:

suckers at the end, which aid the animal in its progressive motion.

The mouth of this animal is armed with a most complex apparatus of calcareous jaws, arches, and teeth, consisting of twenty-five separate pieces. For the movement of these parts separate muscles are provided, of which the anatomy has been minutely described by Cuvier. In the shells of the echini which are cast on shore, this frame-work is often found entire in the inside of the case, and Aristotle having found in it a resemblance to a lantern, it has therefore been called "the に lantern of Aristotle." But there are other echini which are entirely destitute of this apparatus, being without teeth, and having at the mouth only a narrow transverse slit. From this variation of form, it is natural to suppose that the food of the different species of echinus is also various. Mr. Kirby, speaking of the common echini, informs us, that their station is often near the shore upon submerged ledges of rocks, and that they feed upon whatever animals they can seize, sometimes turning upon their back and sides, and sometimes moving horizontally. "This enables them more readily to secure their food, with the aid of the numerous suckers in the vicinity of their mouth, which, when once they are fixed, never let go their hold till the animal is brought within the action of their powerful jaws. Lamarck thinks that they do not masticate, but only lacerate their food; but as two faces of each of their pyramidal organs answer those of the two adjoining ones, and these faces are finely and transversely furrowed, this looks like masticating surfaces. Bose, who appears to have seen them take their food, says it consists principally of young shell-fish and small crustaceous animals. As the latter are very alert in their motions, it is difficult for the sea-urchins to lay hold of them; but when once one of these animals suffers itself to be touched by one or two of the tentacles of its enemy, it is soon seized by a great number of others, and immediately carried towards the mouth, the apparatus of which developing itself soon reduces it to a pulp.'

6

The development of the echini from the time of their first leaving the egg, has not come within the observation of any naturalist, but the young of Echinus esculentus has been examined when only one-eighth of an inch in diameter, and found to have the form and armature of the full-grown animal. The prickles were toothed along their edges; but those spines which in the perfect state have three prongs, as already described, were only provided with two. The globular form was perfect in the young animal; but the shell was composed of few pieces. It may appear contrary to the general law which regulates the mode of increase in these animals, that the shell should be thus perfectly formed in miniature, for it appears necessary whenever cells, intended for the lodgment of soft organs, are to be formed of hard materials, that the foundation should be laid upon a scale suited to the after-growth of the animal, otherwise the soft parts within would be so confined and contracted that they must cease to grow altogether. But in the case of the echini, provision is made for the expansion. of the shell itself, for each shell is divided into a number of small pieces, and each piece has that polygonal form which is best suited to the perfect junction of the whole. Small additions are therefore constantly being made to the margins of each of these polygonal pieces, and the expansion goes on exactly in proportion to the growth of the soft parts of the animal within.

The roe of this animal occupies much space within the shell, being very large in proportion to the animal and its other viscera; and it is in the spring, when the roe is fully developed, that this animal is in some places used as food, as its name imports. An old writer speaks of the sea-egg, as being eaten by the poor in many parts of England, and by the better sort abroad. It is recorded that these animals formed one of the favourite dishes among the Greeks and Romans. "They were dressed. with vinegar, honied wine, or mead, parsley, and mint; and esteemed to agree with the stomach. They

were the first dish in the famous supper of Lentullus, when he made Flamen Martialis priest of Mars. By some of the concomitant dishes they seemed designed as a whet for the second course to the holy personages, priests, and vestals invited on the occasion." They are also mentioned at the marriage feast of Hebe. 6 Thither came crabs and urchins, unable to swim in the sea, but travelling only on the ground." In the Wasps of Aristophanes, likewise, the hero of the piece repeats a fable respecting an urchin, who, when his shell had been cracked by a woman, summoned witnesses to prove the assault. He is interrupted by the remark, that it would have been wiser of the animal to buy a bandage than to spend his time in proving the assault. Horace mentions the echinus several times as very good eating.

The seas of warm and tropical countries are the most productive of these animals; but notwithstanding the number of living species, the fossil remains very far exceed them. These are found principally in the chalk and oolite formations, and are so abundant, and so well preserved, that there are few collections of fossils in which we may not meet with numerous specimens.

The empty shells of the echini are sometimes found in considerable numbers on our western coasts, especi

storms.

ally after the Atlantic has been much agitated by They are shaped more like an apple than an egg, having a small aperture at the top, and another at the opposite extremity. In this state the projecting suckers, spines, and bristles, have all been broken off, leaving the minute apertures reaching from one end to the other in regular rows, something in the same manner as the meridians of a globe.

METALLOCHROMY, OR THE ART OF

COLOURING METALS.

Ir is generally supposed that the blue colour imparted to steel, results from a thin film of oxide formed upon the surface of the metal when exposed to a certain temperature. About fifteen years ago, Professor Nobili, of Reggio, offered a new explanation of this fact, and discovered a method of imparting colour to metallic surfaces: as his results are extremely beautiful and varied, a brief notice of them may be acceptable.

The following is one of the principal experiments connected with what the inventor appropriately terms the Art of Metallochromy. A plate of platinum is A plate of platinum is placed horizontally at the bottom of a glass or china vessel. A platinum point is suspended vertically over this, in such a manner, that the distance between the point and the plate may be about half a line. A solution of acetate of lead is next poured into the vessel, so as not only to cover the plate, but to rise two or three lines higher than the point. The plate and the point are now brought into communication, the former with the positive, and the latter with the negative pole of a voltaic battery. At the moment when the voltaic circuit is closed, a series of coloured rings appear on the surface of the plate precisely under the point. These rings are similar to those described as Newton's rings in our Philosophy of a Soap-bubble*, but in an inverse order: Newton's rings begin at the centre; Nobili's at the circumference, where, from the nature of the electrochemical process, the thinnest layers are deposited: the thickest layers are evidently those at the centre.

This fact, which could not fail to strike any one observing it for the first time, led to the discovery of others. "Science never consults its interests so truly," remarks Professor Nobili, "as when it aims at some useful object connected with the arts." He foresaw the advantages the arts were likely to derive from this new method of colouring metals, and attended seriously to its application. His object was, instead of producing rings of various colours upon a plate of metal, to cover its surface uniformly with any desired tint. The colours being obtained by the effect of very thin plates applied to the surface of metals, it is easy to conceive how difficult it was to preserve such plates of a uniform thickness over the whole of an extensive surface. "Great, however, as the difficulties were, I thought I owed it both to art and to science, to do my utmost to surmount them. I thought it due to art, because this would be extended by means of the uniformity of the tints; and to science, because in the tints produced by plates of a particular thickness, the experimental philosopher would find the means of investigating, with peculiar advantage, the nature and properties of colours."

By substituting plates for the platinum point which forms the coloured rings, it was found that a surface of metal could be covered with one uniform tint. In 1828, Professor Nobili presented several such productions to the French Institute, and afterwards to our Royal Society, and particular attention was excited by the beauty and vividness of the tints, the precision of the outlines, and the softness of their blendings.

Although the efforts of this ingenious philosopher were attended with complete success, his methods so easy in their practical application, and the results so beautiful; although, too, the attention of scientific men was directed to the subject, it is remarkable that this new art was practised by its inventor only up to the time of his death, since which it appears to have been quite forgotten. Not only would this art be valuable to workers in metal generally, but the artist would find in it a wide field for observation and study. Professor Nobili has arranged the tints produced by his method in their natural order, so as to form a scale, or gamut, which he

* See Saturday Magazine, Vol. XV., pp. 199, 204, 222, and 231.

has designated by the epithet chromatic. This scale consists of forty-four tints, each of which is applied to a plate of steel. The tints are disposed in the same order as the layers or films by which they are produced: the colour of the thinnest film is placed first, and the others follow in the order of the progressively increasing thickness of the plates. In this arrangement the layers or films which produce the several colours are all applied by the same electro-chemical process. The voltaic bat tery, the solution of acetate of lead, the distances, all remain exactly the same. There is nothing variable but the duration of the action, which, in respect to the layer No. 1, is very short, a little longer in respect to the second, and increases progressively from the lowest to the highest number.

CHROMATIC SCALE.

44. Rose lake
43. Reddish yellow green
42. Yellowish green
41. Green

40. Greenish violet 39. Violet lake 38. Rose lake 37. Rose orange 36. Greenish orange 35. Orange green 34. Yellow green 33. Yellowish green 32. Green

31. Greenish purple 30. Bluish lake 29. Purpled lake 28. Brilliant lake 27. Lake

26. Orange lake 25. Orange red 24. Red orange 23. Reddish orange 22. Orange

21. Orange yellow 20. Brilliant yellow 19. Yellow

Corresponding to the Fourth ring.

Corresponding to the Third ring.

Corresponding

to the Second ring.

Corresponding

to the

First ring.

The effect produced by these tints, when disposed in the above order, baffles description; it bears a resem blance, however, to that produced on the ear by a scale of semitones, executed by a perfect voice. "I have shown my scale to several, and especially to those erudite visit at Reggio. In all it excited but one feeling of delight and learned persons who have favoured me with a passing So gradual, indeed, is the transition from one tint to another, and such the harmony with which they are blended, that if the eye be accidentally turned away, it reverts in a moment, as if moved by an irresistible desire to gaze still longer on the display. This statement is no exaggeration. It is but the mere fact, in respect to which a language much more glowing would be perfectly consistent with truth; so overpowering is the charm which, if I may use the expression, pervades the scale of our coloured plates."

In an admirable memoir, (which has been translated into the first volume of TAYLOR's Scientific Memoirs,) Professor Nobili examines and compares with natural phenomena all the colours which compose his chromatic

As we are

scale. If the reader be at all interested in the subject of colour, either artistically or scientifically, we strongly recommend him to study this memoir. about to inform the reader of an easy method of producing these beautiful colours, we can find space for only a few short extracts. This we do the more readily because in our notices of the Soap Bubble, already referred to, a popular account is given of the principles upon which colour is produced by thin plates or films. The colours which the clouds assume, are, in general, Black, or very pure ash-colour; White, or very light ash-colour; The colour of smoke or coffee; Red, more or less fiery;

Blue, very deep, and sometimes approaching to violet. These are exactly the tints that would constitute the first ring, were we to include in it the first two colours of the second ring. The tints of smoke result from the more or less thorough blending of the blond and the tawny; those of fire from Nos. 8, 9, and 10; the deep blue is produced by the Nos. 10, 11, and 12, which are the deepest tints of the scale.

The first blond is properly that of light hair in childhood, and it is a fact worthy of remark, that as children grow older, it becomes progressively deeper and deeper, in the order of the Nos. 2, 3, and 4, in the scale. The perfect resemblance of the first tints on the scale to those which we observe about the moon when she is surrounded by clouds, is equally remarkable; it seems in fact that this luminous appearance may be thus definitively explained. Tints of this kind do not arise from refraction and diffraction, they are produced only by means of thin plates; the luminous halo seen round the moon when overcast with

fog or light clouds, is therefore a phenomenon produced by thin plates.

This result appears to Professor Nobili so decidedly certain as to be entitled to a place in science.

says,

In speaking of the tints of the second ring, he We have the sky, their type in nature, constantly before our eyes; for who is there that knows not the dawn, "with rosy forehead and golden feet"? Beginning with No. 12 of the scale, let us run our eye over it as far as No. 28, and we shall find that the tints of the sky are disposed there in the order in which they present themselves in the magnificent spectacle of the dawning day. This succession, as we have already observed, is the most beautiful of all: Newton's second ring gives no idea of it, because its colours are not, and cannot be, sufficiently developed to produce the effect. Painters, if I mistake not, will do well to avail themselves of this part of the scale; they will find it a faithful copy of the beautiful tints of the morning, and endeavour to transfer them to their compositions. Natural philosophers will not fail to remark, that among the various tints of the sky there is no trace of green. This would heretofore have been found a perplexing circumstance, but it may now be satisfactorily explained, merely by reflecting that the tints of the sky belong to the second order, in which also there is no tinge of green. From the blue to the yellow, the transition is through a very faint gradation of azure-yellow, and this is observed to be exactly the case in nature.

The tints produced by the vapours and clouds belong to the second order. They contain in general more fire than the natural tints of the sky, but this quality is nothing in comparison with the purity, vividness, and variety, displayed in the tints of the second order. The appearance of the sun is never so magnificent as when the air is perfectly pure. Toward evening the lower regions of the atmosphere

once the action has commenced.

According to Professor Nobili the colours of which we now speak belong to the same class as those produced by tin plates, and he concludes that no oxide is formed upon the surface of the steel, because, 1st, the metal retains, beneath the deposited layer, its natural brilliancy; 2nd, this layer produces the phenomenon of the coloured rings in all its beauty; and 3rd, instead of oxidizing or rusting the metal, the coloured film contributes to secure it against rust in every part to which it is applied, as was proved by exposing two steel plates, one only of them being coloured, in the open air, to all the vicissitudes of a rainy autumn; when at the end of a month the uncoloured plate was all rusted, the other had lost a little of its colour, but was free from rust.

Considering then, that by the electro-chemical means stated at the beginning of this article, the films can be deposited with equal facility upon platinum, a metal difficult to be oxidized, as upon iron and steel, which belong to a class of metals most easily oxidized, as well as from the results of numerous collateral experiments, it appears that oxygen and certain acids may adhere to the surfaces of metals without producing the slightest chemical change in them. It may be laid down as a general proposition, that the oxygen of the atmosphere produces the colours on metals by the action of fire, not, as is supposed, by oxidizing the surface of the metal, but, by becoming fixed in the form of a thin plate, or film, similar to those produced by the electro-chemical process.