Imágenes de páginas
PDF
EPUB

height, with cold water, and let them be placed upon a stool with glass legs, near the conductor of the electrical machine, and as far apart as the stool will admit. Let a chain, or rod, be fixed, to the conductor, and put into the jar which contains the cold water, and then turn the cylinder to ele&trify the cold water. If a person presents any conducting substance to the knob of the coated jar, ftanding in the vessel filled with hot water, it will give a spark; and if a communication be formed between the knob of the coated vial, and the cold water in the other vessel, there will be a discharge, a spark, and a fhock. In this experiment the electric Auid must pass from the denfer to tho rarer medium, i, e. from the vessel with cold water to the jar with hot water; and if it does not pass through the pores, or over the surface of the glass, I must leave it to the friends of the Franklinian theory to thew, how the knob, of the coaced vial can acquire a sufficient quantity of the electric fuid to give a Sparka'

This strange experiment, which is followed by another nearly of the same kind, is more likely to puzzle than to entertain? the reader ; who surely will not readily comprehend the defigo for which it was made.--It is indeed a circumstance peculiarly characteristic of this work, that, wbile other philosophers aim at the greatest fimplicity in the planning of their experiments, the Author seems to delight in throwing, an air of obfcurity over the simplest phenomena,' by employing the most complicated apparatus. One would think, that his principal defigo in this performance was not to overturn a particular theory, or to esta. blish any other ; but to perplex the Franklinift with a set of electrical enigmas, to exercise his wits upon them; fo as to puzzle him to find which side of a jar or flask, which he has put into some strange fituation or other, is positive, and which iš negative. A great part of his apparatus is certainly better adapted to confound than to convert him: and in fact, we rather fufpect that the Author himself has been often bewilderede in the mazes into which he has been led by his own maa chinery;

A variety of other subje&s is treated of in the present work ; such as Signor Voita's Electrophorus ; where the Author conti-. nues to maintain the doctrine of electric permeability: the electric phenomena produced by the fri&ion of folk, and other bodies of a rare texture; as first observed by Mr. Symner, and afterwards particularly attended to by: M. Cigna, and Signor i Beccaria :-electrical cohefion, and various other phenomena, relative both, co naqural and artificial electricity. On all these subjects the Author produces a great number of original experiments, In this, as well as in the preceding parts of the work, we cannot help observing, and indeed commending the Au

th r's

thor's industry; not, however, without remarking, that his inquiries in this branch of philosophy would undoubtedly have been more successful, had he not evidently prosecuted many of them under the influence of prejudice; or had he more carefully studied the system which he opposes, and imitated the fimplicity of its inventor. We mean no disrespect to the Author, even when we recommend to his perusal the following short article, or rather the little manual which is the subject of it; the leading doctrine of which he opposes, particularly in his 4th chapter, and in his eleventh, when he treats of the Électrophon rus, as well as elsewhere.

W

ART. II. A Short View of Eleåricity : By Benj. Wilson, F.R.S.

and Member of the Imperial Academy of Sciences at Petersburgh, &c. &c. 400. 25. Nourse. 1780.

HATEVER may be thought of Mr. Wilson's pecu-*

liar opinions, with respect to the controversy concern-, ing the comparative utility of blunt and pointed conductors; he has in this little tract performed an acceptable service to those who would undertake the ftudy of electricity. The experiments contained in it, which are in general extremely simple, and of courfe easily to be comprehended, are contrived with a view to explain some of the principal phenomena presented by electrified bodies. The principle on which the greater part of these experiments depends, though not particularly specified by the Author, is one of the most fertile and important in the whole science; and we think, that the Author would have done right to have inserted it among his General Observations' prefixed to the experiments. The proposition to which we allude is, in short, this ;-that electrified bodies, without parting with their own electricity, ad upon other bodies in their neighbourhood, in such a manner as to produce in them an electricity contrary to their own,

We have frequently taken occasion to speak of this principle; particularly in our Review for December 1779, pag. 408, and in the other parts of our Journal there referred to. The principal phenomena of the Leyden vial, and of the Electrophorus, as well as those exhibited by thunder clouds, receive from this principle the most satisfactory explanation. And though it be difficult to conceive in what manner one body can act upon another, at a distance, through glass, or air, without actually pafing through these substances (which are affirmed to be impervious to it, except when it forces its way through the former by perforating it, or through the latter in the form of a spark, or blaft); yet the truth of the principle is too well established by experiment to admit of doubt.

With

B4

With respect, however, to the impermeability of glass, we should'observe that, from an expression that occurs in this treatife, where the Author applies the results of the preceding experiments to the explanation of the phenomena of the Leyden vial, the Reader is led to suppose, that he allows that glass is permeable to the electric fluid; and that he confiders it as differing from metals, wood, &c. only in the circumstance of refistance.'_ More time,' he says, is required for the electric fluid to pass through a given length, or thickness of glass, than through metal or wood of the same length or thickness.'— Though we recollect, that the Author formerly maintained the opinion, that glass was permeable to the electric matter; we are rather inclined to suspect only an inaccuracy of expression in this quotation. For if, in the charging of a Leyden vial, the electric matter passes through the thickness of the glass' at all ; it most undoubtedly passes through it with the greatest freedom and celerity: for; during the process of charging it, sparks successively appear at its outside surface, equal both in number and in strength to those seen at the wire that communicates with its infide. On the whole, whether the electric fuid can pass through glass, under certain circumstances, or no; it appears evident, that it does not pafs through it in the Leyden experiment: 'at least no theory has yet been imagined, by which the charging of the vial has been accounted for, on the hypothesis of the permeability of glass.

In the first set of experiments given in this short tract, one, two, or more cylinders of wood, to which pith balls are annexed, are insulated by refting on a wine glass; and an excited glass cylinder is held at a small distance. The cylinders are either rounded or pointed at the ends, and are successively placed, end to end, in different situations with respect to each other : and the effect of the excited glass, in producing a change in the electric state of the wooden cylinders, is readily ascertained by, an examination of the balls suspended to each.-Thus, to give only one instance exemplifying the principle above mentioned ;

four wooden cylinders, round at both ends, are placed in contact with each other, in the order of the letters, A, B, C, D, The excited glass cylinder is held at a small distance over A, from which it forces out a part of its natural quantity of electricity into B. This additional quantity, accumulated in B, in its turn forces out part of the natural quantity belonging to C, into D. That these changes have been produced is rendered evident, on separating the cylinders just before the excited glass is removed from A : for A is found to be in a negative, B in a positive, C in a negative, and D in a positive state.

The Author makes a practical application of the result of one of his experiments to the celebrated trials formerly made

by

by him, on a larger scale, at the Pantheon ; in order to determine the much litigated question, whether a point or a ball will be struck at a greater distance. To ascertain this matter more precisely, he proposes that a broad and sufficiently thick plate of glass or wax should be interposed between the great insulated conductor, or artificial cloud, and the pointed or blunt conductor which is to receive the stroke : and when the artificial cloud is sufficiently charged, the interpofed plate may be suddenly removed, either by means of a spring, or some other simple mechanical contrivance.

The Author next gives a fimilar set of experiments made with solid glass cylinders, either pointed or rounded at the extremities, and which, like the wooden cylinders above mentioned, perform the office of conductors of the electric fluid. The phenomena are here somewhat different, because the Auid meets with more resistance in glass,' (or rather over its furface, as we are inclined to suppose) than in metal or wood; and its motion accordingly is flower. Those experiments, confidered together with those of the preceding set, very fatisfactorily illustrate the phenomena of the Leyden vial.

These trials are succeeded by others, made with a plain glass, on which is laid a plano-convex glass, with its convex surface in contact with it, after they have both been separately charged ; in order to observe the alterations that might be produced in the appearance of the coloured rings, in consequence of the positive or negative state of the two surfaces. These experiments cannot easily be rendered intelligible, without the affistance of the plate annexed to them; nor indeed to any one who is not previously acquainted with the Author's former publications : particularly with his attempts to explain the phenomena of electricity by the ether of Sir Isaac Newton, or the action of a subtile medium at the surface of bodies, which is supposed to resist the entrance and exit of this ether. We must refer the mathematical Reader likewise to the pamphlet itself for the Author's theorems and demonstrations relative to the density of the elec, tric Auid at the surfaces of bodies, the action of pointed bodies, and the cases in which the electric current is accelerated, or not.

ART. III. Obfervations on Fevers, especially those of the continued

Type; and on the Scarlet Fever attended with Ulceraied Sore Throat, as it appeared at Newcastle upon Tyne in the rear 1778, &c. By John Clark, M. D. &c. 8vo. 5 s. Boards. Cadell. 1780. 'HE Author of this performance formerly published a work of

merit, entitled, Observations on the Diseases in long Voyages to bot Countries; a particular account of which was given in our 49th

Volume,

THE

Volume, September 1773, pag. 173. In the present work, he confines himself wholly to the consideration of fevers, particuJarly those which fell under his own observation at Newcaftle, in the years 1777 and 1778, where he kept a regular and daily account of the particular cases, and of the effects of the medicines which he prescribed.

In a preliminary fe&ion, treating of the difference of fevers, he admits of only one genus; the only species of which, that can be well ascertained are, the intermittent, the remittent, and the continued; considering even the latter, as he had observed in bis former performance, as being liable to alleviations and exaçerbations. He accordingly afirms, that it is the nature of every fever to remit, if not accompanied with local inflammation;' and that all prinary fevers are attended with the fame effential symptoms; the only difference being, that ' in some of them, the intermissions and remiffions are perfect ; in others to obfcure, as justly to entitle them to the name of continued.' The genera into which these last are usually divided, under the titles of inflammatory, nervous, and putrid, he confiders as only proper to express different states of fever, as the fymptoms which characterize them equally attend fevers of the intermittent and remittent type.

The peculiarity which principally distinguishes the Author's treatment of continued fevers, consists in the early and liberal exhibition of the Peruvian bark. The use of this remedy in intermittent and remittent fevers, in all their varieties, is unie versally acknowledged. In bis former publication, the Author had maintained, that this noble febrifuge may be given, with the greatest success, not only in the remifions of fevers, in het climates, but even when they become continued. In consequence of his subsequent experience in this country, he declares, that bere likewise, as soon as the ftrong action of the vessels has been reduced, by means of antimonials, aperients, and diluents, we, ought not to wait till fymptoms of debility ensue, but immediately proceed to exbibit the bark, in as large doses as the stomach of the patient will admit, without regard to the remissions, or exacerbations. If this method be timely employed, he declares, with confidence, that the disease will seldom terminate in the other states of fever; or in other words, become nervous or putrid.

He further observes, that in some cases of continued fever, after the use of antimonials, the affair may be trusted to nature. I have frequently done fo,' he adds, when the direase has shewn no disorder of the nervous system ; and where there has been no suspicion of its having arisen from contagion. But, in doubtful cases, after the fever has continued to the end of the fourth day, it is the safest meshod to commence immea

diately

« AnteriorContinuar »