It was some time later, however, before this suggested to him any useful application of the process. Another accident made him appreciate the full value of his discovery. This time he carelessly dropped some varnish on a strip of copper which he was going to use in the same way he did the penny. Upon removing the copper from the battery he observed that there was no deposition of copper on those parts of the strip where the varnish had dropped. Spencer then conceived the idea of applying this principle to the arts by coating a piece of copper with varnish or wax and engraving a design in the coating, thus exposing the copper strip in the engraved lines. He did this, and then deposited copper in the design so engraved. Upon removing the coating the design was exposed in relief on the piece of copper. On September 13, 1839, Spencer read a paper before the Polytechnic Institution of Liverpool, which he accompanied with specimens of both electrotypes made by this process and of printing from these electrotypes. The publication of this paper acted like an electric shock upon society. Developing his process, Spencer first used lead as the plastic medium in which to mold printing surfaces, and it is to be noted in this connection that in doing so he anticipated Dr. Albert's lead mold by considerably over three quarters of a century. Spencer impressed a form of type on a planed piece of sheet-lead and subjected both of them to the action of a screw-press. A perfectly sharp mold of the type form was thus made in the lead. This lead mold was placed in a battery, and at the end of _eight days_ a copper shell one eighth of an inch in thickness had been deposited. It was then removed from the apparatus and the rough edge of the deposited copper filed off. Being subjected to heat, the copper shell loosened from the lead-mold. Spencer called this a "copper stereotype." [Illustration] The next step in developing the electrotyping process, after Spencer had demonstrated the practical application of the electro-chemical deposition of a copper shell on a mold, was made by a Mr. Robert Murray. Mr. Murray was the first to use plumbago, or black-lead, to give the surface of non-metallic bodies electro-conductive properties. He discovered that he could coat a mold of bees-wax with black-lead and deposit thereon a copper shell. This was in 1840. In the same year Smee's battery was invented. This was a marked improvement and was a most important step towards making electrotyping a commercial possibility. Thus in 1840, four hundred years after the probable date of the invention of printing from individual movable cast-metal type, and over forty years after the foundation of electrotyping was laid by Volta, electrotyping, as a practical method of reproducing a commercial typographical printing surface, came into existence.