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over a spirit-lamp, until it begins to burn*, and then plunge it into oxygen, a vivid combustion ensues; wash out the spoon with a little water, dip in a bit of turmeric-paper, the yellow is instantly changed to brown, indicating the formation of the alkali potassa, or the alkaline oxide of potassium.

If you take out the manganese from the retort, insert a fresh charge, and proceed to evolve oxygen again; another experiment can be made, which is highly instructive: take a large bladder, soak it well in water, tie on to its neck a common gas stop-cock that will fit the end of the pewter pipe, and as soon as oxygen comes off, squeeze the bladder, so that no air remains in it, then fix it on to the pipe with the cock open. If the cock does not screw on, you can make it do so, by putting some paper between; but it is much better to fit it with paper so as to slide on, because when the bladder is full of gas you can instantly remove it by sliding the cock off the pipe-screwing it off is not so handy. When the bladder is full of gas, shut the cock, and quickly remove it from the end of the pipe.

Another bladder may be filled in like manner :-to the stop-cock of the first, fix on a plain brass blowpipe (which can be easily procured at the ironmongers'), and make the joints tight with slips of wetted bladder as before directed.

Now take a piece of charcoal, about six inches long and an inch in diameter, make a small cavity in it with the point of a knife, big enough to hold half a pea, put a bit of lighted amadou in this, and by opening the cock, and gently compressing the bladder, force a stream of oxygen slowly from the beak of the blow-pipe upon the tinder, this will soon heat the cavity of the charcoal red-hot; and when this happens, instantly drop in a bit of steel, broken from the end of a file, or a castiron “sparable” will do :-urge on the gas, an intense heat results, the metal melts, and presently burns with the emission of a shower of brilliant sparks, exactly like the celebrated fire-work, called a gerbe; when these cease, shut off the gas, let the globule cool, and then examine it: you will find it very brittle and easily reduced to powder. Try it with the test-papers. It is neither acid nor alkaline, it is a neutral metallic oxide of iron.

Now these three experiments are very instructive ones, they show you intense chemical affinity existing between bodies of the most opposite natures, they show you oxygen, forming an acid, an alkali, and an oxide, all again new and distinct substances; they also show you how energetically oxygen supports combustion, and about the time that oxygen was adopted as the “ universal acidifying principle,” it was also adopted as the “universal supporter of combustion.” This was another grand error, for we now know that there are instances of combustion in which no oxygen, or compound of oxygen, is present, and of others in which its presence so far from inducing, actually prevents combustion ; but it takes

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* The potassium must be wiped free | is the naphtha burning off; and you must from the naphtha in which it is usually not plunge it into the gas until a purple kept, and when you hold it over the spirit flame appears ; that is, the flame of potaslamp, very likely you will see a flame like sium. coal-gas quickly rise from the globule, this

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place the moment that oxygen is withdrawn. At the time of the discovery of oxygen (1774), philosophers had hardly shaken off the trammels of alchymy, and it was the fashion to draw sweeping and general conclusions from a few experiments; at the present day, our operations and reasonings are conducted with more refinement and precision; hypotheses and theories are carefully examined before they are so generally adopted as heretofore. Philosophers now endeavour to act up to the Baconian precept, which says, “ Conclusions are, in all cases, to be drawn after the comparison of a sufficient number of facts, with a due regard to objections.”

Oxygen is an elementary substance, permanently gaseous at all known temperatures and pressures: it is most abundantly distributed throughout the three kingdoms of nature, but always in combination with other elementary substances; it has never yet been found in a free or uncombined state.

The black oxide of manganese is the metal manganese saturated with oxygen, and therefore a peroxide, (see vol. i., p. 298,) consisting of manganese 28 + oxygen 16 = 44; when this is heated red-hot, the affinity between the two substances is weakened to some extent. Part of the oxygen escapes in the free gaseous form, leaving a sesquioxide of a brownish colour, consisting of manganese 28+oxygen 12 = 40 sesquioxide of manganese; and in this compound the affinity between the two substances is so strongly exerted, that heat alone cannot overcome it, therefore no more oxygen can be extracted from the sesquioxide. This, however, must not induce you to throw it away as useless; it should be preserved for some experiments hereafter to be mentioned.

Oxygen supports respiration: Priestley put a mouse into a jar containing it, and he found that the animal lived about thrice as long as when confined in an equal bulk of common air.

You must not conclude from this, that it is eminently fitted for maintaining the functions of vitality, for the contrary is the case; an animal caused to breathe pure oxygen for any length of time, at last falls a sacrifice to its stimulating agency, and upon examination after death, the blood in the veins is found as florid as that in the arteries.

It is well worth notice and recollection here, that although we have many gases

be breathed for a considerable time without hurting life, yet we have no gas or mixture of gases fit for its perfect support, save atmospheric air, which you will hereafter find to be a mixture of oxygen and nitrogen.

that may

192

EXPERIMENTS ON THE STRENGTH OF IRON BEAMS.

The experiments of Mr. Eaton Hodgkinson, of Manchester, are said to have led to a practical economy of material in the construction of the great iron girders or beams, so generally used, there and in other manufacturing districts, in the building of factories, amounting to not less than 20 per cent. of their weight.

It is very rarely that the application of principles of exact science to the uses of society is attended with success like this. And we feel that the pages of this magazine cannot be occupied more in accordance with the purposes for which it was established, than by giving publicity to these experiments.

If a beam of iron, or any other elastic or flexible material, be bent by a weight which it supports, it is manifest that the part of it lying near that side which supports the weight, will in the act of flexure be compressed, whilst that on the opposite side will be extended *. It is at that part of the beam which is nearest to its extended side that the extension is greatest, and at the part nearest to the compressed side that the compression is greatest. Between the point of greatest extension, and the point of greatest compression, the extension diminishes continually up to a certain point, where it is nothing; and beyond that point the compression commences, and continues to increase up to the other side of the beam where it is greatest. The point of the beam where the extension of its material terminates, and the compression of it begins, and where there is, therefore, neither extension nor compression, is called its neutral point. It is not at one point only, however, of the beam, that this neutral state of its compression and extension exists, but manifestly throughout all the points of a line crossing the whole width of the beam, and passing through the neutral point. This line is called its neutral axis.

The forces which oppose themselves to the rupture of the beam are the resistance of its material to extension on one side of its.neutral axis, and to expansion on the other. Its power of resistance to either of these yielding, it will be broken. Thus, if the one side be so far extended that its material separates, the beam will fail, although the other side may still retain its power of resistance to compression. Or if the one side be so far compressed that it crushes, the beam will fail,

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* This may be seen in a very simple has been made, this process will be further experiment. Let a piece of deal be gra- indicated by the appearance of the broken dually bent, and the part where the prin- ends which will on one side be jagged, cipal flexure takes place observed. It indicating there a rupture of the fibre by will be plainly seen that, on the side from tension or tearing asunder,--and on the which the Aexure is made, the fibres other side, comparatively smooth, as they elongate, and that on the other side they would be if compressed. compress; and when a complete fracture

although the other side is still able to resist the extension to which it is subjected.

In the first case, the beam opening on the extended side, the compressed side would form a fulcrum, about which the separated part of the extended side would turn.

In the second case, the compressed side of the beam immediately beneath the weight, being no longer capable of resisting the compression to which it was subjected, would be crushed in pieces, or otherwise displaced, and the beam entirely broken; although, perhaps, the tensile resistance of the opposite side of it had never yieldled.

This power of resistance to compression, and the power of resistance to extension, constituting the strength of the beam; and the yielding of either of these being of necessity followed by its entire rupture, it is manifest that the material of which the beam is composed will be distributed so as to make it the strongest when it is so distributed, that the one side shall be about to yield by compression, when the other is about to yield by extension. For, if either rupture is about to take place when the other is not about to take place, a portion of the beam might be removed from the stronger side, without causing that side to be in a state bordering on rupture, and added to the other side, so as to take that out of the state bordering on rupture. And thus if the powers of resisting compression and extension be unequal, the strength of the beam may be increased by a new distribution of the material.

This being admitted, the question of the best form of the beam resolves itself into this:-How can the material be distributed on the two sides of it so that the resistance to the compression to which the one side is subjected, and the resistance to the extension of the other may be equal? A principal element in this inquiry is manifestly this:Is the power of a given quantity of material to resist compression the same as its power to resist extension?—if it be the same, it seems probable that the object would be gained by any arrangement by which the part which is subject to compression should be made exactly equal and similar to that which is subject to extension. It appears, however, from the experiments of Mr. Rennie, that at any rate in respect to castiron, this law does not obtain. These experiments, and others of the same kind, which had before, and have been subsequently made, show very clearly that the cast-iron will resist a much greater force tending to compress it, than it is able to resist when the force tends to extend it. And that thus to produce an equal power of resistance on the two sides of the beam, a larger quantity of material should be collected on the extended than the compressed side. This idea suggested itself first, it appears, to Mr. Hodgkinson; and he contrived the following ingenious experiment to serve as a verification of it.

He caused two castings to be made, 5 feet in length, and whose cross-section was of the form represented in the figure; the width, A B, being 4 íó inches, the depth of the rib, DE, 16 inches, and the thickness, BC, of the metal throughout inch. Now, it is manifest Vol. II.

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that this casting being placed in the position shown in the first of

the accompanying figures, with the rib downwards, and being loaded, the portion, ABCG, of the section of fracture would be subjected to compression, and the whole, or the lower part, of the rib DEF would be

subjected to extension. Moreover, the surface, AGCB, resisting the force of compression, being so much greater than the rib

DEF,
which
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itself to the force of extension, it is clear that when the casting yielded, it would, under these circumstances,

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yield by the extension of DEF. Again, if it were placed as in the second figure, with the rib upwards, and loaded in the middle, the compressed portion of the section of rupture would be the rib, FED, or the upper portion of it, and the extended portion ABCG. The surface sustaining the forces of compression would, therefore, in this case, be less than that sustaining the forces of extension; nearly, perhaps, in the proportion in which the area, EFD, is less than ABCG: and if FED were sufficiently small as compared with ABCG, the rib would of necessity yield to the forces which compress it, before the part ABCG yielded to the forces which extend it. Thus, in both cases, the casting would break by the yielding of the rib, EFD. But, in the first case, it would yield by the extension of that rib; and, in the second, by the compression of it. Now, it was found that the disproportion between ABCD and Fed was in these castings sufficiently great to produce these results, i.e. to cause the bar used in the second experiment to yield to the compression of the rib, whilst that in the first yielded by its extension.

If, then, it be true that the same material in a beam yields more easily when it is subjected to extension than when subjected to compression, the casting ought, in the second case, when the rib was compressed until it broke, to have borne a greater weight than in the first, when it was extended until it broke.

In each experiment the supports were placed 4 ft. 3 in. asunder, and the load exactly over the middle point. In the first case, when the rib was broken by extension, the beam just bore 2 cwt., and the breaking load was 22 cwt. In the second case, where the rib was broken by compression, the beam bore 8cwt., and was broken by 9 cwt.

Thus, then, a beam of this form and these dimensions, when turned with the rib upwards, will bear nearly four times as much as when placed

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