308 On Electrotyping Operations of the U. S. Coast Survey. 
This force is very powerful between some bodies, while be- 
tween others it is very slight. Air adheres very strongly to 
metals, as before referred to; hence a film of air may unite two 
copper plates, even though they are separated beyond the dis- 
tance at which cohesive attraction takes place. 
Wax is a common ingredient in cements; its adhesive "- 
ties have become proverbial; its use is evidently improper. 
Therefore a substance having a strong adhesive attraction for the 
plates must not be on the face, and the cohesive foree of the sur- 
face particles must be suspended by other methods than making 
the deposited metal deficient in mechanical properties. 
_ It was hoped that a substance could be found that would act uni- 
formly and gently on the surface of the engraved plate, and which 
in destroying the homogeneous attraction of the surface particles, 
would, by chemical union with them, form an insoluble and fri- 
able compound, having but a slight adhesion to the plate. Iwas 
led to select iodine for the experiment on account of its sparing 
solubility in water, its high equivalent number, and innoxious 
qualities. A copper plate was well cleaned, exposed to the vapor 
of iodine, and electrotyped ; the deposit separated from it readily. 
This was repeated some hundred times with invariable success. 
_ It was found, in cleaning large plates for the application of the 
iodine vapor, that while one part of the plate was being cleaned, 
another part would tarnish, and hence a uniform action of the 
iodine could not be obtained. This led to silvering the plates 
before iodizing, which facilitated the cleaning and exhibited the . 
action of the halogen. A silvered plate was washed with an 
alcoholic solution of iodine and electrotyped; the electrotype 
separated from the matrix yet more readily than before, the iodid 
of silver serving better to prevent adhesion than the iodid of 
copper. ‘ 
_But it was soon observed that a plate prepared on a dull day 
did not separate so readily as one prepared under a bright sky, 
and on experimenting it was found that a plate iodized and eX- 
posed to sunshine would separate with very great facility; while 
a plate iodized ona rainy day, and placed in a dark room for a 
ew hours before introducing it into the vat, might stick so har 
as to require some of the old resorts of heating and jarring * 
Separate it from the matrix. 
_ The process of iodizing and exposing to light has now been 
applied to a very great extent of finely engraved surface, and ft 
no case has the least difficulty been found in lifting one plate 
by intervening between the : ity of 100) 
: 0g bet: he plates; but the quantity i wi 
ed | plate must be thought insufficient to effect it by 
mere mechanical separation when 
