to have been generated by the revolution of
two of the circles about the line joining their centers. During such
revolution, the lines tangent to the two circles describe a conical
surface. We have, therefore, three spheres and three conical surfaces.
Now the plane, which is tangent to the three spheres, is also
evidently tangent to the three conical surfaces; and therefore the
vertices of those conical surfaces are all in the tangent plane. Now
those vertices are the points (1), (2), (3). But the same points are
also in the plane passing through the centers of the three spheres,
which is the same with the plane of the paper on which the figure is
drawn. Those points, being in two planes at the same time, must
therefore be in the intersection of those planes, that is to say, in a
straight line.

(9) C.W.H. asks: Can dyeing or coloring be done in cold water? A. Many
of the coal tar colors may be used in this way: For animal
fibers--wool, silk, | etc.--the affinity of these colors is so great
that, in most instances, no mordants are necessary. The baths are
usually made slightly acid. With vegetable fibers, however, a fast dye
is not assured without mordanting. Some of the finer goods are
prepared by treating with steam coagulated albumen (animalizing),
gelatin, various tannates, tin salt, alum, and other metallic salts.
The following is, the usual method of treatment, except with goods
intended for very light shades: Pass the goods through a strong
decoction of sumac or other tannin solution for an hour, and
afterwards for an hour or two through a weak solution of stannate of
soda; wring out, dip into a dilute solution of sulphuric acid, and
rinse well in water. The goods are then ready to be passed through the
color bath, slightly acidulated. For different tints, these baths are
worked at different temperatures.

(10) F.W. says: I wish to lay the face tier of a brick wall in black
mortar. How can I make the coloring material and mix it? A. Some
prefer to use red mortar and afterwards pencil the joints with black.
Color the ordinary white mortar with Spanish brown for red mortar, and
with ivory black for black, by mixing in enough of the color in a
powdered state to give a good deep tone.

(11) H.A.S. asks: 1. How many prisms are required in a spectroscope to
detect mineral elements in presence of all the ash ingredients of
organic bodies? A. If we understand you, one 60 deg. prism will answer. 2.
What is the best a