Oculus Mundi, or Lapis Mu- tabilis, a kind of precious stone highly esteemed among the ancients, but little known to the moderns till Mr Boyle made his observations upon it. Its specific gravity is about 2.048; its colour of an opaque whitish brown; it is not soluble in acids nor affected by alka- lies, but is easily cut and polished. Sometimes it gives fire with steel, sometimes not. It is infusible per se; but when urged by a blow-pipe, changes to a brownish brittle substance. It is found in beds over the opals in Hungary, Silelia, and Saxony, and over the chal- cedonies and agates in Iceland. These stones in gen- eral are either of a yellowish green, milky grey, or of a yellow like that of amber.

The most remarkable property of this stone is, that it becomes transparent by mere infusion in any aque- ous fluid; but gradually resumes its opacity when dry. There are three of these stones in the British museum at London; the largest of them about the size of a cherry stone, but of an oval form. It is opaque and coloured like a common yellow pea; it may be scratched, though not without difficulty, by a common knife, notwithstanding which it seems to leave a mark upon glass. It does not ferment with nitrous acid. When it has lain some hours in water, it becomes transpa- rent, and of a yellow amber colour. The change be- gins soon after the immersion, and at one end in form of a little spot; but in a small one of the same kind, the transparency begins round the edges. By degrees the spot increases, until the whole stone becomes uni- formly clear throughout: when out of the water it loses its transparency, first at one end, and then gradu- ally over the remainder, until the whole has become opaque; which change happens in less time than it takes to become transparent. This change is not entirely peculiar to the hydrophanes. Bergman informs us, that some teatites produce the same effect; and M. Magellan, that the crust of chalcedonies and agates frequently produce the same appearance.

Messrs Bruckman and Veltheim were the first who particularly inquired into the nature of this stone, and investigated its properties, many of which were brought to light by their endeavours. Their account is to the following purpose. As soon as the stone is put into water, it exhales a musky smell, several air-bubbles ar- rise, and it becomes gradually transparent. Some of the stones become colourless as soon as they are tho- roughly transparent, others have a more or less deep yellow colour; some acquire a beautiful ruby colour; and, lastly, others gain a fine colour of mother-of-pearl, or of a bluish opal. Whatever be the colour of the liquor in which the hydrophanes is immersed, it gains only its usual degree of transparency with the colour peculiar to it. When we look at it in its moist state, we perceive a luminous point, varying its situation as the position of the eye is altered. This luminous point is not, according to Mr Bruckman, the immediate image of the sun, but a reflection of that image refracted in the substance of the stone itself; a phenomenon which probably gave rise to the name of oculus mundi. Mr Bruckman left a piece of this stone weighing 35 grains seven hours in water, the space requisite to make it perfectly transparent; and in that time he found that it had gained three grains in weight. The hydrophanes becomes much sooner transparent when put into hot water; and the same happens if it be dipped in a very dilute acid, or rather a very dilute solution of alkali. When dipped in oil of vitriol, it becomes very quickly transparent, and will continue so, on account of the strong attraction of that acid for moisture, which takes as much from the atmosphere as is necessary to keep the stone transparent; but its opacity will return if it be dipped in an alkaline liquor and afterwards dried.