste with water a piece of chalk you will 
find upon microscopical examination that it is composed almost 
entirely of the tiny shells of foraminifera. Such are the chalk beds 
of England, which, often more than 1,000 feet in thickness, extend 
across the island for 280 miles and at the coast line rise in those 
white cliffs to which England owes her name of Albion. But the 
chalk bed stretches far beyond the coast of England, over much of 
France, through Denmark and Central Europe, south to Africa, 
and even into Central Asia. Chalk and limestone are carbon com- 
pounds, and the overwhelming evidence is that, wherever found, they 
are the product of aquatic life in regions once submerged. 
CARBONATES 
The Latin word for a coin was nummus, and, for a reason which 
will presently appear, it gives its name to the nummulitic limestones, 
which extend over vast areas of North America and, in a band often 
1,800 miles in breadth and of enormous thickness, from the Atlantic 
shores of Europe and Africa through western Asia to northern India 
and China. Of it the pyramids were builded, and from a knoll-hke 
outcropping was fashioned that other memorial of antiquity, the 
Great Sphinx, before which even the centuries seem to pause. 
This variety of limestone has been formed by the slow accretion 
in marine deposits of innumerable billions of the shells of forami- 
nifera of the genus Vummulites, which is characterized by shells of 
extraordinary complexity of structure and a disk or coinlike form. 
But limestone has been formed by other agencies, and its formation 
is still proceeding on a grand scale through the activities of the 
coral polyp in many of the warmer waters of the globe. Two and a 
half million square miles of ocean bottom are covered by coral mud 
and sands. The gigantic structures of the coral islands and the 
barrier reefs, of which one extends for 1,000 miles along the Aus- 
tralian coast, are the work of tiny bits of animated jelly, which 
