THE YOUNG SCIENTIST. 
27 
that it lighted up the bottom of the lake, and to 
add to the brilliancy the mirror was held so as 
to throw down all the rays possible. 
After two or three trials in different places we 
heard the Professor exclaim "I see it, steady 
boys!" and steady it was. The magnet was 
lowered, a little more magnesium was burned 
to enable the Professor to adjust it properly, 
and up came the rifle. 
On the way home the Professor told us that if 
the water had been rippled the blanket dipping 
into it would have kept a still place. If too 
rough, he could have made a water tight tube 
in two divisions and with a glass bottom. Down 
one division he could have sent the light, while 
he looked through the other. 
Sunlight, of course, is best, but when sun- 
light is not to be had, magnesium is a very good 
substitute, and where magnesium cannot be had, 
a light may be made by filling a paper tube 
with blue light mixture* and burning that. The 
materials may be had in any country drug 
store An Old Boy. 
give time to trim mouldings, etc. For fine work 
the plaster may be sifted again after being 
boiled. It may be kept for any length of time, 
if kept perfectly dry and in a place or vessel 
where it cannot imbibe moisture from the at- 
mosphere. Should this take place, no boiling or 
burning will again revive it.— English Mechanic. 
Plaster of Paris. 
Before the establishment of cement mills, 
plaster was only used by stucco men, and these 
had generally to prepare it for their own use ; 
the gypsum was never burnt, at least by those 
who did fine work ; the term was " boiled," not 
"burnt;" the raw gypsum was pounded and 
sifted, and the water of crystallization was 
driven off in an open pot or pan, the powder 
being stirred the while; during which opera- 
tion it assumed all the appearance of a fluid, 
owing to the rapid escape of the steam) especi- 
ally from the lower stratum, which kept the 
whole mass alive, so that when the surface was 
disturbed it immediately assumed its own level. 
When all the water was expelled this peculiar- 
ity ceased, the gypsum was given a good heat, 
so as to make sure of its being thoroughly 
dried, but not by any means to burn it or let it 
get red-hot, which was reckoned to spoil it, as if 
overheated it set too quickly, or merely acted 
as common lime. 
In mixing for use, plaster ought to be added 
to water, not the water poured on to the powd- 
ered plaster. And if any coloring matter be 
used, such as red ochre, it ought to be first 
mixed with the water, so as to insure equable 
distribution ; if a hard material be required, add 
alum or sulphate of sodium (Glauber salts) to 
the water. These cause the plaster to set with 
great rapidity, but should it be wished to delay 
the setting, mix with a weak solution of glue or 
gelatin. The stucco men use this in order to 
How to Loosen Screws. 
Few things are more vexatious than obstinate 
screws, which refuse to move, much less to be 
drawn out, and in the struggle against screw- 
driver power suffer the loss of their heads, like 
conscientious martyrs, rather than take a single 
half-turn backwards from the course they have 
followed, and from the position they have been 
forced into. Like obstinate children, they must 
be coaxed, or rapped pretty hard on the head, 
according to circumstances; in fact, whoever 
has a tight, obstinate screw, to " draw out," must 
keep his temper down and his resolution up, 
quite as much to the sticking-point as the screw 
does. If the screw is turned into iron and not 
very rusty, it is only necessary to clear the head 
with the wedge of the driver, and let a few 
drops of oil penetrate to the threads ; but, find- 
ing that excessive heat or rust has almost fixed 
the screw immutably, then heat, either by 
placing a piece of hot iron upon it or directing 
the flame of the blow-pipe upon the head, and, 
after applying a little oil, turn out gently; 
but care must be taken not to let the tool slip so 
as to damage the notch. If, however, the screw 
refuses to come out, try to force it back with a 
blunt chisel, smartly but carefully tapped with 
a light hammer ; but if evidently nothing can 
dislodge the enemy, it is best to cut the head 
away and drill out the screw. When an obsti- 
nate screw happens to be in wood, merely give 
it a few taps on the head ; but failing that, heat 
it with a piece of hot iron, when it may be easily 
turned. 
* Directions for making all kinds of lights will 
be found in the " Workshop Companion." 
Keeping- Mice From Seeds.— Any one de- 
sirious of keeping seeds from the depredations 
of mice, can do so by mixing some pieces of cam- 
phor gum in with the seeds. Camphor placed in 
drawers or trunks will prevent mice from doing 
them injury. The little animal objects to the 
odor, and keeping a good distance from it, he 
will seek food elsewhere. 
Marking: Tools.— Much trouble can often be 
saved by marking tools with their owner's name, 
which can be easily done in the following man- 
ner:— Coat over the tools with a thin layer of wax 
or hard tallow, by first warming the steel and 
rubbing on the wax warm, until it flows, and let 
