equal to the angle of reflection; and, second, the much more doubtful
observation that twilight continues until such time as the sun,
according to a simple calculation, is nineteen degrees below the
horizon. Referring to the diagram, let the inner circle represent the
earth's surface, the outer circle the limits of the atmosphere, C being
the earth's centre, and RR radii of the earth. Then the observer at the
point A will continue to receive the reflected rays of the sun until
that body reaches the point S, which is, according to the hypothesis,
nineteen degrees below the horizon line of the observer at A. This
horizon line, being represented by AH, and the sun's ray by SM, the
angle HMS is an angle of nineteen degrees. The complementary angle SMA
is, obviously, an angle of (180-19) one hundred and sixty-one degrees.
But since M is the reflecting surface and the angle of incidence equals
the angle of reflection, the angle AMC is an angle of one-half of one
hundred and sixty-one degrees, or eighty degrees and thirty minutes.
Now this angle AMC, being known, the right-angled triangle MAC is 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 s