any I have met with, but to delight in the: information I give you relating to chemical things.

Here I returned my Chemift many thanks, and profeffed my eternal obligation to him; that I could liften for years to him; and wifhed it was poffible to become his difciple, that I might fee him by experiment facilitate the study of a fcience, more entertaining, instructive, and extensively useful than any other. But how, dear Sir, am I to use this ink, you are fo vaftly good as to give me, to make it more useful than any other ink could be?

I will tell you (Ribble replied:) you must write with this lilach-coloured liquor, on good well gummed paper, that does not fink; and the fingularity of the ink, confifts in its property of difappearing entirely, and becoming invifible, though it be not touched with any thing whatever: And: this distinguishes it from all others: The writing muft dry in a warm air, and while it is cold no colour can be perceived: but gently warming it before the fire, the writing gradually acquires a greenith blue colour, which is vifible as long as the paper continues a little warm, and difappears entirely when it cools. When other fympaVOL. IV.

F

thetic

thetic inks are made to appear by proper application, they do not difappear again; but this liquor from the ore of bifmuth muft have the fire or heat kept to it, to render it legible. If a man writes to his mistress, fuppofe, or to a minifter of state, with lemon juice, once the writing has been warmed by the fire, and the letters by that means appear, the epiftle may be afterwards read at any time and place; but if the lady's father should by accident get your letter, written in lilach-coloured liquor, it muft ftill remain a fecret to him: For if on getting it, and opening the feal, he could fee no writing, and therefore ima-gining it was writ with lemon juice, or fome other fympathetic ink, he fhould hold it himself to the fire, or bid his fervant hold it to the heat, that the letters might be produced, and made vifible, yet the moment bifmuth-ink is taken away from the fire, and begins to cool, it is as invifible again, as a fheet of white paper. How ferviceable this may be on various occafions, may be easily conceived.

Of Zinc.

But as to our third femimetal, called Zinc, this is fo like bifmuth to appearance, that some have confounded it with Zinc; though it differs from it effentially in its properties, and will

unite with all metalline fubftances, except bifmuth. It is volatile by fire above all things, and makes a fublimate of the metallic fubftances with which it is fufed. Zinc mixed with copper in the quantity of a fourth part, produces brafs. If the Zinc is not very pure, the compofition proves tombac, or Prince's metal.

The nature of regulus of Arfenic.

Regulus of arfenic, the fourth femi-metal, has a colour refembling lead, unites readily with metallic fubftances, and renders them brittle, unmalleable, and volatile. The calx of it produced by fire, may be made volatile by more fire, and in this differs from the calx of all metalline fubftances; for all other calx's are fixed, and cannot be moved. It has likewife a faline character, in which its corrofive quality or poifon confifts: a quality from which the other metallic fubftances are free, when they are not combined with the faline matter. Thefe things being noticed, in relation to metals, and femi-metals in general, I will now proceed to relate a few curious cafes, in refpect of the metals.

Gold, our firft metal, has ten fenfible "criterions. It is the heaviest and densest

The characters of Gold.

of all bodies: the most fimple of all bodies: the most fixed of all bodies: the only body that cannot be turned into fcoriæ, by antimony and lead; the most ductile of all bodies fo foft as to be fcarcely elaftic or fonorous: must be red-hot to melt: is diffolvable by fea-salt and its preparations, but remains untouched by any other fpecies of falts; and of confequence not liable to ruft; as aqua regia and fpirit of seafalt do not float in the air, unless in laboratories, or chemifts fhops, where we find them fometimes: It unites fpontaneously with pure quickfilver: It never wastes by emitting effluvia, or exhalations. These are the ten fenfible properties or characteristics of this metal. It is certainly pure gold, if it has these criterions, and they are of great ufe in life; especially to perfons who have to do with that fubtil tribe, the alchemists.

As to the weight of gold, it is more than nineteen times heavier than water, bulk for bulk, and this property is infeparable from it; it being impoffible to render gold more or lefs heavy; and for this reafon, the fpecific gravity of gold, if it had no other criterion, might demonftrate real gold. To make gold, other metals must be rendered equiponderant to it; And therefore, if an alchemist should offer to obtrude a metal

no

on you for gold, hang an equal weight of pure, and of fufpected gold by two threads to a nice balance, and on immerging them in water, if the alchemift's gold be pure, the water will retain both pieces in equilibrio; otherwife, the adulterate metal will rife, and the pure defcend.

The reafon is, all bodies lofe fome of their weight in a fluid, and the weight which a body lofes in a fluid, is to its whole weight, as the fpecific gravity of the fluid is to that of the body. The fpecific gravity of a body is the weight of it, when the bulk is given; 38 grains of gold weighed in the air, is not the true weight of it for there it lofes the weight of an equal bulk of air: It weighs only 36 grains in the water, and there it lofes the weight of as much water, as is equal in bulk to itself, that is, two grains, and as the gold weighs 38 grains, it follows, that the weight of water is to that of gold, bulk for bulk, as 2 to 38, that is, as the weight lost in the fluid is to the whole weight.

And fo, if a piece of gold, and a piece of copper, are equiponderant in air, yet in water the gold will outweigh the copper; because their bulks, tho' of equal weight, are inverfely as their specific gravities, that F 3

is,