"I was preparing a slab of stone for engraving, when my mother asked me to write a memorandum of things she was about to send to be washed. The washerwoman was waiting impatiently whilst we searched in vain for a piece of paper, and the common writing ink was dried up. Having no other writing materials, I wrote the washing bill on the stone I was about to prepare for engraving, using for the purpose my ink made of wax, soap, and lamp-black, intending to copy it afterwards on paper. Whilst looking at the letters I had written, the idea all at once occurred to me how it would do to cover the stone, with the writing upon it, with aqua-fortis, so as to leave them in relief, and then to print from them in the same manner as woodcuts, with a common letter press. The attempts I had hitherto made to engrave upon stone had taught me that the relief of the letters thus obtained would not be much. Nevertheless, I made the attempt. I mixed one part of aqua-fortis with five parts of water, and poured it on the stone to the height of two inches, having previously walled it round with wax in the usual manner. The diluted aqua-fortis was permitted to rest on the stone five minutes. I then examined the effect, and I found that the letters were raised above the stone about the thickness of a card. Most of the lines were uninjured, and retained their original size and thickness. This gave me the assurance that writing, sufficiently traced, especially if the letters were in printed characters, would have still greater relief."[17]