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(336.) The history of chemistry (on which, however, we are not about to enlarge,) is one of great interest to those who delight to trace the steps by which mankind advance to the discovery of truth through a series of mistakes and failures. It may be divided, 1st, into the period of the alchemists, a lamentable epoch in the annals of intellectual wandering; 2dly, that of the phlogistic doctrines of Beccher and Stahl, in which, as if to prove the perversity of the human mind, of two possible roads the wrong was chosen; and a theory obtained universal credence on the strength of an induction, valid as such, but wrongly interpreted, which is negatived, _in every instance_, by an appeal to the balance. This, too, happened, not by reason of unlucky coincidences, or individual oversights, but of necessity, and from an inherent defect of the theory itself, which thus impeded the progress of the science, as far as a science of experiment can be impeded by a false theory, by perplexing its cultivators with the appearance of contradictions in their experiments where none really subsisted, by destroying all their confidence in the numerical exactness of their own results, and by involving the subject in a mist of visionary and hypothetical causes in place of the true acting principles. Thus, in the combustion of any substance which is incapable of flying away in fumes, an increase of weight takes place,--the ashes are heavier than the fuel. Whenever this was observed, however, it was passed carelessly over as arising from the escape of phlogiston, or the principle of inflammability, which was considered as being either the element of fire itself, or in some way combined with it, and thus essentially _light_. It is now known that the increase of weight is owing to the absorption of, and combination with, a quantity of a peculiar ingredient called _oxygen_, from the air, a principle essentially _heavy_. So far as weight is concerned, it makes no difference whether a body having weight enters, or one having levity escapes; but there is this plain difference in a philosophical point of view, that oxygen is a real producible substance, and phlogiston is no such thing: the former is a _vera causa_, the latter an hypothetical being, introduced to account for what the other accounts for much better.

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(337.) The third age of chemistry--that which may be called emphatically modern chemistry--commenced (in 1786) when Lavoisier, by a series of memorable experiments, extinguished for ever this error, and placed chemistry in the rank of one of the exact sciences,--a science of number, weight, and measure. From that epoch to the present day it has constantly advanced with an accelerated progress, and at this moment may be regarded as more progressive than ever. The principal features in this progress may be comprised under the following general heads:--

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1. The discovery of the proximate, if not the ultimate, elements of all bodies, and the enlargement of the list of known elements to its present extent of between fifty and sixty substances.

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2. The developement of the doctrine of latent heat by Black, with its train of important consequences, including the scientific theory of the steam-engine.

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3. The establishment of Wenzel’s law of definite proportions on his own experiments, and those of Richter, a discovery subsequently merged in the more general wording and better development of Dalton’s atomic theory.

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4. The precise determination of the atomic weights of the different chemical elements, mainly due to the astonishing industry of Berzelius, and his unrivalled command of chemical resources, as well as to the researches of the other chemists of the Swedish and German school.

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5. The assimilation of gases and vapours, by which we are led to regard the former, universally, as particular cases of the latter, a generalization resulting chiefly from the experiments of Faraday on the condensation of the gases, and those of Gay-Lussac and Dalton, on the laws of their expansion by heat compared with that of vapours.

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6. The establishment of the laws of the combination of gases and vapours by definite volumes, by Gay-Lussac.

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7. The discovery of the chemical effects of electricity, and the decomposing agency of the Voltaic pile, by Nicholson and Carlisle; the investigation of the laws of such decompositions, by Berzelius and Hisinger: the decomposition of the alkalies by Davy, and the consequent introduction into chemistry of new and powerful agents in their metallic bases.

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8. The application of chemical analysis to all the objects of organized and unorganized nature, and the discovery of the ultimate constituents of all, and the proximate ones of organic matter, and the recognisance of the important distinctions which appear to divide these great classes of bodies from each other.

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9. The applications of chemistry to innumerable processes in the arts, and among other useful purposes to the discovery of the essential medical principles in vegetables, and to important medicaments in the mineral kingdom.

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10. The establishment of the intimate connection between chemical composition and crystalline form, by Haüy and Vauquelin, with the successive rectifications the statement of that connection has undergone in the hands of Mitscherlich, Rose, and others, with the progress of chemical and crystallographical knowledge.

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(338.) To pursue these several heads into detail would lead us into a treatise on chemistry; but a few remarks on one or two of them, as they bear upon the general principles of all scientific enquiry, will not be irrelevant. And first, then, with reference to the discovery of new elements, it will be observed, that philosophical chemistry no more aims at determining the one essential element out of which all matter is framed--the one ultimate principle of the universe--than astronomy at discovering the origin of the planetary movements in the application of a determinate projectile force in a determinate direction, or geology at ascending to the creation of the earth. There may be such an element. Some singular relations which have been pointed out in the atomic weights of bodies seem to suggest to minds fond of speculation that there is; but philosophical chemistry is content to wait for some striking fact, which may either occur unexpectedly or be led to by the slow progress of enlarged views, to disclose to us its existence. Still, the multiplication of so-considered elementary bodies has been considered by some as an inconvenience. We confess we do not coincide with this view. Whatever they be, the obstinacy with which they resist decomposition shows that they are ingredients of a very high and primary importance in the economy of nature; and such as, in any state of science, it would be indispensably necessary to be perfectly familiar with. Like particular theorems in geometry, which, though not rising to the highest point of generality, have yet their several scopes and ranges of extensive application, they must be well and perfectly understood in all their bearings. Should we ever arrive at an analysis of these bodies, the chemical properties of the new elements which will then come into view will be known only by our knowledge of these, or of other compounds of the same class, which they may be capable of forming. Not but that such an analysis would be a most important and indeed triumphant achievement, and change the face of chemistry; but it would undo nothing that has been done, and render useless no point of knowledge which we have yet arrived at.

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(339.) The atomic theory, or the law of definite proportions, which is the same thing presented in a form divested of all hypothesis, after the laws of mechanics, is, perhaps, the most important which the study of nature has yet disclosed. The extreme simplicity which characterizes it, and which is itself an indication, not unequivocal, of its elevated rank in the scale of physical truths, had the effect of causing it to be announced at once by Mr. Dalton, in its most general terms, on the contemplation of a few instances[53], without passing through subordinate stages of painful inductive ascent by the intermedium of subordinate laws, such as, had the contrary course been pursued by him, would have been naturally preparatory to it, and such as would have led others to it by the prosecution of Wenzel’s and Richter’s researches, had they been duly attended to. This is, in fact, an example, and a most remarkable one, of the effect of that natural propensity to generalize and simplify (noticed in 171.), which, if it occasionally leads to over-hasty conclusions, limited or disproved by further experience, is yet the legitimate parent of many of our most valuable and soundest results. Instances like this, where great and, indeed, immeasurable steps in our knowledge of nature are made at once, and almost without intellectual effort, are well calculated to raise our hopes of the future progress of science, and, by pointing out the simplest and most obvious combinations as those which are actually found to be agreeable to the harmony of creation, to hold out the cheering prospect of difficulties diminishing as we advance, instead of thickening around us in increasing complexity.

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(340.) A consequence of this immediate presentation of the law of definite proportions in its most general form is, that its subordinate laws--those which limit its generality in particular cases, which diminish the number of combinations abstractly possible, and restrain the indiscriminate mixture of elements,--remain to be discovered. Some such limitations have, in fact, been traced to a certain extent, but by no means so far as the importance of the subject requires; and we have here abundant occupation for chemists for some time.

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(341.) The determination of the atomic weights of the chemical elements, like that of other standard physical data, with the utmost exactness, is in itself a branch of enquiry not only of the greatest importance, but of extreme difficulty. Independent of the general reasons for desiring accuracy in this respect, there is one peculiar to the subject. It has been suggested (by Dr. Prout), and strongly insisted on (by Dr. Thomson), that all the numbers representing these weights, constituting a scale of great extent, in which the extremes already known are in proportion to each other, as 1 to upwards of 200, are simple even multiples of the least of them. If this be really the case, it opens views of such importance as to justify any degree of labour and pains in the verification of the law as a purely inductive one. But in the actual state of chemical analysis, with all deference to such high authority, we confess it appears to us to stand in great need of further confirmation, since it seems doubtful whether such accuracy has yet been attained as to enable us to answer positively for a fraction not exceeding the three or four hundredth part of the whole quantity to be determined: at least the results of the first experimenters, obtained with the greatest care, differ often by a greater amount; and this degree of exactness, at least, would be required to verify the law satisfactorily in the higher parts of the scale.

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(342.) The mere agitation of such a question, however, points out a class of phenomena in physical science of a remote and singular kind, and of a very high and refined order, which could never become known but in an advanced state of science, not only practical, but theoretical,--we mean, such as consist in observed relations among the _data_ of physics, which show them to be quantities not _arbitrarily_ assumed, but depending on laws and causes which they may be the means of at length disclosing. A remarkable instance of such a relation is the curious law which Bode observed to obtain in the progression of the magnitudes of the several planetary orbits. This law was interrupted between Mars and Jupiter, so as to induce him to consider a planet as wanting in that interval;--a deficiency long afterwards strangely supplied by the discovery of _four_ new planets in that very interval, all of whose orbits conform in dimension to the law in question, within such moderate limits of error as may be due to causes independent of those on which the law itself ultimately rests.[54]

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(343.) Neither is it irrelevant to our subject to remark, that the progress which has been made in this department of chemistry, and the considerable exactness actually attainable in chemical analysis, have been owing, in great measure, to a circumstance which might at first have been hardly considered likely to exercise much influence on the progress of a science,--the discovery of platina. Without the resources placed at the ready disposal of chemists by this invaluable metal, it is difficult to conceive that the multitude of delicate analytical experiments which have been required to construct the fabric of existing knowledge could have ever been performed. This, among many such lessons, will teach us that the most important uses of natural objects are not those which offer themselves to us most obviously. The chief use of the moon for man’s immediate purposes remained unknown to him for five thousand years from his creation. And, since it cannot but be that innumerable and most important uses remain to be discovered among the materials and objects already known to us, as well as among those which the progress of science must hereafter disclose, we may hence conceive a well-grounded expectation, not only of constant increase in the physical resources of mankind, and the consequent improvement of their condition, but of continual accessions to our power of penetrating into the arcana of nature, and becoming acquainted with her highest laws.

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CHAP. V.

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OF THE IMPONDERABLE FORMS OF MATTER.

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_Heat._

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(344.) One of the chief agents in chemistry, on whose proper application and management the success of a great number of its enquiries depends, and many of whose most important laws are disclosed to us by phenomena of a chemical nature, is HEAT. Although some of its effects are continually before our eyes as matters of the most common occurrence, insomuch that there is scarcely any process in the useful arts and manufactures which does not call for its intervention, and although, independent of this high utility, and the proportionate importance of a knowledge of its nature and laws, it presents in itself a subject of the most curious speculation; yet there is scarcely any physical agent of which we have so imperfect a knowledge, whose intimate nature is more hidden, or whose laws are of such delicate and difficult investigation.

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(345.) The word heat generally implies the sensation which we experience on approaching a fire; but, in the sense it carries in physics, it denotes the cause, whatever it be, of that sensation, and of all the other phenomena which arise on the application of fire, or of any other heating cause. We should be greatly deceived if we referred only to sensation as an indication of the presence of this cause. Many of those things which excite in our organs, and especially of those of taste, a sensation of heat, owe this property to chemical stimulants, and not at all to their being actually _hot_. This error of judgment has produced a corresponding confusion of language, and hence had actually at one period[55] crept into physical philosophy a great many illogical and absurd conclusions. Again, there are a number of chemical agents, which, from their corroding, blackening, and dissolving, or drying up the parts of some descriptions of bodies, and producing on them effects not generally unlike (though intrinsically very different from) those produced by heat, are said, in loose and vulgar language, to burn them; and this error has even become rooted into a prejudice, by the fact that some of these agents are capable of becoming actually and truly _hot_ during their action on moist substances, by reason of their combination with the water the latter contain. Thus, quicklime and oil of vitriol both exercise a powerful corrosive action on animal and vegetable substances, and both become violently hot by their combination with water. They are, therefore, set down in vulgar parlance as substances of a hot nature; whereas, in their relations to the physical cause of heat, they agree with the generality of bodies similarly constituted.

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(346.) The nature of heat has hitherto been chiefly studied under the general heads of--

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1st, Its sources, or the phenomena which it usually accompanies.

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2d, Its communication from its sources to substances capable of receiving it, and from these to others, with a view to discover the laws which regulate its distribution through space or through the bodies which occupy it.

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3d, Its effects, on our senses, and on the bodies to which it is communicated in its various degrees of intensity, by which, means are afforded us of measuring these degrees.

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4th, Its intimate relations to the atoms of matter, as exhibited in its capability of acquiring a latent state under certain circumstances, and of entering into something like chemical combinations.

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(347.) The most obvious sources of heat are, the sun, fire, animal life, fermentations, violent chemical actions of all kinds, friction, percussion, lightning, or the electric discharge, in whatever manner produced, the sudden condensation of air, and others, so numerous, and so varied, as to show the extensive and important part it has to perform in the economy of nature. The discoveries of chemists, however, have referred most of these to the general head of chemical combination. Thus, fire, or the combustion of inflammable bodies, is nothing more than a violent chemical action attending the combination of their ingredients with the oxygen of the air. Animal heat is, in like manner, referable to a process bearing no remote analogy to a slow combustion, by which a portion of carbon, an inflammable principle existing in the blood, is united with the oxygen of the air in respiration; and thus carried off from the system: fermentation is nothing more than a decomposition of chemical elements loosely united, and their re-union in a more permanent state of combination. The analogy between the sun and terrestrial fire is so natural as to have been chosen by Newton to exemplify the irresistible force of an inference derived from that principle. But the nature of the sun and the mode in which its wonderful supply of light and heat is maintained are involved in a mystery which every discovery that has been made either in chemistry or optics, so far from elucidating, seems only to render more profound. Friction as a source of heat is well known: we rub our hands to warm them, and we grease the axles of carriage-wheels to prevent their setting fire to the wood; an accident which, in spite of this precaution, does sometimes happen. But the effect of friction, as a means of producing heat with little or no consumption of materials, was not fully understood till made the subject of direct experiment by count Rumford, whose results appear to have established the extraordinary fact, that an unlimited supply of heat may be derived by friction from the same materials. Condensation, whether of air by pressure, or of metals by percussion, is another powerful source of heat. Thus, iron may be so dexterously hammered as to become red-hot, and the rapid condensation of a confined portion of air will set tinder on fire.

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(348.) The most violent heats known are produced by the concentration of the solar rays by burning glasses,--by the combustion of oxygen and hydrogen gases mixed in the exact proportion in which they combine to produce water,--and by the discharge of a continued and copious current of electricity through a small conductor. As these three sources of heat are independent of each other, and each capable of being brought into action in a very confined space, there seems no reason why they might not all three be applied at once at the same point, by which means, probably, effects would be produced infinitely surpassing any hitherto witnessed.

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(349.) Heat is communicated either by _radiation_ between bodies at a distance, or by _conduction_ between bodies in contact, or between the contiguous parts of one and the same body. The laws of the radiation of heat have been studied with great attention, and have been found to present strong analogies with that of light in some points, and singular differences in others. Thus, the heat which accompanies the sun’s rays comports itself, in all respects, like light; being subject to similar laws of reflection, refraction, and even of polarization, as has been shown by Berard. Yet they are not identical with each other; Sir William Herschel having shown, by decisive experiments, verified by those of Sir H. Englefield, that there exist in a solar beam both rays of heat which are not luminous, and rays of light which have no heating power.

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(350.) The heat, radiated by terrestrial fires, and by bodies _obscurely_ hot, by whatever means they have acquired their heat (even by exposure to the sun’s rays), differs very materially from solar heat in their power of penetrating transparent substances. This singular and important difference was first noticed by Mariotte, and afterwards made the subject of many curious and interesting experiments by Scheele, who found that terrestrial heat, or that radiated from fires or heated bodies, is intercepted and detained by glass or other transparent bodies, while solar heat is not; and that, being so detained, it heats them: which the latter, as it passes freely through them, is incapable of doing. The more recent researches of Delaroche, however, have shown that this detention is complete only when the temperature of the source of heat is low; but that, as that temperature is higher, a portion of the heat radiated acquires a power of penetrating glass; and that the quantity which does so bears continually a larger and larger proportion to the whole, as the heat of the radiant body is more intense. This discovery is very important, as it establishes a community of nature between solar and terrestrial heat; while at the same time it leads us to regard the actual temperature of the sun as far exceeding that of any earthly flame.

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(351.) A variety of theories have been framed to account for these curious phenomena; but the subject stands rather in need of further elucidation from experiment, and is one which merits, and will probably amply repay, the labours of those who may hereafter devote their attention to it. The theory of the radiation of heat, in general, which seems to agree best with the known phenomena, is that of M. Prevost, who considers all bodies as constantly radiating out heat in all directions, and receiving it by a similar means of communication from others, and thus tending, in any space filled, wholly or in part, with bodies at various temperatures, to establish an equilibrium or equality of heat in all parts. The application of this idea to the explanation of the phenomenon of dew we have already seen (see 167.). The laws of such radiation, under various circumstances, have been lately investigated in a beautiful series of experiments on the cooling of bodies by their own radiation in vacuo, by Messrs. Dulong and Petit, which offer some of the best examples in science of the inductive investigation of quantitative laws.

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(352.) The communication of heat between bodies in contact, or between the different parts of the same body, is performed by a process called conduction. It is, in fact, only a particular case of radiation, as has been explained above (217.); but a case _so_ particular as to require a separate and independent investigation of its laws. The most important consideration introduced into the enquiry by this peculiarity is that of time. The communication of heat by conduction is performed, for the most part, with extreme slowness, while that performed by direct radiation is probably not less rapid than the propagation of light itself. The analysis of the delicate and difficult points which arise in the investigation of this subject in its reduction to direct geometrical treatment has been executed with admirable success by the late Baron Fourrier, whose recent lamented death has deprived science of an ornament it could ill spare, thinned as its ranks have been within the last few years. This acute philosopher and profound mathematician has developed, in a series of elaborate memoirs presented to the French Institute, the laws of the communication of heat through the interior of solid masses, placed under the influence of any external heating and cooling causes, and has in particular applied his results to the conditions on which the maintenance of the actual observed temperature on the earth’s surface depends; to the possible influence of a supposed central heat on our climates; and to the determination of the actual amount of the heat, derived to us from the sun, or at least that portion of it on which the difference of the seasons depends.

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(353.) The principal effects of heat are the sensations of warmth or cold consequent on its entry or egress into or out of our bodies; the dilatation it causes in the dimensions of all substances in which it is accumulated; the changes of state it produces in the melting of solids, and the conversion of them and of liquids into vapour; and the chemical changes it performs by actual decompositions effected in the intimate molecules of various substances, especially those of which vegetables and animals are composed; to which we may add, the production of electric phenomena under certain circumstances in the contact of metals, and the developement of electric polarity in crystallised substances.
