thrown up into a small receiver of common air over mercury, immediately changed colour, and became brown. When the powder was heated out of the contact of air it gave off ammonia, and the chloride remained.

I have not examined the action of ammonia upon the other chlorides; with some of them it would probably form neutral compounds, with other combinations similar to those described. Nearly all those mentioned are formed by the exertion of an affinity so weak that it is overcome by the attraction of water for the ammonia, and yet in one instance it is capable of giving a definite crystalline form.

The facility with which many of them afford dry ammoniacal gas at low temperatures in considerable quantities, may perhaps in some cases make them convenient sources of that substance; 19 grains of the compound with chloride of lime which had been made many days, gave 19.4 cubical inches of gas. They also offer a convenient means of ascertaining the specific gravity of ammonia, by the quantity of gas given off, and the loss of weight in the substance.

ART. IX. Observations on the Rays which compose the Solar Spectrum.

THE

HE third volume of the Memoirs d'Arcueil contains a paper by M. Berard on the properties of the different kinds of rays existing in solar light; the object of the Memoir is to point out by delicate experiments the relative situation and intensity of the heating rays, the rays of light and the chymical rays, and to show the analogy which exists between them in their general physical properties.

The results contained in that part of the paper which relate to the arrangement of the three set of rays, in the prismatic spectrum, are well known here, and require no farther notice than to say that they agree nearly with the experiments of Herschel and others in this country; but the experiments

on the polarization of the rays of heat, and the chymical rays are not so well known, though they deserve every attention for their association with the brilliant discoveries made regarding the nature of light.

Having substituted a prism of calcareous spar for the one of glass used in previous experiments, M. Berard found that in each image formed by the prism, the red extremity was hotter than the violet, and this induced him to suspect that the rays of heat underwent a double refraction in the manner of the rays of light. This idea was strengthened and ultimately confirmed by further experiments, which we shall translate from his own description.

"I received all the solar rays reflected by the mirror of the heliostata, on a glass plate, at an angle such that the reflected rays were polarized, and these were again received in their turn on a second glass properly inclined. I reunited the rays reflected from this second glass by a metallic mirror, in the focus of which I had placed an air thermometer. I found that when the second glass reflected the polarized rays, the thermometer rose, and when the glass did not reflect the polarized rays, the thermometer rested stationary."

This experiment proves in an evident manner that the heat which accompanies the solar light is polarized at the same time with the light, and nearly under the same angle.

To ascertain whether the rays of heat proceeding from heated bodies were also polarized in the same manner, M. Berard proceeded as follows: "I placed in the focus of a metallic mirror 3 decimeters (about 11.8 inches) in diameter, a lighted taper, I inclined the mirror so that the parallel rays reflected from it made an angle of 19° 10′ with the horizon. I will suppose, to give clearness to the explanation, that these rays proceeded in the plane of the meridian, from south to north. I received them on a glass, 30 centimeters, (11.8 inches) long and 22 (8.7 inches) wide: this glass was disposed so that it reflected the light of the taper downwards in a perpendicular direction; and beneath it a second similar one was placed parallel to it, which reflected the rays again from the south towards the north. I received these last rays on another

metallic mirror, in the focus of which was an air thermometer, having a blackened bulb and a long tube. This mirror and the second glass were fixed together, so that they could be turned round horizontally without changing their relative position, or the inclination of the glass." The apparatus being thus disposed, it was easy, by turning the lower glass and mirror round, to ascertain that the light was constantly concentrated on the bulb of the thermometer, sufficient being reflected even at the position where the greatest quantity was absorbed by the second mirror, to render the focus evident; the taper was then removed, and the whole suffered to cool.

"In the course of some minutes (M. Berard says) I placed a heated ball of copper, about the size of an egg, exactly in the previous position of the taper, and at the moment, the air thermometer rose about 50 centimeters (19.7 inches); then turning the second glass towards the west, the thermometer sunk more and more as it approached that point. I left it some time in that position, and the thermometer returned to within 2 centimeters of its first point. I continued to turn the second glass, and as it approached the south the fluid again rose, where having left it about a minute, it had mounted to 45 centimeters; (17.7 inches) continuing the motion of the glass, the thermometer cooled gradually until it had reached the east, where remaining two minutes, it had gained its original temperature."

This experiment, which was repeated a great number of times, proves, that radiant heat reflected by a glass, at an angle of about 35°, and falling on a second glass, making the same angle with its surface, is reflected by this second plane when it is turned in two positions opposite to each other, and is not reflected in two other positions equally opposed, each being intermediate and equally distant from the two first. Radiant heat, therefore, like light, may be polarized.

As to the angle at which radiant heat is most completely polarized, I have not found the means of determining it exactly: but the preceding experiment proves that it does not differ much for the same reflecting substance from that at which light is polarized.

M. Berard then substituted for the two glass reflectors, two

polished metallic surfaces, and found that in every position of the second surfaces, the thermometer rose nearly to the same degree. The angles of incidence were varied without any superior effect being observed towards the east than in the south; from whence it is concluded, that to heat, as with light, metallic surfaces cannot communicate the same singular properties as are given by surfaces of glass.

The rest of the section on the rays of heat, refers to experiments well known here on their absorption by opaque bodies, and their reflection and transmission by transparent sub

stances.

The second section of the paper is employed on the chymical rays found in the solar light. Having noticed the properties of these rays in effecting certain chymical changes, and marked out the degree of effects produced by them in the different parts of the spectrum, the author says, I received the chymical rays directed in the plane of the meridian on a glass surface, at an angle of incidence of 35° 6'. The rays reflected by this first glass were received on a second at the same incidence. I found that when this was turned towards the south, the muriate of silver exposed to the reflected invisible rays was blackened in less than half an hour, whilst if turned towards the west, it was not at all discoloured in ten hours.

The chymical rays, therefore, may be polarized like the rays of light, by surfaces of glass under a certain angle, and this angle appears to be nearly the same for both kinds of rays. It is therefore to be presumed also, that the chymical rays will suffer double refraction in passing through certain diaphanous bodies.

The paper terminates by a series of conclusions drawn from the experiments described in the body of it. With the exception of those on polarization, they are so well known from the experiments of Herschel, Sir H. Englefield, Dr. Wollaston, and others, that they require no particular notice. Those relating to polarization are, that the calorific or heating rays may be polarized by glass surfaces, and that they are affected by metallic surfaces similarly to the rays of light.

Radiant heat emitted by hot bodies, is polarized by glass surfaces, when reflected at an angle equal to that at which light is polarized. Metallic surfaces have a similar action on both kinds of rays.

The chymical rays may be polarized by glass surfaces, and possess all the general physical properties of the luminous rays.

ART. X. On the Colours of Waters. By G. W. Jordan, Esq., F. R. S.

THE colours of waters in the scenes amid which they are diffused, and of the atmosphere, and bodies existing in the atmosphere from which they are derived, have heretofore only incidentally been referred to; never, that I know of, generally stated or accounted for. Poetry has her blue and green seas, and Geography her black, white, red, and yellow seas.

The internal colour of the sea on the British coasts is green, from vegetable colouring matter brought down into it by the waters of the land. Beyond the limits of this green colour, at distances remote from land, the waters of the ocean have generally been considered as blue, from the almost constant exhibition of this colour at its surface. But neither the waters of the coast, nor of the ocean, are essentially or exclusively green or blue.

Water and sea water are in themselves perfectly transparent and colourless. The colours which they exbibit in their states of accumulation, as rivers, lakes, and seas, depend upon, and are produced by, other bodies, from without and from within the waters, and are as various as those bodies and their reflections. They are not of the water; those from without are rather of the air, being reflected by the air incumbent on the surface of the water, and the water itself being altogether incapable of reflecting from and by its surface of external incidence; those from within by bodies within the waters.