It must be admitted, that this
conjectural explanation is liable to very considerable
difficulties; for, although the converse operation of taking an
impression from a liquid surface has a parallel in the art of
marbling paper, the possibility of transferring the ink from the
copper to the fluid requires to be proved.
Another and more plausible explanation is founded on the
elastic nature of the compound of glue and treacle, a substance
already in use in transferring engravings to earthenware. It is
conjectured, that an impression from the copperplate is taken
upon a large sheet of this composition; that this sheet is then
stretched in both directions, and that the ink thus expanded is
transferred to paper. If the copy is required to be smaller than
the original, the elastic substance must first be stretched, and
then receive the impression from the copperplate: on removing the
tension it will contract, and thus reduce the size of the design.
It is possible that one transfer may not in all cases suffice; as
the extensibility of the composition of glue and treacle,
although considerable, is still limited.
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