58 
THE YOUNG SCIENTIST. 
it is said that some famoas sword blades are 
tempered in this way. 
No special skill is required for hardening 
small tools, but thin plates and large pieces 
require great care or they will crack or warp. 
They should therefore be handed over to some 
one of experience. Having hardened the tool, 
the next step is to temper it, since if left hard it 
would be brittle. It must therefore be heated 
until it has been sufficiently softened, and this 
varies with the use to which the tool is to be 
put. For drilling or cutting very hard sub- 
stances, the temper should be high; soft sub- 
stances may be cut with tools that are not so 
hard. 
The degree of softness is judged by the color 
which the steel assumes when heated. A piece 
of hard steel, which has been made bright on 
a stone, becomes, during the process of heat- 
ing, first straw color, then brownish yellow, 
then light purple, then dark purple, and lastly 
blue. The straw color, in its various shades, 
answers for tools intended to cut metal; tools 
for cutting wood should be brought to a brown- 
ish yellow; tools that are to be struck with a 
hammer should be made a little softer than 
those that act by pressure. 
The temper of the cutting part may be let 
down in several ways, but the easiest and best 
is to cool or harden only the end of the tool, 
and leave the rest hot. In a few moments 
the heat will have been conducted from the 
hot part to the hardened part, and when the 
latter has assumed the proper color, the whole 
of the tool is cooled, and the tempering is 
complete. 
Our Ten-Cent Battery. 
The numerous letters that we have received, 
asking for further information in regard to this 
little piece of apparatus is evidence to us of 
the deep interest which many of our readers 
take in the subject, and we therefore answer in 
a general way the inquiries received. 
The battery is simply a Daniel's battery, in 
which the tin can plays the part of both jar and 
copper sheet; the paper cup serves as a porous 
cell, and the zinc remains as in all batteries. 
The use of the wooden block in making the 
porous cell is simply to serve the purpose of a 
mould. If such a cylinder is not at hand, the 
paper may be rolled upon itself into a tube, 
three or four thicknesses of paper being used. 
and this tube being placed upright on a smooth 
surface (a board, for example), liquid plaster 
may be poured into it to a depth of half an 
inch, and allowed to set. And, by the way, 
let us say that the plaster is never used by 
dealers to fasten chimneys on lamps, as was in- 
advertently stated in our last, but to fasten the 
brass necks to the bowls. The object of the 
plaster is to form a heavy bottom to the paper 
cell. The wood is, of course, removed as soon 
as the cell is finished— that is, as sooji as the 
plaster has become hard. 
It is, of course, always better to amalgamate 
the zinc, especially if a durable battery is 
wanted, but very good results may be obtained 
without amalgamation. To amalgamate a new 
zinc, rub it with mercury and dilute sulphuric 
acid until it is as bright as a new dollar. When 
zinc, well amalgamated, is used with earthen 
cells and glass jars, the best liquid to use next 
the zinc is dilute sulphuric acid, but with un- 
amalgamated zinc, paper cells and a tin can, it 
is best to use a strong solution of sulphate of 
soda, rendered very slightly acid. 
The liquids must not be left in the battery 
until after the tin can has become heavily 
coated with copper. 
Do not let the paper become dry, except after 
it has been washed out in clean water. 
This battery is not a mere creature of the 
imagination. Some years ago we put together 
a considerable series, constructed substantially 
as we have described, and we obtained very 
satisfactory results. 
Iron Shot. 
Ed. Young Scientist — In your article on iron 
bird shot, in the January number of your valu- 
able journal, you say that, owing to the light- 
ness of iron, shot made of that metal would 
reduce the range of a gun nearly one-half. It 
is evident that if the shot are made of such a 
size as to be of the same weight as leaden ones, 
the only loss of force would be that due to a 
slight increase of resistance of the air, owing to 
their greater size. Let us see how much this 
difference would be. As the specific gravity of 
lead is 11.25, and that of iron is 7.78, and as 
spheres are to each other as the cubes of their 
