I did not continue trials with zinc, because just then I obtained a handsome piece of Kellheimer stone for the purpose of rubbing down my colors on it; and it occurred to me that if I painted this stone plate with my wax ink, it would serve as well for practicing as copper or zinc, with very little labor in grinding and polishing. The experiments succeeded, and though I had not thought originally that the stone itself might be used for printing (the samples I had seen hitherto of this Kellheim limestone were too thin to withstand the pressure exerted in printing), I soon began to believe that it was possible. It was much easier to do good work on the stone than on the copper. I observed also that I needed weaker and much diluted aqua fortis. A stone mason told me that he could provide me with this sort of limestone in plates from one inch to eight inches thick. Thus I needed not to fear cracking of the stone; and the only thing that I needed to invent, in order to use the stone just like copper, was either a way to give the stone a better polish, or else a tint which would be easier to rub away than the ordinary copper-plate printing-ink. The stone will not take the polish that is demanded with ordinary printer's ink,--and perhaps this is the reason why the stone has not been used long before my time as substitute for copper, for I imagine that such attempts must have been made. I tried all possible kinds of polishing and grinding without attaining my purpose completely. The result was best when I poured a mixture of one part of concentrated oil of vitriol and four or five parts of water over the stone after polishing it. This mixture, which is very sharp, has the property of boiling immediately when poured over the stone, but ceasing instantly, so that one is tempted to believe that the vitriol has sated itself and lost its power. This is not so, however; for the same fluid, placed on an untouched part of the stone, boils again at once. The reason is that a firm skin of gypsum forms at once on the stone, and this remains impervious to the fluid. If now the etching fluid is poured off and the stone is rubbed lightly with a rag, it attains a shining polish. Unfortunately this is so thin and weak that one can make barely fifty impressions without repeating the process, which involves some loss to the drawing. But if one desires to print in the present chemical style, that is, wet, and the stone is polished before the drawing, one can make several thousand imprints, which will be described in the proper place.