should be increased or diminished, according to the quantity of food or air that the creatures consume; but we do not find this to be the case; for in those situations where they are sometimes found to be most luminous, they are deprived, in a great measure, of these assumed sources of their light.

In fact, the luminous exhibitions of living animals are not only independent of all foreign light, but are frequently destroyed by the latter. I have always found the shining of the medusa to cease upon the rising of the moon, or at the approach of day; and when out of the sea, I never could excite them to throw out light until they had been kept for some time in the dark; all the luminous insects likewise secrete themselves as much as possible during the day time, and go abroad only at night. I have, it is true, found that the scolopendra electrica will not shine unless it has been previously exposed to solar light; but I have observed that it shone as brilliantly and as frequently after being kept a short time in a light situation, as when left uncovered the whole day. The circumstance of the scolopendra requiring exposure previous to its giving out light, is very unaccountable, as the insect, when left to itself, always seeks as much as possible concealment during the day, indeed it is the opinion of some naturalists that it is killed by the light of the sun. :

The opinions of Brugnatelli and Carradori are connected with some general doctrines, respecting the nature of light, which I shall not at present venture to discuss. It

appears to me, that the question is still unresolved, whether light bas a substantial existence, or is a phænomenon depending upon certain operations or conditions of the ordinary forms of matter. But the highly ingenious researches of Count Rumford, on the laws of what have been called subtile fluids, and the extraordinary advances lately made by Mr. Davy, on the decomposition of substances that were hitherto looked upon as elementary, give us reason to hope that future investigations may unfold views of the material world, of which we can at present have only an indistinct conception; that new modes of analysis may enable us to see things, not "through a glass darkly," but more nearly as they are; and that the boundaries of physical and metaphysical science, now so far asunder, may be made to approach each other.

In the present state of our knowledge, our business should be to collect, arrange, and compare phænomena, rather than to speculate upon their nature. Nevertheless, I cannot refrain from observing, that the circumstances attending the luminous appearance of living animals, are much more favourable to the supposition of light being a property than a substance. The quantity of light emitted by an animal in a certain time, (admitting it to be matter) far exceeds that which could be possibly supplied by the sources from whence it is usually supposed to be derived. Thus the luminous appearance of some medusa may be continued with the intermission of short intervals for an indefinite time, notwithstanding the creature Tt 4

be

be kept in darkness, and without any other food than what a small quantity of filtered sea-water would afford. The uninterrupted and long continued light that is sometimes evolved by the luminous sacs, and the ova of the glowworm, is also inconsistent with the notion of an accumulation and subsequent dispersion of a material substance.

I shall terminate this paper by an enumeration of the several conclusions, that are the result of the observations I have been able to make upon the phænomena of animal light.

The property of emitting light is confined to animals of the simplest organization, the greater number of which are inhabitants of the sea-The luminous property is not constant, but in general exists only at certain periods, and in particular states of the animal's body. The power of shewing light resides in a peculiar substance or fluid, which is sometimes situated in a particular organ, and at others diffused throughout the animal's body. The light is differently regulated, when the luminous matter exists in the living body, and when it is abstracted from it. In the first case, it is intermitting, or alternated with periods of darkness; is commonly produced or increased by a muscular effort; and is sometimes absolutely dependent upon the will of the animal. In the second case, the luminous appearance is usually permanent until it becomes extinct, after which it may be restored directly by friction, concussion, and the application of warmth; which last causes ope

rate on the luminous matter (while in the living body,) only indirectly, by exciting the arimal-The luminous matter, in all situations, so far from possessing phosphoric properties, is i combustible, and loses the quality of emitting light, by being dried, or much heated.-The exhibition of light, however long it may be continued, causes no diminution of the bulk of the kminous matter. It does not require the presence of pure ar and is not extinguished by, other gases.

The luminous appearance & living animals is not exhaust by long continuance, or frequest repetitions, nor accumulated by exposure to natural light; it is therefore not dependent upon any foreign source, but inheres as a property, in a peculiarly organized animal substance, or fluid, and is regulated by the same laws which govern all the other functions (4 living beings.

The light of the sea is always produced by living animals, sal most frequently by the preselite of the medusa scintillans. When great numbers of this species approach the surface, they sometimes coalesce together, and cans that snowy or milky appearance of the sea, which is so alarming to navigators. These animab, when congregated on the surface of the water, can produce a flash of light, somewhat like an elec tric coruscation. When the l minous medusæ are very nume rous, as frequently happens in confined bays, they form a considerable portion of the mass of the sea, at which times they

render the water heavier, and more nauseous to the taste; it is therefore adviscable to always strain sea water before it is drunk.

The luminous property does not appear to have any connec

tion with the ceconomy of the animals that possess it, excepting in the flying insects, which by that means discover each other at night, for the purpose of sexual congress.

USAFUL

Improvements in the Aquatinta Process, by which Pen, Pencil, and Chalk Drawings can be imitated. By Mr. J. Hassell.

[From the Transactions of the Society for the Encouragement of Arts, &c. Vol. XXVIII, for 1810]

PERCE

SIR, ERCEIVING the various methods of imitating drawings and sketches in the graphic art, fall short of an accurate imitation of the black-lead pencil, I determined on an attempt some years since, which, after repeated experiments, I flatter myself, I have fully established.

The manner is totally new, and solely my own invention. By the method I adopt any artist can sketch, with a black lead pencil, his subject immediately on the copper; and so simple and easy is its style, that an artist can do it with five minutes study.

By this manner, the trouble in tracing on oil paper, and other retracing on the etching ground is avoided, and the doubtful handling of an etching-needle is done away, as the pencilling on the copper is visible in the smallest

touch. It has also another perfection, that by using a broader instrument it will represent black chalk, a specimen of which I procured Mr. Mann, the landscapepainter, to make a trial of. I have herewith sent the said specimen, marked C, and Mr. Munn's name is affixed to the same. This subject he actually drew upon copper, under my inspection, in less than twenty minutes, the time he would have taken, perhaps, to do the same on paper; in fact, it can be as rapidly executed on copper as on paper.

It is particularly pleasant for colouring up, to imitate drawings, as the lines are soft, and blend in with the colour. It is a circumstance always objectionable in the common method of etching, that those so tinted can never be sufficiently drowned, nor destroyed, and always present a wiry hard effect.

It is equally adapted to historical sketching, and might be the means of inducing many of our eminent painters to hand down to posterity their sketches, which, at present, they decline from the irksome trouble attending the repeti

tion of retracing their performances, and the doubtful handling of the etching-needle, which can never give a sufficient breadth and scope to their abilities.

I have, sir, forwarded, in an annexed paper, the different specimens, for the inspection of the gentlemen forming the Society of Aris, &c. &c.

In making my specimens I have thought it necessary to show, if by any accident a part might fail, that it could be retouched a second time, and oftener if wanted; in this particular its simplicity stamps its use.

To elucidate the foregoing proposition, I purposely caused a part of the distance to fail in specimen AA; this is repaired, you will perceive, in specimen B, and the sharp touches wanted to perfect the sketch are added.

I beg also to state, it is not the style usually termed soft ground etching : that process is always uncer tain, cannot be repaired, and will only print about two hundred impressions; whereas the specimens herewith sent will print upwards of five hundred with care.

Should the Society for the Encouragement of Arts, &c. deem the subject worthy of their reward, I shall feel proud in communicating its process, and flatter myself the arts and artists will feel a peculiar addition and pleasure in its utility. Permit me, sir, to subscribe myself, with all respect,

Your obedient humble servant, JOHN HASSELL, Landscape Draftsman, 11, Clement's Inn, Strand. March 26, 1810. To C. Taylor, MD. &c. &c.

Process of Drawing upon Copper, to imitate Black-lead Pencil, or Chalk.

A remarkable good polish must be put on the copper with an oilrubber and crocus-martis well ground in oil; after which it must be cleaned off with whiting, and then rubbed with another clean rag.

You are then to pour over your plate the solution to cause ground, which is made as follows:

No. 1.-Three ounces of Burgundy pitch.

One ditto of frankincense. These are to be dissolved in a quart of the best rectified spirits of wine, of the strength to fire gunpowder when the spirits are lighted.

During the course of twentyfour hours this composition must be repeatedly shook, until the whole appears dissolved; then filter it through blotting-paper, and it will be fit to use.

In pouring on this ground, an inclination must be given to the plate, that the superfluous part of the composition may run off at the opposite side; then place a piece of blotting-paper along this extremity, that it may suck up the ground that will drain from the plate, and in the course of a quar ter of an hour the spirit will evaporate, and leave a perfect ground that will cover the surface of the copper, hard and dry enough to proceed with.

With an exceeding soft blacklead pencil sketch your design on this ground, and when finished take a pen and draw with the following composition, resembling ink if you wish your outline to be thin and delicate, cause the

pen