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PREFACE.

SCARCELY a century has elapsed since Electricity was but a modest chapter in treatises on Physics. The Leyden jar, then, had just been discovered; and the power of the electric spark had already presented itself to our notice. But Franklin had not as yet established the identity between lightning and Electricity. It was merely known that certain bodies, after having been rubbed, become capable of exercising an attraction around them, — that some substances are conductors, and others non-conductors of Electricity, – that in Electricity there are two different principles, which attract each other, whilst those which are similar repel each other, — that the uniting of the two opposed principles gives rise to sparks, the power of which may attain to a remarkable degree of energy, by means of proper apparatus.

The philosophers of 1750 knew very little beyond that of the nature and properties of Electricity. Who, at that time, could have supposed that Meteorology would ere long discover in Electricity the cause of the grand phenomena of the atmosphere ? — that Heat would borrow from it its most perfect instruments, and the means of manifesting its most important laws ? — that Molecular Physics would have employed it for the purpose of penetrating into the intimate constitution of bodies; and would have caused it to concur with polarised light in the manifestation of the relations that exist between ponderable matters and the imponderable ether? — That Chemistry would have been indebted to it for the discovery of new elements, the formation of new compounds, its most powerful means of analysis and synthesis, and its most satisfactory theories? - That Mineralogy and Geology would in a great measure have found in it the explanation of the origin of their crystals and of their strata ? — that Physiology would have deduced from it a more intimate knowledge of the forces that rule over animated matter, and the secret of acting on such matter almost as life acts ? - that Medicine would have discovered in it resources against maladies hitherto assumed to be incurable ? -- that the Metallurgic Arts would have found in it new processes for extracting, moulding, and applying metals ? — that, finally, it would have furnished to Mechanics a force, as prompt as thought, equally independent of time and space, — would have enabled intelligence to escape from its limited envelope, to dart at pleasure with the rapidity of lightning into the most distant regions.

Such, however, are the marvels that Electricity has accomplished in less than a century ; such are the links that now unite it in an indissoluble manner with all the parts of the physical sciences. And hence the study of it has become indispensable to all who cultivate these sciences, - to the chemist, as well as to the natural philosopher, to the geologist, as much as to the physiologist, — to the engineer, as well as to the physician. All are compelled to meet Electricity in their path; all consequently have need of becoming familiarised with it. I have been induced to think that a work designed to satisfy a want which is universally felt, would be received with favour. The very abridged manner, and the unmethodical style in which Electricity is for the most part treated in the treatises on Physics is not at all calculated for attaining that end; and, on the other hand, the special works that are devoted to it,

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enter too much into detail for persons who do not desire to make it the principal object of their labours. To initiate into the study of Electricity all the category of men of Art and Science, to whom it is necessary, an exposition is required that shall be at once substantial and elementary, complete and abridged; the subject must be presented in a logical order, and not, as has been hitherto the case, in an order that is almost purely historical ; in a word, the character of a true science must be given to this, as to the other branches of Physics, of which character it will always be deficient so long as it shall remain a simple, and, as is generally the case, a confused compilation of hap-hazard theories and disconnected facts.

I have endeavoured to express the spirit with which I have been actuated, while preparing the Treatise that is now submitted to the public. I have not attempted to make it a treatise for men of the world, although I have endeavoured to render it comprehensible to all who have received a reasonable amount of intellectual culture. I have endeavoured to address myself essentially to men who cultivate science, and who consequently are already acquainted with its language and its processes ; but, at the same time, I have not assumed them to possess previously any profound knowledge of this or that particular branch of science. On this account, I have collected in some final notes, placed at the end of each volume, the mathematical developments, of which certain particular points are susceptible; but which are in no degree essential to the knowledge of the whole.

It remains for me to explain the order that I have followed, and to justify it, by a rapid glance over the vast subject to which the Treatise on Electricity is devoted.

When viewed in relation to the successive progresses that it has manifested, Electricity offers so prodigious a variety of scientific features, that the mind runs a risk of being lost when, by following the historic order, it is most commonly obliged to contemplate them all at once. Whilst it is following with Coulomb the researches of the laws, to which static electricity is subjected, it is called upon to scrutinise with Galvani the mysteries of animal electricity, and to travel with Volta to the discovery of the pile. Then again, whilst it is endeavouring to comprehend the beautiful calculations, on which Poisson has founded the theories of electricity, it is seized with admiration at the magnificent as well as unexpected results that Davy obtained from the Voltaic pile. But it is when setting out from the year 1820, which Oersted's discovery has made so remarkable in the history of the sciences, that the task of him who wishes to follow day by day the movement impressed upon electricity, becomes still more difficult. In the first place, it assists in the creation of that new branch of the science, which includes under the name of Electro-dynamics the general laws of electricity in motion:the very interesting study, to which it leads, of the works of Arago, of Ampère and of Faraday, the founders of this part of physics, is constantly interrupted by discoveries of a very different order. Here is Seebeck, who discovers thermo-electric currents, here is Becquerel, here is Nobili who analyse them, at the same time that they are laying the foundations of electro-chemistry. Here are Marianini, Matteucci and Dubois-Reymond, who, in taking up the labours of Galvani and of Volta on animal electricity, give to this part of physiology a development, which threatens entirely to usurp it. Then there are the works upon the theory of the pile and its effects, which have called into the field very many philosophers, as Ohm, Pouillet, Fechner, Faraday, &c., and amongst whom is included the author himself of the present treatise. Then, again, there is an uninterrupted succession of new researches -- the magnetic, the chemical, the calorific, and the luminous pheno

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