[ 353 I

curiosity, as affording the foundation of one of the writingscalled invisible or sympathetic. For this purpose, vinegaris strongly impregnated with lead, by boiling with li-tharge, cerusie, or other calces of the metal; or whatamounts to the same thing, the common preparation calledsugar of lead is dissolved in water: solutions of lead inaqua fortis answer the same end, except that, when writ-ten with, they are apt to corrode the paper. The sulphure-ous liquor is commonly prepared by boiling some orpi-ment, which is a natural mixture of sulphur and arsenic,in water with quicklime, till the water is strongly im-pregnated with the orpiment: in the room of this prepa-ration may be used a saturated solution of common brim-stone, made by boiling the brimstone either with quick-lime, or in strong alcaline ley.. Characters written withthe lead solution, which when gently dried in the air arenot to be distinguished from the rest of the paper, becomeof a legible deep colour, on pasting over them a pencildipt in the sulphureous solution. Those who amuse them-selves with this experiment, have black characters in theneighbourhood of the invisible ones; which black onesare drawn with burnt cork, or other charcoal blacks,mixed only with water : by the wet pencil, these are wash-ed off, at the same time that the others are made toappear.

If any acid be added to the sulphureous solution, a veryoffensive smell arises; and a solution of sulphur made instrong volatile spirits, prepared with quicklime, exhales alike smell. This penetrating diffusive vapour, particularlythat of the last of these preparations, gives colour to theinvisible writing with the lead solution at a considerabledistance : though the writing be placed in the middle of aquire of paper, or of a pretty thick book, the vapour willsoon reach it, and stain it brown or black. The colour

is