same deflexion in the fourth experiment as in the first, although the depths were only in the ratio of 3 to 4.

The third, that the quality of elasticity as distinguished from flexibility, or the difficulty of giving a set to the beams was in a much higher ratio than is assigned to it in ordinary beams, requiring in experiments 2 and 4, more than one-half the breaking-weight. Experiment (1) presents in reality no exception to this remark, because the depth there is exceedingly small, and the first set which is that given in the table was exceedingly small, and did not affect the elasticity of the material, being followed by no other set until the weight was nearly doubled.

Now in ordinary beams, it appears (Tredgold, page 79), that there is a sensible injury of the elastic force with one-third the breakingweight.

Mr. Hodgkinson concludes, therefore, that in these beams the elasticity remains perfect under loads greater in comparison with those under which they break than in the ordinary beams.

Various other experiments were made after the first publication of these on a considerably larger scale; for the details of them the reader is referred to the fifth volume of the Transactions of the Manchester Phil. Society. It need only here be stated, that these experiments on a larger scale have in every way supported the conclusions drawn from those on a less scale. In conclusion, we shall give the very simple rule which Mr. Hodgkinson has deduced from these experiments to estimate the strength of beams cast on his construction.

RULE.

If A be the area of a section of the bottom rib in the middle of the beam, and if D be the depth of the beam there, and L the length or distance between the points of support, all these dimensions being in inches, then will the ultimate strength of the beam in tons, when it is cast erect, be represented by the formula,

26 X AX D

L

And when it is cast on its side by

25 X AX D

L

That is, if taking the dimensions in inches, we multiply the area of the middle section of the lower rib by the depth of the beam, and divide this product by the length of the beam, then 26 times this product will represent the number of tons' weight which will just break the beam when it is cast upright, and 25 times the product when it is cast on its side.

It is manifest that the principles which Mr. Hodgkinson has established for the best sections of girders, are applicable, with the proper modifications, to every form under which cast-iron is employed to sustain a transverse strain of the material.

INSTANTANEOUS LIGHTS.

THE tinder-box has been employed from time immemorial for the purpose of obtaining a light: it is now, however, nearly expelled from our dwellings by a host of ingenious chemical contrivances, called "Instantaneous Lights." A popular account of some of these may, perhaps, be acceptable to the readers of this Magazine.

About the year 1673, phosphorus was discovered, and its singular inflammability was taken advantage of, as a refined chemical method of getting a light quickly. A small portion of phosphorus, rubbed between the folds of brown paper, instantly bursts into a flame, which will kindle a common brimstone match. This method was eagerly adopted by all such persons as could procure so great a chemical curiosity; for phosphorus was by no means plentiful until the year 1680, when Godfrey Hanckwitz manufactured and sold it in large quantities at his laboratory, in Southampton-street, Strand. [This laboratory, and much of its curious apparatus, is yet in existence.] The marvellous properties of phosphorus excited much attention in this country, and Godfrey set out on his travels abroad, to exhibit and vend the article; he, therefore, has the merit of generally introducing the first chemical method of getting a light.

Improvements were, of course, soon made upon it; one of which was to coat the wicks of small wax-tapers with phosphorus, in glass tubes, hermetically sealed; thus constituting what were called "phos`phoric tapers." When a light was required, one end of the tube was cut off with a file, and the taper being quickly drawn out, it took fire upon touching the air; but this plan, although ingenious, was not so simple as the original method of friction; it was inconvenient, sometimes even dangerous, and never came into general use. It was succeeded by placing a bit of phosphorus in a small phial, and then stirring it about with a hot iron-wire, thus partially burning it in a confined portion of air, and covering the interior of the phial with oxide of phosphorus: it was then corked up tightly after removing the wire, and preserved for use. When a light was wanted, a common brimstone-match was put into the bottle, and a small portion of the phosphoric compound withdrawn upon its brimstone tip; flame instantly resulted, from the strong affinity of the sulphur for the phosphorus. This method, from its simplicity and durability, had a long run, and may even now be met with. Another plan with phosphorus was, to take a small bit of it on the point of a brimstone-match, and then to rub it on a cork or piece of soft wood; the friction caused the union of the phosphorus and sulphur, with the evolution of flame.

These methods with phosphorus were succeeded by a substance called pyrophorus: it was a black powder, produced by the calcination of flour, sugar, and alum, and having the singular property of taking fire upon mere exposure to air. A small bottle of pyrophorus, well prepared, lasted a considerable time, and was a good method of getting a light. The theory of its action was long a mystery; but we now know, that the fire of the pyrophorus, or fire-bearer, results from the attraction of oxygen for potassium, which inflammable metal is elicited from the

potash of the alum by the action of the charcoal of the flour and sugar. "Homberg's pyrophorus," for so it was named, had a long day, but chiefly with scientific curiosos. The invention which first bore the name of "Instantaneous Light Machine," was the "inflammable air-lamp of Volta;" an extremely elegant and scientific apparatus, consisting of a glass reservoir filled with hydrogen-gas (or inflammable air, as it was then called), which could be subjected to the pressure of a column of water upon turning a stop-cock. The pedestal upon which the reservoir was placed, contained an electrophorus (a variety of the electrical machine), the apparatus being so adjusted by connecting wires, that upon turning the cock a small stream of hydrogen rushed out, and met with a spark of electric fire, which caused its combustion; and this flame kindled a wax-taper, placed directly against it. This machine was soon modified into a variety of ornamental forms, and quickly found a place in the study of almost every scientific man. One fatal objection to its general introduction was, its tendency to explode,—a most disagreeable tendency certainly, especially if the light-seeker was in a great hurry to seal a letter, to say nothing of the squirting of the acid-water over papers, books, and furniture, and perchance the fracture of a looking-glass or window, by a flying fragment of the gas-reservoir. Such an accident often happened, and the Light Machine was denounced as an Infernal Machine."

66

The researches concerning the evolution of heat, by the compression of air, led to the introduction of its agency for obtaining a light.

A small stout brass tube, about six inches long, and half an inch in diameter, closed at one end, and fitted with a hollow air-tight piston, containing in its cavity a scrap of amadou or German tinder, constituted the apparatus which was called, “The Pneumatic Tinder-box, or Light Syringe," and which was used as follows:-The piston was suddenly driven into the tube by a strong jerk of the hands; the air in the tube thus compressed, had its capacity for heat diminished, and therefore parted with it in sufficient quantity to cause the ignition of the tinder; and upon quickly drawing out the piston, the glowing tinder would, of course, kindle a match*.

The Light Syringe is even now frequently used on the continent, although we very seldom see it here, unless upon the lecture-table of the chemist. Considerable practice is required to learn the right method of using it, and a novice has to undergo sundry abrasions of the knuckles, fingers, sprains of the hands, &c., before he lights upon the proper knack of suddenly and successfully compressing the air. The agency of Voltaic electricity was next pressed into the "light company;" it was found that a plate of zinc, and a double plate of copper, when dipped into a dilute acid, evolved sufficient electricity to ignite a fine platina-wire connecting them. This apparatus, being very simple and scientific, was adopted by many, but more especially by philosophers. It took up very little room, as a single pair of plates, two inches square, and a little cistern of acid, about the size of a snuff-box, was quite adequate to the ignition of a fine

*This apparatus, somewhat modified, was employed by the French as a substitute for the gun-lock on fire-arms, before the introduction of percussion-caps.

filament of platina-wire, at which a bit of touch-paper could be ignited, and then a match from this. It would not answer to apply the match directly to the ignited wire, as the sulphur combined with the platina. All the contrivances which have been now spoken of, quickly vanished into the shades, before the sudden blaze of "The Oxymuriate Matches." It was discovered that the singular salt called oxymuriate, or chlorate of potash, when mixed with sugar or other inflammable matters, caused them to enter into sudden combustion upon the contact of a drop of vitriol.

Small portions of such mixture in powder were first employed, but it was soon mixed with gum-water into a paste, (together with a little vermilion, for the sake of colour), and with this thin slips of deal were tipped, to constitute oxymuriate matches, which were put into a neat little case of crystallized tin, or moirée métallique (for that beautiful manufacture was just come into fashion), with a bottle containing a bit of asbestos soaked in strong oil of vitriol: cases thus fitted up were improperly, but almost universally, called "Phosphorus-boxes." The addition of a wax-taper was made soon afterwards, and the matches, bottles, and boxes, appeared in swarms, of all colours and dimensions. Competition at last became so great, that from fifteen, ten, and five shillings, the price fell to half-a-crown for a small box, and this price continued steady for some years.

The theory of the action of these "phosphorus or instantaneous light-boxes," consisted in the sudden decomposition of the chlorate paste by the oil of vitriol, and the evolution of oxygen, which entering into energetic combination with the inflammable elements of the sugar, produced flame. It was a very complex action, and understood by none but chemists; but every one could appreciate its practical application, and nothing more was wanted.

Sometimes this production of fire took place so instantaneously that the wood of the match had not sufficient notice to light; at others, in consequence of the weakness of the acid, or badness of the composition, not a spark of fire was evolved, but the red tip merely fizzed, and spurtled the vitriol over ladies' silk-dresses, to their utter spoilation. By the way, about fifteen years ago, when this invention started, ladies were not so "darkly, deeply, beautifully blue," in chemical matters, as they are now-a-days; and very frequently, when the match did not light instantaneously, they innocently returned it to the case amidst the others, and selected a fresh one: the rejected match still retaining a spot of vitriol, imparted it to the tip of its next neighbour, this being perchance a good one, entered into rapid combustion, which instantly spread throughout the whole legion of fiery spirits, and out they all shot from the box, like a small girandole flight of sky-rockets, to the great terror and discomfiture of the female experimenter. This firework happened when the tips of the matches were placed downwards; and complaints of the danger being made to the match-maker, he, to avert it, placed the tips upwards; but this plan was even worse than the other, for if great care was not taken in withdrawing a match from the vitriol-bottle (and especially in the dark), it might touch the combustible tips of the others and set them blazing also; but they would not be projected from the

box, as in the downward plan. However, both these inconveniencies were soon remedied, by making a little tin cover to the match department, and giving printed directions to shut the cover down upon taking out a match; and if this did not light, to throw it away, and never return it to the case.

The chlorate paste was gradually improved, and the matches still more perfected by tipping them, first with sulphur, and then with the paste: thus they lighted with more certainty; frankincense and camphor were sometimes mixed with the composition, and the wood of the match was pencil-cedar, so that a fragrant odour might be diffused during the combustion. It was soon found that the cork of the bottle became corroded and rotten, and the vitriol weakened by exposure to the air. A ground-glass stopper was therefore substituted for the cork, and this again covered with a ground-glass cap, still more effectually to exclude the air: some persons preferred touching the match with the vitriol, instead of the vitriol with the match, and for their accommodation an elongated stopper was provided, which would withdraw a drop of vitriol. Those who did not like the expense of caps and stoppers, were provided with a bottle, having a plug of India-rubber, instead of a stopper or cork, for that substance was not acted upon by vitriol*.

But although the instantaneous-light box was now tolerably perfect and certain in its operation, half-a-crown for a plain box, and a shilling per hundred matches, was a high price, and beyond the reach of every individual. It rapidly fell by the appearance of a placard, with the following announcement:-"Save your knuckles, time, and trouble: use Heurtner's EUPYRION, price one shilling!" and accordingly the Eupyrion found its way, like magic, into the parlour, bed-room, and kitchen. The housemaid, whose chapped knuckles had many a time and oft received the gashing stroke of the flint, when striking a light on a dark frosty morning, was loud in the praises of Hewperryon. In the study or parlour, a letter or billet-doux could be sealed and sent off quietly, without "ringing for a light;" and in the bed-room a light could be obtained in case of illness or sudden emergency. Almost everybody was possessed of the Eupyrion; it even found a place on the shelves of the most celebrated laboratory in this country, and facilitated many of the operations of the "philosopher by fire.”

This invention consisted of two tin cases, about one inch and a quarter square; but the one three inches, and the other about an inch long, they were soldered together thus,the shorter one holding the vitriol-bottle, the other the matches with their tips upwards; by this simple arrangement there was very little chance of any accidental combustion occurring, as the bottle and matches were not on a level; the composition was good, the vitriol strong, and rarely failed to produce an instantaneous light; the cork did not corrode, partly because it was covered with thin lead, and partly because there was no superfluous

* The bottles of the Instantaneous Lights | belonging to the thermometer for measuring accessible heights, were always fitted

with plugs of India-rubber, although this plan has been claimed as a more recent invention.