disk of the planet, Mr. Norman Lockyer has ascribed to the presence of clouds in the planet's own atmosphere.

Mr. Huggins in his valuable paper on the "Spectrum and the Colour of Mars" ("Monthly Notices of the Astronomical Society," March 8, 1867), has referred the absence of colour at the edges of the planet to some peculiar effect of the surface of the planet itself. That the ochreish colour is due to some peculiarity in the surface is, I think, almost proved by Mr. De la Rue's exquisite drawings of Mars, as in these markings of that tint are seen with definite outlines. As I have said, when our atmosphere is free from mist the colour of the equatoreal part is pale, and has not nearly so strongly marked a ruddy tint. This is accounted for by supposing that mist stops the most refrangible rays of light, that is, those towards the blue end of the spectrum, whose waves have the greatest velocity; the red light thus being allowed to preponderate.

When observed under these conditions, the edges of the disk appear Naples yellow, the centre orange, tinged with burnt ochre, while the parts immediately under the dark markings, near the south pole, are whitish, with a tinge of salmon colour. The colour of the dark markings on Mars has been described as greenish or bluish grey; they always appear bluish grey to me, and with this colour I have depicted them. Occasionally the north polar ice has been seen strongly tinted with a bluish colour. I have examined the spectrum of the planet with a direct-vision spectroscope, fitted in the eye-piece of a telescope; the spectrum of the dark markings presented no distinctive peculiarity. This would scarcely prove the entire absence of a blue or greenish shade of colour in the markings, as I have found that if white light be reflected five or six times from surfaces of metallic silver, and then received on white paper, it will be strongly tinged with a chocolate colour. Yet if the light reflected from white paper thus illuminated be examined by means of a spectroscope, no appreciable difference can be seen between the spectrum of this light and that of white light.

In consequence of the effect of irradiation, I have not been. able to make out satisfactorily the outline of the north polar ice. I have frequently seen faint white spots appear on the disk, and as these spots approached the edge of the disk, they increased in brilliancy, until, when nearly at its extreme edge, they almost rivalled the polar snows in whiteness; these white cloudy patches never had any definite outlines. They were generally nearly circular in form, and they always appeared in the region of the equator. One of these white cloudy spots I have shown in the sketch taken on February 8th, at 10.30. The spot is represented passing off the left hand edge of the disk.

Mr. De la Rue has shown the dark markings near the south pole as darkest towards the edge nearest to the centre of the planet, and just below the edge of the dark markings the ruddy colour of the body as much fainter than it is nearer to the centre of the body about the equator. I see these appearances distinctly.

In the drawing taken on January 28th, at 9h, a number of breaks will be seen in the edge of the dark marking near the south pole, and forming a series of light streaks directed towards the pole. Mr. Barnes also sketched this appearance, without seeing my sketch, and as, with the exception of a slight difference in the angle given to the light streaks, the two sketches agreed, I cannot have been mistaken in their appearance having been as I have drawn and described them.

On the 31st of March, at 7h, I obtained an exact repetition of the markings in precisely the same position shown in the drawing taken on February 23rd at 9h.; the two drawings coincide so perfectly that I can only distinguish between them by the dates affixed to them. In these two drawings the mark usually termed the hour-glass mark is represented as having just passed the centre of the disk of the planet. The movement, in the time I have stated, includes a period of 35 revolutions, and the time of a single revolution on its axis deduced from the observed recurrence, would be 24h. 38m. 8s. Beer and Mädler, who have observed the largest number of revolutions, give the period of rotation as 24h. 37m. 23s. My own determination is, however, less than Sir Wm. Herschel's.

Could a repetition of the markings in exactly the same position have been observed after a much greater number of revolutions had been completed, a period of revolution more closely accordant with B. and M.'s determination would probably have been obtained. The very unfavourable weather we have had to contend with for some months, has, I regret, rendered a repetition of the observations impossible.

I have said elsewhere that between my own drawings, Mr. De la Rue's, and Mr. Dawes's, there exists a great similarity, and to one of Secchi's drawings one of mine has a considerable resemblance, but I cannot trace a likeness between any of my views and those of Beer and Mädler. This observation leaves on the mind a suspicion that in the course of time some change may have taken place. With regard to the point that no flattening of the planet at the poles has been detected, even by that admirable observer, Mr. Dawes, I would remark that although if the sphere of the planet were oblate to the extent of onesixteenth, or one-tenth of its diameter, as in the case of Jupiter and the globe of Saturn, the flattening might be easily discerned, yet, if the flattening should not exceed, in proportion,

that of the earth, as would probably be the case from its having nearly the same axial velocity, we could not hope to perceive it, for, under such circumstances, the flattening of the disk of Mars would not exceed twelve miles, and this when the planet is nearest to us, would subtend an angle of only one-tenth of a second.

Professor Phillips's drawings of Mars, of which three are engraved in the "Proceedings of the Royal Society," No. 55, were taken when the north pole of Mars was invisible. Allowing for the difference produced by this cause, the drawings agree with mine in the general form of the markings, but the white margin which I only see in the bays, Professor Phillips shows completely fringing the whole of the markings.

Now half a second is the smallest amount of difference in diameter we could hope to detect, even by the aid of the most delicate micrometric apparatus. The double-image micrometer, devised by the Astronomer Royal, would be the best to employ for this measurement.*

No satellite attending on Mars has yet been discovered. Pursuing the analogy between the planet and our earth, if such a satellite existed of a size proportionate with our moon, as it would be one-quarter the diameter of the primary, we might expect it to be easily visible, but should a satellite exist, not exceeding in size proportionately the second satellite of Jupiter, it would only be visible in very powerful instruments, still it would not, I think, have escaped the notice of the persistent observers who have searched for it hitherto vainly.

The discovery of a satellite to Mars is looked for with interest, as the effect of the sun and the primary in producing perturbations of a satellite would enable the density and mass of the planet to be accurately calculated. At present these are only imperfectly known, but the density is considered to be almost exactly the same as that of the earth.

Unfortunately, observations of value can scarcely be made upon this difficult planet with telescopes of less than six inches aperture, and, unless the observer has extremely good sight, eight or ten inches will be found necessary. During the next opposition I hope to be able to continue my observations with an instrument having a silvered glass speculum twelve inches in diameter.

* In measuring, micrometically, the diameter of Mars, it is very difficult to avoid obtaining too large a result for the polar diameter, the effect of the irradiation causing the white spots near the poles to appear to project slightly from the disk of the planet. The best method, probably, of overcoming this difficulty is by using a single reflecting solar eye-piece.

THE FUNCTIONS OF THE BLOOD.

BY C. W. HEATON,

Professor of Chemistry to Charing Cross Hospital College.

EVERY one who has looked through a microscope at a drop of blood, knows that the red or purple colour is confined to certain minute discs, which resemble pieces of money in form, and which float in a clear yellow liquid. The discs are known to physiologists as the blood-corpuscles, and the liquid as liquorsanguinis. Both these constituents of the blood have their own specific functions to fulfil in the operations of life, and both have been the subject of numberless researches. Very much still remains to be done; but it is not too much to assert that this most wonderful of liquids is slowly yielding up its secrets to the patient workers who have so long sought for them in vain, and that "the blood, which is the life" of the animal, is no longer the utter, hopeless mystery which it has for ages remained.

Careful microscopic measurements have been made of the size of the corpuscles in the blood of different animals, and it is now generally agreed that in the human subject their average length is 1-3200th of an inch, and their thickness 1-12-100th of an inch. Hence it would be possible, if they were packed close together, for 8,126,464 to lie in the compass of one cubic millimetre-a space not larger than a good-sized pin's head. Now the corpuscles occupy, in the aggregate, about one half of the volume of the blood,* and we are, therefore, able to form a good guess at their probable number. Vierordt and Welker have, indeed, gone through the laborious process of counting them; and the former fixes their number at 5,069,000, and the latter at 4,600,000, in the cubic millimetre. It will be seen that these figures agree tolerably well with the rough calculation founded on the size of the corpuscles, and we are, therefore, forced to admit that the tiny red drop obtained from the finger by the prick of a needle, may contain four or five millions of these curious bodies. Such figures, however, give but vague ideas to the mind. A more distinct one is, perhaps, conveyed in the fact, that a room sixty feet long, thirty feet wide, and fifteen feet high, could not contain as many grains of corn as there are corpuscles in a single teaspoonful of human blood, the number being, approximately, eighteen thousand millions!

* This is, of course, only a rough approximation. Their quantity varies extremely in different parts of the body, and even at different times of the day.

It is still doubtful whether the corpuscles consist of red liquid, enclosed by a membrane, or whether they are semisolid, and of uniform structure throughout. Two of the latest investigators, Max Schultze and Ofsiannikof, assert directly opposite opinions upon this point. Whatever they be, however, it is at least certain that they possess a definite term of life. They are incessantly being formed in the chyle and lymph, and also probably in the liver and some other glands. And after the completion of their work, they disappear or are destroyed, this destruction being seen most remarkably in the liver, and in the blood which has traversed muscular tissue.*

The chief function of the blood-corpuscles in the body has long been known, or, at any rate, strongly suspected. They are the carriers of oxygen, the agents of oxidation, in the animal body. During its passage through the lungs, the blood, as every one knows, loses carbonic acid and takes up oxygen. Every 100 volumes of the blood which enters the lungs is capable, according to Claude Bernard, of absorbing twenty-one volumes of oxygen. This is about seven times as much as an equal quantity of water could dissolve, and Berzelius, long ago, showed that serum, which differs but slightly from liquor-sanguinis, was hardly superior to water in this respect. Consequently, it is evident that the great mass of the oxygen must be attracted by the blood-corpuscles. The corpuscles, as before mentioned, constitute about one-half of the bulk of the blood, and, therefore, allowing for the small quantity dissolved by the liquor-sanguinis, we find that they absorb thirty-nine per cent., or thirteen times as much oxygen as water could. That this oxygen is combined in, and not merely dissolved by, the corpuscles, is indicated by the fact observed by Bernard, that pyrogallic acid, a substance that combines eagerly with free oxygen, when it is injected into the veins, will pass out of the body of the animal without undergoing oxidation. It has, therefore, been generally assumed, although upon imperfect proof, that the colouring matter of the corpuscles was capable of combining with oxygen in the lungs, and afterwards of giving that oxygen out again-in small doses, as it were to the substances to be oxidized. This notion has been recently raised to the dignity of a theory by some beautiful experiments which physiology owes to a physicist-Professor Stokes, of Cambridge. Stokes's researches appear hardly to have received from physiologists the attention they deserve, and I, therefore, venture to present a brief description of them here. Hoppe-Seyler had previously recorded the curious fact, that when a ray of white light passes through a weak solution of blood, and is afterwards decom

* Bernard, "Liquides de l'Organisme," i. 460.