130 
THE YOUNG SCIENTIST. 
when the disturbance of the sand above 
warns the watcher below, and he begins to 
toss up the earth from beneath the unwary 
ant or beetle, thus deepening the hole, and 
causing the sand on the shelving sides to 
slide down toward the centre, bearing with 
it the prey to within reach of the extended 
jaws. The lower sides of these are pro- 
vided with tubular channels, that serve as 
ducts to convey the blood of the victim to 
the mouth of the captor. After sucking 
the body dry, the carcass is tossed beyond 
the limits of the pit, which is at once re- 
paired, and made ready for the approach of 
a second visitor. 
Simple Lessons in the Art of Photo- 
graphy. 
FIXING SOLUTIONS. 
Continued from page 119. 
THE last important solution used in 
photogrophy is the fixing solution. 
Without fixing solutions, the image invisi- 
bly printed by the actinic rays of light re- 
flected from the object, and rendered visible 
by the developer, would soon fade away and 
entirely disappear. Experiments indefati- 
gably carried on for more than ten years 
were required before fixing solutions — the 
last keystone in the arch of photographical 
operations — were discovered. The fixing 
solutions at present in use are : cyanide of 
potassium and hyposulphite of soda. 
Cyanogen does not exist either in a free 
or combined state in nature; it is a produc- 
tion of decomposition in which the ele- 
ments contained in it are brought together 
in the nascent state. It is a gas which, by a 
pressure of four atmospheres, is reduced to 
the liquid state of the numerous combina- 
tions which it forms. Hydrocyanic acid or 
prussic acid, and cyanide of potassium, are 
the most important; the latter of the two 
is the only one used in photography. The 
extraordinarily poisonous character of this 
compound renders its use rather dangerous. 
Hyposulphite of soda, which possesses the 
same degree of dissolving power, without 
the poisonous qualities of the former salt, 
is obtained by digesting sulphur in a solu- 
tion of sulphite of soda. 
Formula No. 1, (poisonous). Cyanide of 
potassium, 1 drachm; water, 4 ounces. 
Formula No. 2. Hyposulphite of soda, 2 
ounces; water, 4 ounces. 
The photographic properties of cyanide 
of potassium and hyposulphite of soda 
which are called into action, are their power 
to dissolve the iodide and bromide in the 
sensitized collodion film, which have not 
been afi'ected by light. Cyanide is not only 
a solvent, but also a reducer, a fact which 
accounts for the whiteness in the silver film 
when the plate has been fixed by its appli- 
cation. For this reason it is regarded as 
the fixing agent peculiarly adapted for collo- 
dion positives; whereas in the negative, 
where the whiteness of the silver film is of 
little or no consequence, hyposulphite of 
soda is regarded as the proper fixer. 
Negatives fixed by the latter salt require 
a very thorough washing, since if any trace 
of it is left in the film, the image will be 
destroyed by crystallization taking place on 
its surface. 
How to Sharpen a Screwdriver. 
IN driving a screw into wood, the force 
used to press the screwdriver against 
the head of the screw, tends to aid the 
latter in penetrating the wood, but when 
we attempt to extract a screw, every pound 
of pressure that we apply tends to render 
it more difficult to get the screw out. It 
therefore becomes very important that the 
screwdriver should be so formed that it may 
be kept in the nick of the screw by the 
exertion of the very least degree of force; 
for if it has any tendency to slip out, we 
can keep it in place only by applying pres- 
sure, in which case we run great risk of in- 
juring the nick and rendering it impossible 
to draw the screw. If we examine a screw- 
driver in the condition in which it is or- 
dinarily found, we shall find that it pre- 
sents a section like that shown in Fig. 1, in 
which the sides of the wedge, in which all 
screwdrivers terminate, are curves with the 
convex sides outwards, Now, the effect of 
