r, brass, etc. A mixture of oxygen and hydrogen when
ignited constitutes the hydrogen blowpipe, invented by Dr. Hare of
Philadelphia.

What is Magnesium Light?

When the metal magnesium is rolled out into a fine ribbon and heated
to red heat it burns with a dazzling light.

Which is the most powerful artificial light?

The so-called Electric light. This light, whether produced by a series
of galvanic cells or by dynamic power, is the most brilliant and
useful.

What is a Rainbow?

The rainbow is that beautiful semi-circular band or arc of different
colors in the clouds during the occurrence of rain in sunshine. When
the clouds opposite the sun are very dark and rain is falling from
them, the rays of the sun are divided by the raindrops as they would
be by a prism. There are often two rainbows at the same time, because
the primary bow is again reflected to another layer of clouds.

What is a Prism?

A triangular solid piece of glass, on which if a ray of light be cast
it will be distinctly divided into the seven colors we see in a
rainbow. By this fact we see that white light is composed of different
rays which have different reflective susceptibilities.

What is a Spectrum?

It is this beautiful band of seven colors obtained by the refraction
of a ray of light through the prism.

Whence come the colors in the objects we see in nature?

They all come from light; every object has a power to absorb certain
rays and to reflect others. A red cloth, for example, absorbs all the
other colored rays except red, and this it gives off, thus appearing
red.

Why are the leaves of plants green?

Because a peculiar chemical substance called chlorophyl, formed within
their cells, absorbs all other rays of light, reflecting only blue and
yellow--which mixture produces the different green tints.

What is Photography?

The word means "light drawing." It is a mode of fixing on certain
substances the lights and shades of any object by means of a lens
inserted in a camera obscura. This process was first called
Daguerreotype from the name of the inventor, Daguerre. A plate of
copper thinly coated with silver is exposed to the vapor of iodine,
then placed in a camera obscura, where an image of the object to be
presented through a lens is cast upon it. Ambrotype is the same
application to glass. There are now different variations of method in
the use of the same agents. Now photography consists in taking th