i aa a a a a cr te ti a 
On Electrotyping Operations of the U. S. Coast Survey. 309 
of strong alcohol. If one grain of the solution is required to 
Wet a square foot, it will give but ;;1,,;th of a grain of iodine 
a square foot. But as the iodine evaporates rapidly with the 
alcohol, probably the actual quantity on a square foot does not 
exceed one-hundred-thousandth part of a grain. 
king the weight of a cubic inch of iodine at 1,250 grains, 
and supposing that it remains on the silver surface in its elemen- 
lary state, instead of forming iodid of silver, then we have 1,250 
x 144 100,000 = 18,000,000,000, only one-eighteen-thousand- 
millionth part of an inch for the thickness of the coating of 
iodine. Even if we suppose that the solar rays decompose the) 
lodid of silver, and leave the iodine in vapor on the plate, it will 
I only one-forty-four-millionth part of an inch—a thickness 
to be taken as nothing in a mechanical view. 
_To test the effect of the chemical method of preventing ad- 
hesion on the sharpness of the engraved lines, an engraving was 
“ven times successively transferred from plate to plate, when the 
: Closest inspection failed to show any inferiority of impressions 
from the last plate as compared with those from the first. 
Ti ime and expense of electro-casting.—Next in importance to 
Securing a certain and easy separation of the matrix and casting 
is bringing the entire time and expense of electrotyping within 
the narrowest limits. 
4. Smee and others have shown that the quality of electro- 
metal is determined by certain relations between the rapidity of 
“ming the plate and the strength of the solution in which it is 
.l€d. Both the common operations of the electro-metallur- 
gist, and the improvements he proposes, must conform to these 
relations, : 
_ As small quantities of electricity are easily set in motion, small~ 
Sized electro-castings are readily made in six or eight days. ‘To 
oped € castings in a short time requires a powerful current, 
elem om Plish the corresponding augmentation 1n the effective 
ite action has proved a somewhat difficult matter. 
. At the date of the “Aide Mémoire to the Military Sciences,” 
Itis Stated that in the ordnance survey one pound of copper was. 
the ted in twenty-four hours on a plate of eight square feet, 
by Plates being made ductile enough to bear hammering only 
? Continued agitation of the electrolytic solutions. 
