THE YOUNG SCIENTIST. 
117 
the plants are dry. In the first place, a 
thoroughly dry plant is generally rigid, 
unless it be very long and weak. Feeling 
of the plant by the lips, or placing the 
hand on the sheet of paper from which it 
has just been taken, are other tests, but in 
these cases you must distinguish between 
coldness and dampness. Generally speak- 
ing, you may rest satisfied that if the speci- 
men has been under pressure, with dry and 
frequently changed papers, for eighteen or 
twenty days, it is likely to be quite dry, 
unless its nature is such as to make the 
matter doubtful. 
Now comes the mounting, which should 
not be deferred, as postponing it probably 
means spoiling the specimens and losiug the 
labels, and certainly means an accumulation 
of work at some future time. For fixing 
the plants, some recommend the applica- 
tion of hot glue; but this is very trouble- 
some to manage, and as the operation of 
carefully gluing a large specimen is rather 
a long one, the glue is not likely to be very 
hot when the time comes for fastening it 
down. Strong gum answers quite as well 
and is more convenient. To make the 
gum, take one ounce of picked gum arabic, 
as colorless as possible, powder it, and stir 
it with a clean stick or a glass rod in an 
ounce and a half of cold water until it is 
dissolved; add a quarter of an ounce of 
powdered gum tragacanth ("gum dragon ") ; 
and lastly, add two grains of corrosive sub- 
limate, previously dissolved, along with two 
grains of sal ammoniac, in one drachm of 
water. A metal stirring- rod should not be 
used, as it is apt to discolor the liquid. The 
latter, if carefully made, is quite colorless, 
and does not show much, if any be acci- 
dentally smeared. (N. B. — Corrosive sub- 
limate is very poisonous, and the bottle of 
gum containing it should be labeled " Poi- 
son.") 
To he continued. 
— Prince Leopold, the youngest son of 
Queen Victoria, has written a book upon the 
Polarization of Light, which is to be pub- 
lished during the present year. 
Simple Lessons in the Art of Photo- 
graphy. 
THE SILVER BATH. 
Continued from page 92. 
'^PHE cause that so many amateurs fail 
completely in photographing may, in 
nine cases out of ten, be traced to the silver 
bath. Any one not practically acquainted 
with photography will undoubtedly be 
greatly surprised on being informed that 
more mistakes will occur in making the sil- 
ver bath than in compounding any other 
liquid used in photography. The whole 
secret of making a good silver bath that 
will produce clear negatives and keep its 
good properties for a reasonable time, lies 
in the application of heat, in one form or 
other, in order to effect a chemical mixture 
of the nitrate of silver in water. 
It is true that nitrate of silver is readily 
dissolved in water in greatly varying pro- 
portions, but without the action of light or 
heat the mixture will rather be mechanical 
than chemical, and the results obtained 
with such a bath will be entirely unsatis- 
factory. Nitrate of silver of chemical pur- 
ity can be bought either from druggists or 
dealers in photographic materials at such a 
comparatively low price that it will not pay 
to incur the trouble of making it. Distilled 
water is not absolutely necessary, but under 
no circumstances should spring water be 
used. Ice water or rain water do very well. 
The best way to proceed in making a sil- 
ver bath is as follows: First clean from all 
impurities, by means of sulphuric or nitric 
acids, the bottle which is going to serve as 
a temporary receptacle for the silver solu- 
tion. Select a bottle of white glass. Fill 
the bottle with 12 ounces of water, and dis- 
solve in it 1 ounce nitrate of silver. The 
bottle should now be placed for ten or 
twenty hours in the full sun, where the 
liquid will gradually change into red brown, 
or even black, and finally, after the settle- 
ment of some dark-colored sediment on the 
bottom of the bottle, the fluid will exhibit 
the purest crystalline transparency. The 
settlement of the sediment is the sign that 
