Letters and divers other mixt discourses in natural philosophy many of which were formerly published in the Philosophical transactions of Mr. Oldenburg, and part in the Philosophical collections of Mr. Hooke and else where : all which are now revised, augmented, and to them are added very many other matters of the same nature, not before published : also an intire treatis of the nature and use of colours in oyl. painting / written by M. Lister, F. of the R.S.

II. Of an odd figured IRIS.

I have not observ'd any Rock or sort of stone, whether Metalline or more Vulgar, which hath not its differ∣ent sort of Sparr, shot in some part or other of its bed or seams. And these Sparrs differ not only in their Co∣lours and other accidents, but eminently too in their Figure. To pass by divers, which I have collected, I shall describe one of a very curious Figure, and which (though very common in our blew-Lime-stone Rocks, out of which plently of Lead-Ore is got,) yet is not, that I know of, mention'd by any Author.

These Crystals are mostly of a black water, like the black flint in Chawk-hills; but there are of them, which have a purplish or amethystine colour; and there are as clear as crystal. They adhere to the seams of the rock, be it betwixt bed and bed, or where-ever there are cross and oblique veins through the very substance of the bed.

The smaller the veins, the less the Iris. You will find of them as small as wheat-corns, and others an hundred times bigger. They shoot from both sides the the seam, and mutually receive one the other.

They are figured thus, viz. a column consists of three quin-angular plains, very little rais'd in the middle: these plains too are very unequal. Let them hug one another, or be any ways straightned and compressed in their shooting; yet the number of plains meution'd, both of the column and top, is most certain. The pla∣ces, where infinite of them may be had, are Rainsbo∣rough Scarr upon the Rible; also in a Stone-quarry near Eshton Tarne in Craven.