easily resolved, since the side AC of that triangle, being the
radius of the earth, is a known dimension. Resolution of this
triangle gives us the length of the hypotenuse MC, and the
difference between this and the radius (AC), or CD, is obviously
the height of the atmosphere (h), which was the measurement
desired. According to the calculation of Alhazen, this h, or the
height of the atmosphere, represents from twenty to thirty miles.
The modern computation extends this to about fifty miles. But,
considering the various ambiguities that necessarily attended the
experiment, the result was a remarkably close approximation to
the truth.

Turning from physics to chemistry, we find as perhaps the
greatest Arabian name that of Geber, who taught in the College of
Seville in the first half of the eighth century. The most
important researches of this really remarkable experimenter had
to do with the acids. The ancient world had had no knowledge of
any acid more powerful than acetic. Geber, however, vastly
increased the possibilities of chemical experiment by the
discovery of sulphuric, nitric, and nitromuriatic acids. He made
use also of the processes of sublimation and filtration, and his
works describe the water bath and the chemical oven. Among the
important chemicals which he first differentiated is oxide of
mercury, and his studies of sulphur in its various compounds have
peculiar interest. In particular is this true of his observation
that, tinder certain conditions of oxidation, the weight of a
metal was lessened.

From the record of these studies in the fields of astronomy,
physics, and chemistry, we turn to a somewhat extended survey of