THE YOUNG SCIENTIST. 
135 
The necessity, therefore, of removing this 
scale and chilled surface, becomes readily ap- 
parent, and all mechanics who give any con- 
sideration to the proper and economical use of 
the file, will be careful to see that the scale and 
sand are first removed by pickling, and the 
surfaces which have become chilled, by grind- 
ing, before applying the file. 
Pickling the Work. — The pickle for gray iron 
castings is generally made by mixing sulphuric 
acid and water, in the proportion of two or 
more parts of water to one of acid, and is 
usually kept for this purpose in a trough lined 
with lead. 
The articles to be pickled are sometimes im- 
mersed in this bath, where they are allowed to 
remain for a short time; they are then removed, 
and the acid is allowed to act upon their sur- 
faces until the scale is loosened, when they 
are washed off with water. More often, how- 
ever, the pickle is dipped from the trough, and 
poured over the castings, which are placed on 
a sloping platform (thus allowing the acid to 
return to the trough), where, after remaining 
for a sufficient time, they are then washed. 
When dry, the castings are either rattled, or 
scraped and cleaned with old files and wire 
scratch-brushes, until the surface is freed from 
scale and sand. 
To pickle brass, or gun-metal castings, a 
mixture of nitric acid and water may be used, 
in the proportion of, say one part acid to five 
of water, the treatment being the same as that 
of the iron castings. While not in general use 
upon the coarser kinds of brass work, the 
pickle is desirable for smaller castings, or those 
requiring to be protected with lacquer. 
When Oil should not he Used. — All files, when 
they leave the manufactory, are covered with 
oil to prevent them from rusting. While this 
is not objectionable for many uses to which the 
file is put, there are cases where the oil should 
be thoroughly removed, as when the file is to 
be used in finishing the larger cast iron sur- 
faces, which are of a glassy nature, the princi- 
pal difficulty being, to make the file bite," or 
keep sufficiently under the surface to prevent 
glazing; otherwise the action not only hardens 
or burnishes the surface operated upon, but 
dulls the extreme points of the teeth, thus 
working against the desired end in both direc- 
tions. 
When Oil may he Used. — Oil may, however, 
be used to good advantage on new files, which 
are put immediately to work upon narrow 
fibrous metals of a harder nature; in such 
cases, it is not uncommon, with good work- 
men, to fill the teeth with oil and chalk. 
Oil is also useful on fine files, in the finish- 
ing of wrought iron, or steel, as by its use the 
teeth will not penetrate to the same degree, 
and the disposition to "pin," and scratch the 
work, is materially less than when used dry. 
Cement for Stone or Marble. The best ce- 
ment for mending marble or any kind of stone, is 
made by mixing 20 parts of litharge and 1 part of 
freshly burned lime in fine dry powder. This is 
made into a putty by linseed oil. It sets in a few 
hours, having the appearance of light stone. 
To Make Sailclotli Impervious to Water, 
and yet Pliant and Durable—Grind 6 lbs. 
English ochre with boiled oil, and add 1 lb. of black 
paint, which mixture forms an indifferent black. 
An ounce of yellow soap, dissolved by heat in half 
a pint of water, is mixed while hot with the paint. 
This composition is laid upon dry canvas as stiff as 
can conveniently be done with the brush. Two 
days after a second coat of ochre and black paint 
(without any soap) is laid on, and, allowing this 
coat time to dry, the canvas is finished with a coat 
of any desired color. After three days it does not 
stick together when folded up. This is the for- 
mula used in the British navy yards, and it has 
given excellent results. We have seen a portable 
boat made of canvas prepared in this way and 
stretched on a skeleton frame. 
Harmless Pliai'oali's Serpents. Many of our 
readers have seen, and no doubt been amused with 
the little balls sold as "eggs of Pharoah's ser- 
pents," which, when ignited, turn into a long 
twisted rod of fused ashes, having a striking re- 
semblance to a snake. Unfortunately the materials 
of which they are made are highly poisonous, and 
the fumes, being mercurial, are also dangerous^ 
The following mixture is nearly as good, can be 
had anywhere, and is quite free from danger: Mix 
intimately together two parts of bichromate of 
potassa, one part of nitrate of potassa, and three 
parts of white sugar. This mixture should be 
pressed in paper or tinfoil cones, and, if intended to 
be kept for any length of time, the paper should be 
varnished over with sandarac varnish. A small 
quantity of balsam of Peru may be added to per- 
fume the mixture, so as to cause its combustion to 
be attended with a pleasant odor. The greenish- 
colored very porous mass, which assumes the ser- 
pent shape, is a mixture of carbonate of potassa, 
oxide of chromium, carbon, and a small quantity 
of neutral chromate of potassa. We have tried 
this recipe with very satisfactory results.. 
