« PreviousContinue »
LECTURES AT THE GUERNSEY MECHANICS' INSTITUTION.
AMONGST the varied phenomena, said Mr. Ollivier, which are constantly occurring around us, there is perhaps none more wonderful in itself, nor more interesting on account of its utility, than combustion. And yet this phenomenon appears to be very much disregarded, at least by the bulk of mankind. Very erroneous ideas are generally entertained respecting this really wonderful process. It is, indeed, rather discreditable to the age in which we live, that notwithstanding all the brilliant discoveries of modern times, relative to combustion, and the boast that the school-master is abroad ; that vast numbers are as ignorant of the effects resulting from this process, as the inhabitants of those savage countries who have not been favoured with the light of science. It is indeed lamentable that the phenomena of nature should be viewed with so much apathy and indifference. Dr. Ure appropriately remarks : “A few happy rhymes or musical periods, though conveying sentiments both trite and trivial, will enamour the hearts and kindle the fancies of the million; while the optical revelations of Newton, and the electrochemical magic of Davy, are either neglected altogether, or scanned with frigid tranquillity. The process of combustion has a high claim to our notice, not only from its constant occurrence and great utility, but alsu from its forming a part of nature's volume, which so strikingly displays the exquisite skill and infinite intelligence of its great Author."
I do not pretend in this lecture to explain the process of combustion ; I shall only state facts relative to it. For it must be remarked, that the nature of combustion is yet but very imperfectly understood. Many different theories have been framed, at various times, in order to account for this phenomenon, but they have all been found fallacious, and incapable of explaining satisfactorily all cases of combustion. There is no phenomenon in nature by which the attention of philosophers has been more engaged, nor which has perplexed them more to account for, than this very common operation: nor are their efforts likely to prove successful till the nature of light, heat, and electricity, are better understood. One of the principal difficulties attending the explanation of this process, is the accounting for the evolution of heat and light, and the ascertaining the sources from which they are derived, whether from the combustible body, or from the air, or from both. For we must be aware that, during ordinary combustion, both the air and the combustible body are conce
cerned, as the latter will not burn without the former. Although we cannot at present explain liow the heat and light are produced, yet there are many interesting facts relative to combustion, the examination of which demands our serious consideration. In this lecture, I shall keep these in view, facts, rather than the description of those numerous theories of combustion which have been propounded at various periods, and successively exploded ; being convinced that the description of such subjects, to a mixed audience, would be as tedious as it would be unprofitable. Indeed, a lecture room is not a place well suited for the dissemination of the abstruse speculations of chemical philosophers. It is not favourable to that concentration of thought necessary for their comprehension. I shall attempt, therefore, principally to remove some of the most prevailing prejudices respecting combustion.
It is a popular opinion, that the destruction of the combustible body is a necessary consequence of its undergoing combustion, and that when bodies are burned they are destroyed. But the science of chemistry teaches us the error, and demonstrates the fallacy, of such a conclusion. The beautiful and conclusive experiments
of modern chemistry prove that the destruction of the most minute particle of matter is utterly impossible. Its form, however, may be changed and its combinations varied, but its annihilation is beyond the power of man. This assertion will no doubt, to the common mind, appear at variance with common sense, and every-day observation; and judging from appearances, the cursory observer will be apt to draw a very different conclusion. Probably he will reason thus : How can a candle, or other combustible, burn and not be destroyed ? Have we not palpable evidence that this is the case? Does not the candle disappear gradually, till scarcely a vestige of it remains ?
But the man of science observes the phenomena of nature very differently. Where the superficial observer can discover nothing but apparent waste and destruction, he detects the process of ceaseless renovation. He beholds new substances continually forming ; new compounds, like the fabled Phenix of old, springing from this very decomposition and decay. He sees the elemental particles, of which the combustible is composed, converted into aeriform bodies, which are not the less useful than the original body, in the great laboratory of nature. Not content with judging from appearances, he examines nature by experiment, and pries into her most secret operations.
It is rather a difficult matter to give a good definition of combustion, at least one which can be applied to every case of combustion. It has been defined as the operation of fire upon any inflammable substance, by which it smokes, flames, and is reduced to ashes. Dr. Ure describes it as the disengagement of heat and light, which accompanies chemical combination. Sir Humphrey Davy describes it to be the general result of the actions of any substances possessed of strong chemical attractions, or different electrical relations, and that it takes place in all cases in which an intense and violent motion is communicated to the corpuscles or minute atoms of bodies. And by another chemist, simply as the result of intense chemical action. On these definitions I offer no remarks: when the subject of combustion has been duly investigated, we shall be better enabled to judge what kind of definition is most proper. When a stone or a brick is heated, it undergoes no change, except an augmentation of temperature ; and when left to itself, it soon evols again and becomes as at first. But with combustible bodies the case is very different. When heated to a certain degree in the open air, they suddenly become much hotter of themselves, continne for a considerable time intensely hot, sending out a copious stream of caloric and light to the surrounding bodies. This emission, after a certain period, begins to diminish, and at last ceases altogether. The combustibles appear to be consumed, or, to speak more correctly, are converted into substances altogether new, and which frequently are not apparent to the senses. Thus, when charcoal is kept for some time at the temperature of about 800°, it kindles, becomes intensely hot, and continues to emit light and heat for a long time. When the combustion ceases, the charcoal has all disappeared, except an inconsiderable residuum of ashes ; being almost entirely converted into carbonic acid gas, which makes its escape, unless the experiment be conducted in proper vessels. It is this change of properties, and apparent loss of substance, together with the rapid emission of light and heat, which constitute the process, usually termed combustion.
In the examination of these phenomena we have, therefore, two things to consider ; first, the change which the body undergoes ; and, secondly, the emission of light and heat.
“When bodies are burnt,” says Berthollet, “none of their principles are destroyed; they had previously formed together one kind of compound, and they now separate from each other at the high temperature to which they are exposed,
in order to form others with the vital air in contact with them : and such of their principles as cannot unite with the vital air, viz. the earth, some saline and some metallic particles compose the cinder. The new compounds formed are carbonic acid and water; the proportion of these varies according to the proportion of the carbonic particles, and of the hydrogen that had been contained in the inflammable body.”
Although this passage describes the effects of combustion with great perspicuity and clearness, yet it may not be perfectly understood by one unacquainted with the principles of chemistry. I shall, therefore, enter more minutely into the subject, and elucidate some of those parts which may not appear perfectly intelligible.
Those substances which we use for producing fire and light, such as coal, oil, wood and tallow, are found, when subjected to chemical analysis, to be compound bodies. They are composed of two or more different ingredients. To these different ingredients, when separated from one another, and obtained in a distinct form, the names oxygen, nitrogen, hydrogen, carbon, &c. are applied. They are also called simple bodies, because, unlike coal, wood, oil and tallow, they have hitherto resisted all attempts to decompound them. Some of these ingredients or principles, such as nitrogen and oxygen, are incombustible; and, on the other hand, those called hydrogen and carbon are combustible. We have, therefore, simple combustible bodies and compound combustible bodies. Some of these components can only be obtained in an aeriform state, such as oxygen, hydrogen, nitrogen, but the carbon may be obtained in the solid state. The simple combustibles are as follow: hydrogen, carbon, sulphur, phosphorus, boron, and the metals.
Combustible bodies cannot of themselves support their own combustion. Certain media are necessary to maintain it; these, consequently, are called supporters of combustion. Thus, common atmospheric air is a supporter of combustion, for combustible bodies burn in it. Whereas, nitrogen gas, one of its constituents, is a non-supporter. Hydrogen gas also, although itself combustible, yet is a nonsupporter of combustion. (Here Mr. Ollivier introduced a taper into a bottle of hydrogen gas; it was immediately extinguished, but the hydrogen was inflamed at the mouth of the bottle, because hydrogen is combustible, but a non-supporter of combustion.)
We have thus seen the relation existing between a combustible and a supporter. It remains now to be shewn, that the combustible is not destroyed by undergoing combustion. But we must first premise that matter has no permanent form. Some elements, which constitute a solid body, may, by changing their state of combination, become gaseous or aeriform. When, therefore, the term gas is used, it is only to denote one of the forms of matter, the same as when the terms solid and liquid are used. The reason, therefore, that the greatest part of the combustible body disappears during combustion is, because the portion which disappears assumes the aerial form, or becomes converted into gas. It has been shewn in the preceding lecture, that it is the oxygen which is one of the constituent parts of air, which enables it to support combustion. This it effects by entering into combination with those constituent parts of the combustible body which have an affinity to, or are capable of uniting with, it. Thus, when coal, woud, tallow, &c. are burned, carbonic acid is formed, because carbon, one of their constituents, combines with the oxygen of the air, and forms a new body, called carbonic acid gas. Water is formed, because hydrogen, another of their constituents, by combining with oxygen, forms water. For water is a compound body, being composed of oxygen and hydrogen. As carbonic acid and water are the results of the combination of carbon and hydrogen with oxygen, and as some combustible bodies exist which contain neither of these principles, it necessarily follows that when such bodies undergo combustion, neither carbonic acid nor water will be formed. Thus, when we burn sulphur, which is considered a simple body, and consequently contains neither carbon nor hydrogen, a compound of oxygen and sulphur is formed, called sulphurous acid, which is the cause of that suffocating sensation experienced during the burning of sulphur. Phosphorus, which is also consirlered a simple body, forms, by burning, a compound of oxygen and phosphorus, called phosphoric
acid. The latter part, therefore, of the passage above quoted from Berthollet's chemical statics is correct only so far as it applies to the combustion of bodies containing both carbon and hydrogen.
From what has been stated, it must appear evident that the process of combustion, instead of destroying, merely decomposes the combustible body, and sets its several component parts at liberty, in order to form new combinations.
As the combustible, during combustion, combines with another body, and as the light and heat evolved during this process have no weight, it necessarily follows that the product of combustion will be heavier than the original combustible body, in the proportion of the weight of the oxygen with which it has combined. And this is found to be actually the case when the product of combustion is saved. If a candle be suffered to burn in a glass vessel of globular figure, with a trench round the interior of the bottom to receive the water as it trickles down, and the exterior be kept constantly cold by wet cloths, the water only formed by the combustion of the candle, without taking account of the carbonic acid also formed, will be found heavier than the original candle, in the proportion of the oxygen, which has combined with its hydrogen. One hundred ounces of oil, consumed in a similar manner, will form one hundred and thirty ounces of water.
It is also a very remarkable circumstance that all bodies, thus entering into chemical combination, invariably combine in certain fixed and definite proportions. Thus 12 parts of hydrogen invariably combine with 88 of oxygen by weight, to form water; 28 parts of carbon combine with 72 oxygen, to form carbonic acid; so that, when water or carbonic acid are analyzed, they are invariably found united in the above-mentioned proportions. We can, therefore, calculate beforehand how much carbonic and water will be formed by the combustion of a given weight of tallow or oil, supposing the whole of it to undergo combustion.
For this beautiful discovery we are entirely indebted to modern chemistry. It is most happily expressed, says Murray, in the sublime emphasis of the Sacred Record, which refers to Deity in the creation of material things : “He weighed the hills in scales, and the dust in a balance." There is thus proved to be a sublime literality in this extraordinary passage far surpassing the grandeur of orientalism, reserved for these latter days to illustrate. It is thus true that created forms are by weight and measure; and matter, in its multifarious combinations, reveals at length the important truth.
The merit of the discovery that the combustible body always increases in weight, and that the increase corresponds exactly to the weight of the quantity of oxygen gas which disappears during that process, belongs to Lavoisier. This he proved by several experimente. He found that phosphorus, in burning, absorbs more than one and a half its weight of oxygen gas, 45 grains of it consuming 69 of oxygen; and that the weight of the substance produced, during the combustion, exactly equals the sum of the weight of the phosphorus consumed, and oxygen absorbed. He proved the same with regard to sulphur, charcoal, and several of the metals. Thus, in the combustion of 100 grains of iron, he found that 70 cubic inches of oxygen gas were consumed, and the iron had increased in weight 35 grains. But a cubic inch of oxygen gas weighs just one half grain ; the weight, therefore, of the 70 inches was 35 grains, corresponding exactly to the augmentation of weight in the iron.
The combustibility of bodies which have undergone combustion may again be restored by abstracting the oxygen with which they have combined. This may be effected by presenting to the burnt body a substance having a superior attraction for oxygen; the new body will deprive the burnt body of its oxygen, and its combustibility will be restored. For instance, if we take phosphoric acid, which is burnt phosphorus, or phosphorus combined with oxygen, and expose it to heat along with charcoal, it will become deoxaggenized, the charcoal will combine with its oxygen and escape as carbonic acid, and the phosphorus will be reduced to its original form. The metallic oxides, in like manner, may, by being exposed to heat along with charcoal, be also reduced to their metallic state. Water is a product of combustion, and its base is hydrogen, a combustible substance. To restore the combustibility of the hydrogen, we have only to abstract its oxygen, whicḥ may readily be done by mixing iron or zinc filings and sulphuric acid, with the water; by which means the metal becomes oxidized, and the hydrogen gas is evolved as combustible as ever. Mr. Ollivier then decomposed water by this means, and inflamed the hydrogen as it escaped from the water. The combustibility of metals was also exemplified in the combustion of a watch-spring in oxygen gas, and also of that of a common file with a oxyhydrogen blow pipe.
The combustion of the metal, which was brilliant and striking, afforded a remarkable instance of the energy of oxygen as a supporter of combustion.
In concluding Mr. O. remarked, that the examination of those other subjects connected with combustion, which had not been mentioned, would be reserved for a future occasion.
THE CHAPEL OF LA HOUGUE-BIE, IN THE ISLAND OF JERSEY.
The traditionary origin of this ancient chapel, though shrowded in the legendary Jore of the mythology, contained in “Le Livre Noir de Coutances," is, perhaps, not altogether divested of truth. The account we have is, that this part of the island was infested by a monstrous serpent or dragon,* which desolated the country. Fired with an ardent desire to destroy the dreadful hydra, the lord of Hambye, in Normandy, undertook the adventurous enterprize, and was successful : but in the conflict, this chivalrous nobleman was apparently suffocated by the pestilential breath of his dying antagonist. The lord of Hambye was attended in This enterprize by a supposed trusty domestic, who, perceiving his master reviving from the effects of this conflict, basely assassinated him. Returning to Normandy with the tragical account that his lord did not long survive the encounter, he presented to the disconsolate widow a letter, which he represented to have been written by her lord just before his death, and which contained a dying request that his bride would recompense the faithful servant by conferring her hand upon him. The artifice prevailed, and the “mourning bride” was united at the sacred altar to the murderer of her husband: but, upon the very day on which the consummation of his villainy was complete, he was suddenly seized with a delirious paroxysm, disclosed the horrid truth, and, on recovering, was tried on his own confession and publicly executed.
The lady, as well to testify her grateful thanksgiving to Heaven for this miraculous deliverance, as to establish a lasting memorial of conjugal affection towards her murdered lord, caused a large and high mound of earth to be raised on the spot where the lord of Hambye was buried, on the summit of which she built a chapel, with so lofty a tower as to be visible from her own mansion at Coutances.
Various reliques, interesting to that learned portion of the insular community “ which delighteth" in antiquarian research, have, from time to time, been found among the ruins of the old chapel. We present our readers with the following historical fragment, recently discovered in one of the venerable niches of the large tower. Oh ! list the doleful tragedie,
The rumour of this baneful scourge
Is borne from place to place.
Across the sea,-across the land,
It travelleth right fast; This monument of Gothic art
Till to the ear of Hambye's knight
It doth arrive at last.
His breast quick kindleth with desire
To meet this deadly foe, And wakened by these ruined walls,
Where honor leads, and danger calls,
He cannot choose, but go.
In vain his lovely bride essaies
To check his youthful zeal; In days of glorious chivalrie,
Too well her fears, her beating breast,
And stifled sobs reveal.
“Nay weep not thus, my onelie life,
“My Athelinda dear; Horror and deadly fear was rife
“'Twould surely ill-become a knight Throughout the neighbourhood;
“To bow to woman's fear. For oftimes did this monster prey On human flesh for food.
“The Holy Saints in Heaven above,
“ Who watch o'er thee and me, The widow's weeds,--the orphan's tears, “Will bring me, ere five suns have passed, The mother's shrill lament,
“In safety back to thee." Did track the cruel monster's course, And marked him as he went.
He girds him with his trusty sword,
His lance, and buckler true; Across the sea, in Normandie,
A single page his steps attends(Whence sprung our ancient race,)
His only retinue. * The dragon is probably the impersonation of some piratical marauder, who infested the islands about that time.