110 
THE YOUNG SCIENTIST. 
nary flask is as good as anything. Cover the 
mouth of the flask to keep out dust, and let it 
cool in a quiet place. When cold, the solution 
contains much more of the salt than cold water 
is capable of dissolving; hence it is super-satu- 
rated. If the glass is well cleaned on the inside 
the whole can now be carried about without 
change, but if we drop the smallest particle of 
a crystal of Glauber's salt into it, crystalliza- 
tion at once begins, and the contents of the 
flask become solid. This bit of crystal fur- 
nished a nucleus which determined the crystal- 
lization. The experiment can be repeated in- 
definitely by carefully heating the contents 
again until all the crystals dissolve. 
We give another experiment on the authority 
of Peligot. The proportions are by weight. 
There are two solutions used — the first con- 
tains 150 parts of hyposulphite of soda dis- 
solved in 15 parts of boiling water; the second 
100 parts of acetate of soda dissolved in 15 
parts of boiling water. Pour the first solution, 
still hot, into a tall glass, and then pour the 
second carefully on top, so that the fluids do 
not mix. I>et the whole now cool, protected 
from dust. When we wish to see the crystals 
form, tie a small crystal of the hyposulphite on 
the end of a thread, and lower it into the ves- 
sel until it reaches the bottom solution. It 
will at once determine the crystallization of 
this. Drop now a crystal of the acetate into 
the upper solution, and this also will solidify, 
but the form of the crystals will be very dis- 
tinct from the others. H. 
How to Make an iSSolian Harp. 
One of our subscribers having requested us 
to describe a good method of making an 
^oliau harp, we give the following, which has 
been tested and found to work admirably: Of 
very thin cedar, pine or other soft wood, make 
a box five or six inches deep, seven or eight 
iuches wide, and of a length just equal to the 
width of the window in which it is to be placed. 
Across the top, near each end, glue a strip of 
wood half an inch high and a quarter of an inch 
thick, for bridges. Into the ends of the box, 
insert wooden pins like those of a violin, to 
wind the strings around — two pins in each end. 
Make a round hole in the middle of the top, 
and string the box with small catgut or blue 
first fiddle strings. Fastening one end of each 
string to a metallic pin in one end of the box, 
and carrying it over the bridges, wind it round 
the turning pin in the opposite end of the box. 
The ends of the box should be increased in 
thickness where the wooden pins enter, by a 
piece of wood glued upon the inside. Tune 
the strings in unison, and place the box in the 
window. It is better to have four strings, as 
described; but a harp with a single string pro- 
duces exceedingly sweet notes, which vary with 
the force of the wind. 
Hints on Filing. 
The following practical directions for using 
the file are extracted from "A Treatise on 
Files," recently published by the Nicholson 
File Company, and will be found of great value 
to amateurs: 
Height of Work. — Various ideas very natur- 
ally exist amongst mechanics, as to the height 
at which the jaws of the vise should be set 
from the floor, for use in filing; arising largely, 
no doubt, from the varied nature of the work 
upon which the advocates of the different ideas 
have been accustomed to operate. 
For filing general work, the top of the vise 
jaws should be placed so as to be level with the 
elbow of the workman, which will be found to 
range from 40 to 44 inches from the floor — 
therefore 42 inches may be considered as an 
average height, best suited for all heights of 
workmen, when the vise is to be permanently 
fixed. 
If the work to be filed is small and delicate, 
requiring simply a movement of the arms, or 
right hand and arm alone, the vise should be 
higher, not only in order that the workman 
may more closely scrutinize the work, but that 
he may be able to stand more erect. 
If the work to be filed is heavy and massive, 
requiring great muscular effort, its surface 
should be below the elbow joint; as the oper- 
ator stands further from his work, with his feet 
separated from 10 to 30 inches, and his knees 
somewhat bent, thus lowering his stature; be- 
sides, in this class of work, it is desirable to 
throw the weight of the body upon the file, id 
make it penetrate, and thus, with a compara- 
tive fixedness of the arms, depend largely upon 
the momentum of the body, to shove the file. 
It will therefore be seen that in fixing the 
height of the vise, the nature of the work and 
the stature of the operator should be consid- 
ered, if it is deemed necessary to apply the 
principle correctly. 
