whose powers were to be investigated, between a steady heat and the voltaic pile, when their capacities would be indicated by the rapidity of the action upon the needle. That the heat is actually transmitted, and does not pass by conduction, is proved by the fact that the internal portions of the glass do not instantly become heated, which is demonstrated by placing a glass screen in front of the pile, and intercepting the communication with the source of heat. The posterior surface of the glass plate would radiate the heat conducted from its interior towards the pile, if the hypothesis that the heat is communicated by conduction were correct. But this does not occur, and hence, there is no alternative left but the conclusion that heat permeates bodies directly. Heat and light agree, therefore, in this property, that both possess the power of passing through bodies.

It is proper that each should have such a capacity distinguished by appropriate names, until their identity be proved. Melloni terms the permeating power of heat through bodies, diathermal power, just as we indicate capacity of bodies to transmit light by the names, transparency, opalescence, &c. The diathermal power is subject to similar modifications. Heat, however, differs from light in this respect, that the facility with which it is transmitted by different bodies has no relation to their transparency.

Thus if we suppose the rays of a constant heat to be represented by 100, the only body which appears but slightly to diminish this when interposed as a screen is rock salt, whose diathermal power is 92, but the quantity of heat transmitted through a crystal of smoke-colored quartz will be denoted by 57, and through a crystal of alum by 12, where the difference is so very great as to excite astonishment. This and similar facts have induced Melloni to conclude that heat and light are distinct; but in this opinion Dr. Faraday does not coincide.

Melloni has also examined the diathermal relation of colors, and has found that their powers are in the following order: violet 53, yellowish red 53, purple red 51, bright red 47, pale violet 45, orange red 44, clear blue 42, deep yellow 40, bright yellow 34, golden yellow 33, dark blue 33, apple green 26, mineral green 23, very deep blue 19. Hence, we see that the mineral relations of the colors to their heating power is so completely altered, that the violet ray, which in the spectrum possesses temperature 25 or 30 times below that of the red ray, observes here a higher temperature, but the result seems modified as occurs with light by the nature of the power employed, to illustrate the comparative experiments.

Dr. Faraday exhibited many of the experiments which Melloni has described in his papers, especially in reference to the diathermal properties of rock salt, glass, alum, with screens of which substances he had been supplied. The absorptive power of different colors, in relation to the solar spectrum, was well illustrated by means of the oxy-hydrogen blowpipe. The contrivance of passing the decomposed ray through a volume of disengaged ammonia had a happy effect, the colors of the spectrum being as it were made to float in the air.

He likewise exhibited the method of polarizing light by means of tourmaline, by which fanciful figures are formed, and light transmitted or withheld by merely altering the relative position of the screens properly adapted.

REVIEW.

A Treatise on Wood Engraving; Historical and Practical, with upwards of 300 Illustrations, engraved on Wood, by John Jackson. Knight & Co. Ar length this splendid book, of which the public have heard so much, is before them. We have been especially solicitous to procure a copy, that we might examine it and give to our readers an unbiassed opinion of its merits. The work is one printed of a noble size, excellent paper, and the best of type; and in a strong, handsome bindingthus much of its GETTing up. The contents o. it require a longer notice, too long indeed to form but a single paper; we shall therefore confine ourselves at present to the consideration of a single chapter, that on the practical part of the art; and this we are the more anxious to choose and to elucidate, from the recollection that there is no other book published which even pretends to describe the practice of wood engraving, an art which is now, and perhaps ever will be for the future, so much encouraged; an art which all who draw can easily learn-one which is cleanly, elegant, well paid, costs little for tools and materials— may be practiced in a small apartment; is applicable to both sexes, and to youth; and for the products of which there is a constant demand. How much it is to be regretted, that in this country where female employments are so laborious, so little varied, and ill paid, that a genteel art like this, which may be carried on by ladies in privacy and in the bosom of their families and friends, should have been so long unknown: it need be so no longer. Mr. Jackson's book contains every needful instruction, as to the choice of materials, form and application of tools, and progress of the work, from the simplest to the most difficult parts, with examples throughout to render the meaning of his clear instructions still more clear. We have no doubt that many an excellent wood-engraver of the future age will acknowledge how much he is indebted to Mr. Jackson for his first instructions. There are hundreds, however, who cannot afford to purchase the book, yet who are not less anxious to learn; we will therefore do our utmost to assist them, partly with Mr. Jackson's book, and partly with such remarks as our own experience enables us.

As the explanation of the art requires illustrative cuts, we must content ourselves now with directions on the choice and preparation of wood. Mr. Jackson says, page 637,—

"For the purposes of engraving no other kind of wood hitherto tried is equal to box. For fine and small cuts the smallest logs are to be preferred, as the smallest wood is almost invariably the best. American and Turkey box is the largest, but all large wood of this kind is generally of inferior quality, and most liable to split: it is also frequently of a red color, which is a certain characteristic of its softness, and consequent unfitness for delicate engraving. From my own experience English box is superior to all others; for though small, it is generally so clear and firm in the grain, that it never crumbles under the graver; it resists evenly to the edge of the tool, and gives not a particle beyond what is actually cut out; the large red wood on the contrary, besides being soft, is liable to crumble and cut short; that is, small particles will sometimes break away from the sides of the line cut by the graver, and thus cause imperfections in the work.

"Large red wood containing white spots or streaks, is utterly unfit for the purposes of the engraver, for in cutting a line across, adjacent to these spots or streaks, sometimes the entire piece thus marked will be removed, and the cut consequently spoiled: a clear yellow color, and as equal as possible over the whole surface, is generally the best criterion of box wood."

Mr. Jackson goes on to state many other reasons why the red, or foreign box wood, is inapplicable to fine work, particularly as on account of its greater softness and porosity, it prints much less perfectly, and is more liable to injury from the press, and the liquids used in cleaning the blocks after printing.

"Box when kept long in a dry place, becomes unfit for the purpose of engraving. When the wood does not cut clear, but crumbles as it were too dry, the defect may sometimes be remedied by putting the block into a deep earthenware jug, or pan, and placing such jug or pan in a cool place for ten or twelve hours; when the wood is too hard and dry to be softened in the above manner, I would recommend that the back of the block should be placed in water in a plate or large dish, to the depth of a sixteenth of an inch for about an hour; if allowed to remain longer there is danger of the block afterwards splitting. Box, when not well seasoned, is extremely liable to warp and bend; if not for immediate use it ought to be placed on one of its edges, and not laid down flat. block of this kind be permitted to lay in this manner for a week or two, it is almost certain to turn up at the edges, the upper surface becoming concave and the lower convex. The same thing will occur in the process of engraving, though to a small extent, should the engraver's hand be warm and moist; and also when working by lamp-light without a globe filled with water between the lamp and the block. Such slight warping in the course of engraving is, however, easily remedied by laying the block with its face, that is, the surface on which the drawing is made, downward on the desk or table, at all times when the engraver is not absolutely employed on the subject.

If a

"Many artists who are not accustomed to make drawings on wood, erroneously suppose that the block requires some peculiar preparation. Nothing more is required than to rub the previously planed and smoothed surface with a little powdered Bath brick, slightly mixed with water; as little water as possible is, however, to be used, as otherwise the block will absorb too much, and be afterwards extremely liable to split; when the thin coating is perfectly dry, it is to be removed by rubbing the block with the palm of the hand. No part of the light powder ought to remain, for otherwise, the pencil coming in contact with it will make a coarse and comparatively thick line, which, besides being a blemish in the drawing, is very liable to be rubbed off. The object of using the Bath brick is to render the surface less slippery, and thus capable of affording a better hold to the point of the black lead pencil.

"When the principal parts of the drawing are first washed in upon the block in Indian ink it is of great advantage to gently rub the surface of the block, when dry, with a little dry and finely powdered Bath brick, before the drawing is completed with the black-lead pencil. By this means the hard edges of the Indian ink wash will be softened, the different tints delicately blended, and the subse

quent touches of the pencil be more distinctly seen. Some artists, previous to beginning to draw on the block, are in the habit of washing over the surface with a mixture of flake white and gum water-this practice is by no means a good one. The drawing indeed may appear very bright and showy when first made on such a white surface, but in the progress of engraving a thin film of the preparation will occasionally rise up before the graver, and carry with it a portion of the unengraved work, which the engraver is left to restore, according to his ability and recollection. This white ground also mixes with the ink in taking a first proof, and fills up the finer parts of the cut. If a white wash be used without gum, the drawing is very liable to be partially effaced in the progress of engraving, and the engraver left to finish his work as he can.

"The less that is done to change the original color of the wood, by white or any other preparation, so much the better for the engraver; a piece of clear box is sufficiently light to allow of the most delicate lines being distinctly drawn upon it."

Mr. Jackson proceeds to show, how a block which may have received an injury, or a part of which may require alteration, is to be mended by PLUGGING, that is, by the insertion of a round piece of wood, driven into a hole properly bored to receive it. This is too tedious for us to enter into, though the operation is extremely simple; we shall proceed next week in showing and explaining the various tools employed: at present, having rather exceeded our usual limits, we must conclude, and will do so by an excellent receipt of Mr. Jackson's at page 723, on transferring a print on to the wood to take a fac-simile from it.

"When a duplicate of a modern, or a fac-simile of an old wood-cut is required, the best mode of obtaining a correct copy is, to transfer the original if not too large or valuable, to a prepared block; and the mode of effecting this is as follows:-The back of the impression to be transversed, is first moistened with a mixture composed of equal parts of concentrated potash and essence of lavender; it is then placed above a block whose surface has been slightly moistened with water, and rubbed with a burnisher. If the mixture be of proper strength, the ink of the old impression will be loosened, and be transferred to the wood. impressions of a wood-cut, before the ink is set, may be transferred to a block without any preparation, merely by what is technically termed, rubbing down.' In order to transfer impressions from copper-plates, it is necessary to use the OIL of lavender instead of the essence; if a very old impression apply the preparation to its face."

MISCELLANIES.

Recent

Most powerful Electro-Magnet.-The Rev. N. J. Callan, Professor of Natural Philosophy in the Roman Catholic College, Maynooth, has described in Sturgeon's Annals of Electricity, &c., for July, 1837, an electro-magnet, which appears to be by far the most powerful instrument ever constructed. The iron bar of which it is composed weighs fifteen stone, is two and a half inches in diameter, and more than thirteen feet in length, it is bent into the form of a horse-shoe, and the distance between the poles is seven inches. A copper wire one-sixth of an inch in diameter, is coiled once round the whole length of the iron bar. This wire is divided

into seven parts, each about seventy feet long. A thin copper wire about one-fortieth of an inch in diameter, is soldered to one of the thick wires at about a foot from one of its extremities. The thin wire is about ten thousand feet long, it is wound round the magnet in the same direction as the thick wire, and in one continuous coil. By connecting the opposite ends of the seven thick wires with the opposite poles of a powerful galvanic battery, an extraordinary magnetic power is communicated to the iron bar; and, by breaking the battery communication, an electric current of enormous intensity is excited in the long thin wire. The electric power of Professor Callen's magnet, was shown not only by a brilliant combustion of charcoal, but also by the destruction of animal life. As often as the connection between charcoal points attached to the thick wires and the battery was broken, the succession of sparks was so rapid, that they formed a continued blaze of vivid light; and when, by means of an electro-magnetic repeater, a rapid succession of the currents excited in the long coil, was passed through the body of a large fowl, instant death was produced.

New Method of Working Cauotchouc.-The employment of either spirits of turpentine, the volaitle caoutchouc, balsam of copavia, and the oils obtained from gas-works, as solvents of India-rubber, have the disadvantage of being expensive, and of producing a varnish which dries with much difficulty. For some time past ammonia has been used with advantage. The gum elastic, cut up into shreds, is covered with caustic ammonia, and left in this state several months. The ammonia becomes brown, and the gum assumes a brilliant and silky appearance, resembling a fresh nerve, the caoutchouc swells, but is still elastic, and resembles very closely beautiful silky threads, when drawn out, but it breaks more easily than raw caoutchouc. In treating this swelled caoutchouc with spirits of turpentine, it is easily converted by agitation, into an emulsion, and in a short time it swims on the surface like butter on milk-after this, it acts like varnish. But a much smaller quantity of spirits of turpentine is sufficient to dissolve it than when it has not been softened by ammonia.

Meteoric Paper which fell from the Sky.On the 31st of January, 1686, a great mass of a paper-like black substance fell with a violent snow storm from the atmosphere, near the village of Rauden, in Courland; it was seen to fall, and after dinner was found at places where the laborers at work had seen nothing similar before dinner. This meteoric substance, described completely and figured in 1686, was recently again considered by M. Von Grotthus, after a chemical analysis, to be a meteoric mass; but M. Von Berzelius, who also analyzed it, could not discover the nickel said to be contained in it; and Von Grotthus then revoked his opinion. I examined this substance, some of which is contained in the Berlin Museum, (also in Chladni's collection), microscopically. I found the whole to consist evidently of compactly matted mass of Conferva crispata, traces of a Notsoc, and of about twenty-nine well-preserved species of Infusoria, of which three only are not mentioned in my large work on Infusoria, although they have since occurred living near Berlin; moreover, of the case of Daphnia Pulex. Of the twenty-nine species of Infusoria, only eight have siliceous shields; the others are

soft or with membranous shields. Several of the most beautiful exceedingly rare Bacillariæ are frequent in it. These Infusoria have now been preserved 152 years. The mass may have been raised by a storm from a Courland marsh, and merely carried away, but may also have come from a far distant district, as my brother, Carl Ehrenberg, has sent from Mexico forms still existing near Berlin. Seeds, leaves of trees, and other things of the kind, scattered through the mass, were, on the examination of larger portions, easily visible. The numerous native Infusoria, and the shells of the common Daphnia Pulex, seem to speak thus much for the substance, that its original locality was not the atmosphere, nor America; but most probably either East Prussia or Courland.-Professor Ehrenberg.

Zinc Milk Pails.-Among the patents lately taken out in America, one is for a process for extracting cream from milk by the use of zinc. It is said that if zinc is put into the milk-pail, or the milk be put into a vessel made of that substance, the same quantity of milk will yield a greater portion of cream or butter.-Repertory of Inventions.

Asphaltic Mastic.-The asphaltic mastic is obtained from Pyrmont, near Seyssel, and brought down the Rhone. It is a compound of carbonate of lime and mineral pitch. After being roasted on an iron plate, it falls to powder, or may be readily pounded. By roasting, it loses about one fortieth of its weight. It is composed of nearly pure carbonate of lime, with about nine or ten per cent. of bitumen. When in a state of powder, it is mixed with about seven per cent. of bitumen, or mineral pitch, found near the same spot. This bitumen appears to give ductility to the mastic. The addition of only one per cent. of sulphur makes it exceedingly brittle. The powdered asphaltic is added to the bitumen when in a melting state; also, a quantity of clean gravel, to give it a proper consistency for pouring into moulds. When laid down for pavement, small stones are sifted on, and this sifting is not observed to wear off. The mass is partially elastic, and M. Simens has seen a case in which a wall having fallen away, the asphaltic stretched, and did not crack. It may be considered as a species of mineral leather. The sun and rain do not appear to have any effect on it; it answers exceedingly well for the floors of the abbatoirs of the barracks in France, and keeps the vermin down; and is uninjured by the kicking of horses. It may be laid down at from eight-pence to nine-pence per square foot.-Railway Magazine.

QUERIES.

66.-Is it a fact that gardeners use soot to change the color of flowers? If so, how do they proceed? Soot is employed as a manure, being mixed with seed in sowing. It furnishes to the young roots a large quantity of carbon, in a state easily to be absorbed. Its nauseous bitter principle is also valuable to keep the seedling plants from slugs and insects; but as to changing the color of flowers, it does not appear to have any such tendency.-ED.

67.-If a thread be twisted tightly round a poker, it will not burn, though held in the flame of a candle. Why is this? Answered on page 104.

68. What is the construction of the Cosmorama ? Answered on page 101.

69.-What is the red varnish for electrical purposes? The best is made by dissolving red sealing wax in strong.spirits of wine. Three or four coats of this varnish will give the appa ratus a beautiful appearance: it dries quickly, and is very durable.-ED.

70.-How may tin plates be variegated and colored?Answered on page 111.

WOOD ENGRAVING.

(Resumed from page 78.)

THE Wood being chosen and prepared, as already described, and the design transferred or drawn upon its surface with a hard fine pointed pencil, it is necessary previous to engraving that it be preserved from injury by being covered with paper, with the exception of that part on which it is intended to begin; upon this Mr. Jackson makes the following remarks :

"Soft paper ought not to be used for this purpose, as such is most likely to partially efface the drawing where the hand is pressed upon the block. Moderately stout post paper, with a glazed surface is the best; though some engravers, in order to preserve their eyes, which become affected by white paper, cover the block with blue paper, which is usually too soft, and thus expose the drawing to injury. The dingy, grey, and overdone appearance of several modern wood cuts is doubtless owing, in a great measure, to the block when in course of engraving having been covered with soft paper.

"Different engravers have different methods of fastening the paper to the block. Some fix it with

gum, or with wafers, at the sides; but this is not a good mode, for as often as it is necessary to take a view of the whole block, in order to judge of the progress of the work, the paper must be torn off, and afterwards be replaced by new wafers or fresh gum, so that before the cut is finished, the sides of the block are covered with bits of paper, in the manner of a wall or shop-front covered with posting bills. The most convenient mode of fastening the paper is to wrap a piece of stiff and stout thread three or four times around the edges of the block, and there fastening it. The paper cover will thus form a kind of moveable cap, which can be taken off at pleasure, to view the progress of the work, and replaced without the least trouble."

Mr. Jackson recommends all persons to avoid the use of magnifying glasses, or spectacles, particularly those who are young, and gives the most powerful reasons for his advice, reasons founded not, he says,

upon any theoretic knowledge of objects, but from his own practical experience: that to use glasses to preserve the sight, is to meet half way the evil, which is thus sought to be averted." Mr. Jackson says, page 650-"I would recommend all persons to avoid the use of glasses of any kind, whether single magnifiers or spectacles, until impaired sight renders such aid necessary; and even then, to commence with such as are of a low magnifying power: the habit of viewing minute objects alternately with a magnifying glass, and the naked eye, applying the glass every two or three minutes, is, I am satisfied, injurious to the sight. The magnifying glass used by wood engravers is similar to that used by watch-makers; and consists of a single lens, fitted into a short tube, which is rather wider at the end applied to the eye-as the glass seldom can be fixed so firmly to the eye as entirely to dispense with holding it; the engraver is thus frequently obliged to apply his left hand to keep it in its place, as he cannot hold the block with the same hand at the same time, or move it as may be required, so as to enable him to execute his work with freedom; the consequence is, that the engraving of a person who is in the habit of using a magnifying glass has frequently a cramped appearance. There are also other disadvantages

attendant on the habitual use of a magnifying glass. A person using such a glass, must necessarily hold his head aside, so that the eye on which the glass is fixed may be directly above the part on which he is at work. In order to attain this position, the eye itself is not unfrequently distorted; and when it is kept so for any length of time it becomes extremely painful. I never find my eyes so free from pain or aching as when looking at the work directly in front, without any twisting of the neck, so as to bring one eye only immediately above the part in course of execution. I, therefore, conclude that the eyes are less likely to be injured when thus employed, than when one is frequently distorted and pained in looking through a glass. I am here speaking merely from experience, and not professedly, from any theoretic knowledge of optics; but as I have hitherto done without the aid of any magnifying power, and am not without reason convinced that glasses of all kinds ought to be dispensed with till impaired vision renders them necessary.'

It is therefore to be observed, that these remarks only apply to the young and clear sighted, who would adopt glasses as preservatives to the eye, rather than those who through age, or infirmity of vision, need glasses under other circumstances. Defence, however, to the eye is absolutely necessary, from the glare of light thrown by the lamp on the wooden block; this is best accomplished by a paper or pasteboard shade tied to the forehead, and hanging over the eyes, as represented in the cut: a second shade is occasionally used for the mouth and nose, as in cold and damp weather the drawing upon the block is apt to be obliterated by the breath settling upon it. Upon shading the eyes from light and the face from heat Mr. Jackson observes ;

"There are various modes of protecting the eyes when working by lamp-light, but I am aware of only one which both protects the eyes from the light, and the face from the heat of the lamp. This consists in filling a large transparent glass globe with clear water, and placing it in such a manner between the lamp and the workman, that the light, after passing through the globe, may fall directly on the block. The height of the lamp can be regulated according to the engraver's convenience. By the use of these globes one lamp will suffice for three or four persons, and each person have a much clearer and cooler light than if he had a lamp without a globe solely to himself.

Note. "The French prefer a bull's-eye lens, of about three and a half inches diameter, flat on one side and convex on the other, to a globe filled with water. This bull's-eye is inclosed in a kind of frame, which can be inclined to any angle, or turned in any direction by means of a ball and socket joint. The light of this is equally good, but the heat affects the head in a disagreeable manner."

Tools.-The next important part of the art is the tools employed and their peculiar application to certain kinds of work; and it is impossible to do this in plainer terms than in the following quotations:

"There are only four kinds of cutting tools necessary for wood-engraving, namely:- gravers, tint tools, gouges or scrapers, and flat tools or chisels. Of each of these four kinds there are various sizes. The cut, (No. 2,) shows the form of a graver, that is principally used for outlining, or separating one figure from another. This tool is very fine at the point, as the line which it cuts ought to be so thin as not to be distinctly percep