This section is from the book "A Library Of Wonders And Curiosities Found In Nature And Art, Science And Literature", by I. Platt. Also available from Amazon: A library of wonders and curiosities.
There are three of these remarkable stones in the British Museum; 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 transparent, and of a yellow amber colour. The change begins soon after the immersion, and at one end, in form of a little shot; 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 uniformly clear throughout: when out of the water it loses its transparency, first at one end, and then gradually over the remainder, until the whole has become opaque, which change happens in less than it takes to become transparent. This change is not entirely peculiar to the hydro-phanes. Bergman informs us, that some steatites produce the same effect; and M. Magellan, that the crust of chalcedonies and agates frequently produce the same appearance. Messrs. Buck man and Veltheim were the first who particularly inquired into the nature of this stone, and investigated its properties. Their account is as follows: - "As soon as the stone is put into water, it exhales a musty smell, several air bubbles arise, and it becomes gradually transparent. Some of the stones become colourless as soon as they are thoroughly transparent; others have a more or less deep yellow colour; some acquire a beautiful ruby colour; and 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 per-ceive aluminous 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 its 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 then dried.