« 
THE YOUNG 
SCIENTIST. 
tern, and into this bits of glass are driven, 
according to tint, by means of a small 
wooden mallet. If the effect produced 
wounds the artist's eye, he can easily 
amend the defect by withdrawing the 
offending piece of enamel and driv- 
ing in another while the cement is still 
wet; and, by observing proper precau- 
tions, it can be kept damp for more than 
a fortnight. When the work is completed 
any tiny crevices which may remain are 
carefully plugged or "stopped" with 
pounded marble, or with enamel mixed 
with wax, and the entire surface of the 
picture is then ground down to a perfect 
plane, and finally polished with putty and 
oil. Byzantine may be broadly distin- 
guished from Roman mosaic by the cir- 
cumstance of the surface of the former 
being left unground or unpolished — save 
w^here there is burnished gold— thus leav- 
ing an irregularity of surface productive 
of great vigor of effect. A picture of the 
Byzantine style can at once be recognized 
as a mosaic, even if it be hung at an alti- 
tude of one hundred feet from the ground ; 
but a perfected mosaic picture, after the 
Roman manner, might easily be mis- 
taken, even at a very short distance, for 
a very elaborately finished and highly 
varnished painting in oils. 
How to Make an Electric Battery. 
Amateur experimenters in electricity will find 
the following battery an exceedingly cheap, yet 
strong one. An^'one with care and thought can 
make it. 
The materials used are as follows: One 
common fruit jar, one piece of zinc, twelve 
inches by three wide, a small flower-pot (un- 
glazed) about three inches deep, piece of gas- 
carbon, two ounces of bichromate of potash, two 
ounces of 'sulphuric acid and three ounces of 
common salt. 
We will begin with the jar. Dip in kerosene 
about one yard of twine and wind it around tlie 
Jar about three inches from the bottom. Ignite 
it and slowly revolve the jar in the hands so as 
to have the flame on all sides of the jar at once. 
When the jar cracks, strike the open end with a 
piece of wood and the jar will crack off where 
the string was placed. Remove the sharp edges 
with n file and the battery jar is finished. 
Solder a piece of copper wire to one corner of 
the zinc and then roll it into a cylinder form 
about the size of the jar. The wire from this 
zinc constitutes the negative pole of the battery. 
With a little melted tar stop the hole in the 
bottom of the flower-pot. 
At the gas factory obtain a piece of gas-carbon 
the size of your wrist anrl three or four inches 
long. Drill a small hole in one end and twist a 
stout piece of copper wire into it. Applj- a little 
melted tar around the wire where it enters the i 
carbon, which will partly protect it from the 
attacks of acid. This wire from the carbon con- 
stitutes the positive pole from which the elec- 
tricitj'' flows. 
To prepare the acid, place a pint of warm 
water in a bottle, and add the bichromate ; when 
nearly dissolved add the sulphuric acid. In 
preparing this acid be very careful as it is very 
poisonous and corrosive. 
In the bottom of the jar place the salt. Then 
place in the zinc. Inside of the zinc place the . 
pot. Into the pot place the carbon with the 
wire projecting upwards. Fill this pot with the 
acid. Fill the jar within one-quarter of an inch 
of the top of the pot with water. In a few 
moments the batterj^ will be ready to work. 
Connect the wires and the circuit is complete 
from the carbon to the zinc. When in a dark 
room a bright electric spark may be seen at the 
moment the wires are disconnected. Therefore, 
electric phenomena is only obtained when there 
is a metallic circuit between the positive and 
negative poles. 
To maintain the battery, recharge the pot with 
acid about once in two or three weeks, accord- 
ing to the use. Renew the salt water about once 
in a month. A number of experiments may be 
performed with this battery, one of which I will 
describe in the next number. 
Geokge Thompson. 
Maiden, Mass. 
Luminous Faint. 
The comparatively recent discovery that 
luminous paint can be applied as ordinary 
whitewash, considerably expands the field of 
its usefulness. Sheets of glass coated with the 
paint form Aladin's lamps, which are in use in 
some of the vessels of the navy, at the Waitham 
Powder Factory, at Young's Paraffine Works, 
and in the spirit vaults of several docks ; but 
now that, by increased production and the use 
of water as the medium, the cost is reduced by 
one-lialf, it will probably be extensively used 
for painting walls and ceilings. The ordinary 
form of oil paint has already been applied in 
inany ways to clock-faces, to name-plates and 
numbers and house doors, and to notice boards, 
such as "mind the step," "to let," etc, The 
paint emits light without combustion, and, 
therefore, does not vitiate the atmosphere. 
Several experimental carriages are now running 
on different railways, the paint being used in- 
stead of lamps, which are necessary all day on 
account of the line passing through occasional 
tunnels. It is reported that a paper at Turin 
called Light is to appear shortly, printed in ink 
