from the fire, and attend diligently to the application of the ether. Dry muriatic acid gas, and common air, do not affect iron in the cold, at least during a period equal to their joint application in the present case, as I have found by experiment in a graduated glass-tube over quicksilver; and as the whole atmospheric air was expelled, before the application of heat, the effect is solely due to the acid gas, as, indeed, its progressive increase, with the duration of the process, sufficiently attest. The oxygen of the atmosphere could have no influence on the result *.

There was found within the tube, near the end farthest from the retort, on the verge of the ignited part, a white pulverulent matter, glistening like snow, and the adjoining laminæ of iron, were encrusted with the same substance, in spangling crystalline plates.

ap

This powder dissolves readily in water. Into recently boiled water, when a little of it was put, and a small fragment of crystallised prussiate of potash was added, a greyish cloud peared, speedily becoming blue. Tincture of galls dropped into a similar solution, gave the characteristic purple tinge of iron. Thirteen parts of it by weight, being ignited in a small platina tray, evolved copiously a dark-brown smoke, smelling of muriatic acid, and left 6 parts, which were red oxide of iron. But 13 of green muriate, by Dr WOLLASTON's Scale, are equivalent to 8 of red oxide. Does the above muriate contain the atomic

* M. GAY LUSSAC, in his Recherches Physico-chimiques, describes a similar experiment, but without the production of water. If the utmost precautions be not taken, to keep the condensing tube at a very low temperature, the expanded and heated acid gas will readily carry off the moisture, as I found in one experiThe neglect of these precautions will account for the difference of M. GAY LUSSAC's result.

ment.

atomic protoxide of iron, hitherto unknown? This question must be answered by future researches.

I tried to determine the proportion of muriatic acid in the above muriate of iron, by nitrate of silver. The quantity on which I operated was, however, too small to permit me, to place much confidence in the result. But one circumstance arrested my attention. A portion of revived silver was found at the bottom of the glass capsule, on pouring off the precipitated muriate.

On the general principle of research, above stated, the product of water or liquid, will be proportionate to the quantity of muriatic acid gas, condensed into muriate of iron. Hence, to obtain large results, it is proper to have a considerable portion of iron lamina, placed just at, or a little beyond, the limit of ignition.

From the whole of the preceding experiments, we may legitimately conclude, that muriatic acid gas hygrometrically dry, contains much combined water. And since that gas results from the union of chlorine and hydrogen in equal volumes, each likewise hygrometrically dry, the above water must be formed in consequence of the hydrogen finding oxygen in the chlorine, for its saturation. Chlorine is therefore Oxymuriatic, or Oxygenated muriatic acid.

My experimental examination of iodine, has further led me to conclude, that this curious substance is not entitled to rank in the same class with chlorine, but with sulphur. The details will form the subject of a separate memoir.

VOL. VIII. P. II.

Y y

XVIII.

XVIII. On the Laws which regulate the Distribution of the Polarising Force in Plates, Tubes, and Cylinders of Glass, that have received the Polarising Structure. By DAVID BREWSTER, LL. D. F. R. S. LOND. & EDIN.

(Read June 17. 1816.)

IN the Philosophical Transactions for 1816, I have described at great length the various phenomena which are exhibited by glass and other substances to which the property of double refraction has been communicated by heat, by rapid cooling, by evaporation, or by mechanical compression and dilatation. In pursuing the same subject, I have observed many singular facts respecting the developement of new axes, by a change in the form and condition of the plates; and by submitting the phenomena to accurate measurement, I have succeeded in determining the laws which regulate the distribution of the polarising force. A brief account of these results will form the subject of the following paper.

1. On Plates of Glass with One Axis of Polarisation.

If we take a plate of glass perfectly circular, and communicate to it the polarising structure, either transiently, by the transmission of heat from its circumference to its centre, or permanently, by cooling it rapidly, when it has been made red hot, we shall find that it will exhibit, when exposed to polarised light, a system of rings traversed by a black rectangular This system of rings is precisely the same, both in appearance and in the character of its tints, as the system seen Y y 2 along

cross.

along the axis of Ice, Quartz, &c. and other crystals of the sitive class.

po

If the circular plate of glass, on the contrary, receive the polarising structure transiently, by heating it uniformly in boiling oil, and allowing it to cool rapidly, it will exhibit a similar system of rings; but this system has a negative polarisation, like the rings formed by Calcareous spar, Beryl, &c. and other crystals of the negative class.

This opposition in the character of the two plates may be finely observed, by combining them together. The resulting system of rings, when two positive or two negative plates are combined, will be the same as that which would have been produced by a plate equal to the sum of their thicknesses; but when the one is positive, and the other negative, the resulting system will be that which would be produced by a plate equal to the difference of their thicknesses. Hence, when the negative system is exactly equal to the positive system, they will destroy each others effects, and the compound plate will have no action whatever upon polarised light.

By comparing the value of the tints with their distances from the centre of the plate,. I have found, that they vary as the squares of their distances from the axis. Hence if T is the tint which corresponds to any distance D, the tint t corresponding to any other distance d, will be found by the formula T d2 t = D

2. On Plates of Glass with Two Axes of Polarisation. When a plate of glass deviates from the circular form, and is either elliptical or rectangular, it has two axes of polarisation, one of which is perpendicular to the plane of the plate, and the other at right angles to it, and lying in the plane of the plate. When the plate has received the polarising structure transiently, by the transmission of heat, or permanently, by

being