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thing in other words, the electric fluid must not be able to pass from one surface to the other, but its afflux on one of the surfaces of the glass must have the power to excite an efflux on the opposite surface.

Glass generally unites these two qualities, but not every kind of glass. There is even glass that the electric fluid passes through, almost as readily as it enters metals. This is a property natural, to some kinds of glass, and accidental to others. It would seem astonishing, that no philosopher had yet thought of seeking out the causes of all these differences, if natural philosophy alone were equal to the task; but there is need of the aid of chemistry, which certainly may throw some light on so interesting a subject.

I would not propose to the chemists to analyze the different kinds of glass, permeable or impermeable to electricity; but to endeavour to imitate them, which would be much easier for them to do.

Pure vitrifiable earth is without doubt the only ingredient in rock crystal, which may be considered as a true natural glass; but art has not yet succeeded in obtaining for us a glass so pure, and there is even very little reason to hope that such perfection can ever be attained.

There is no earth known so vitrifiable as not to require some auxiliary solvent to facilitate its vitrification. Now solvents are distinguished into three principal kinds, which are, saline solvents, metallic solvents, and earthy solvents; for there are different kinds of earths, which, although each singly is refractory, yet serve as mutual solvents, as there are also many kinds of salts, and many kinds of metals, which may be used as solvents for the vitrifiable earths, and which may be combined in different proportions with the same earths. We ought not to be more surprised to find glass more or less permeable to electricity, than to find it pervious and impervious to light. Since there is transparent glass and opaque glass, or glass of various colors, why should there not be glass which is a conductor, and that which is a non-conductor, of electricity?

It would not be a problem of difficult solution for a chemist, but yet it would be a labor requiring considerable time, to furnish us with a comparative table of the different kinds of glass possessing either of these qualities in all their various degrees. The places merely, occupied by your greenish American glass, as well as by the white London glass, would indicate, at the first glance, the mixture of ingredients of which they are respectively composed.

. On the other hand, as the intensity of heat to which the substance of the glass is exposed, whether in melting or annealing, may cause the evaporation of some of these ingredients, and as this heat is not equally powerful in every part of the furnace, it is not very surprising, that you should have found considerable difference between several glass globes from the same manufactory, as you inform us.

Independently of the natural properties of one kind of glass or another, arising from their particular composition, great differences may also result from the different thickness of their masses, were it from this consideration alone, that the heat could not be precisely the same, nor the rapidity of cooling very nearly equal, in the different layers of very thick glass; without taking into the account, that it seems almost impossible, that the action of the electric fluid in motion should be effectually conveyed from one surface to another of a very massive body.

Lastly; it is equally easy to conceive, that a considerable degree of heat, by rarefying the substance of thin glass, should open its pores to the electric fluid; but that the degree of heat must be in proportion to the thickness of the glass; and that Mr. Kinnersley found a heat of only two hundred and ten degrees (the point at which water boils, according to Fahrenheit's thermometer,) necessary to render the very thin glass of a Florence flask permeable to the electric shock, while Mr. Cavendish required a heat of four hundred degrees to make glass a little thicker permeable to the common stream.

My reason for wishing that some chemist would have the goodness to enlighten us upon all these points is, that too much pains cannot be taken to spare the lovers of natural philosophy any unnecessary expense; because this may turn some entirely aside from its pursuit, and somewhat damp the zeal of many others. I am, &.c.


Concerning the Mode of rendering Meat tender by Electricity.

My Dear Friends,

My answer to your questions, concerning the mode of rendering meat tender by electricity, can only be founded upon conjecture; for I have not experiments enough to warrant the facts. All that I can say at present is, that I think electricity might be employed for this purpose, and I shall state what follows as the observations or reasons which make me presume so.

* This letter has no date, but the one to which it is an answer is dated May 1st, 177a

It has been observed, that lightning, by rarefying and reducing into vapor the moisture contained in solid wood, in an oak, for instance, has forcibly separated its fibres, and broken it into small splinters; that, by penetrating intimately the hardest metals, as iron, it has separated the parts in an instant, so as to convert a perfect solid into a state of fluidity; it is not then improbable, that the same subtile matter, passing through the bodies of animals with rapidity, should possess sufficient force to produce an effect nearly similar.

The flesh of animals, fresh killed in the usual manner, is firm, hard, and not in a very eatable state, because the particles adhere too forcibly to each other. At a certain period, the cohesion is weakened, and, in its progress towards putrefaction, which tends to produce a total separation, the flesh becomes what we call tender, or is in that state most proper to be used as our food.

It has frequently been remarked, that animals killed by lightning putrefy immediately. This cannot be invariably the case, since a quantity of lightning, sufficient to kill, may not be sufficient to tear and divide the fibres and particles of flesh, and reduce them to that tender state, which is the prelude to putrefaction. Hence it is, that some animals killed in this manner will keep longer than others. But the putrefaction sometimes proceeds with surprising celerity. A respectable person assured me, that he once knew a remarkable instance of this. A whole flock of sheep in Scotland, being closely assembled under a tree, were killed by a flash of lightning; and, it being rather late in the evening, the proprietor, desirous of saving something, sent persons early the next morning to flay them; but the putrefaction was such, and the stench so abominable, that they had not the courage to execute their

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orders, and the bodies were accordingly buried in their skins. It is not unreasonable to presume, that, between the period of their death and that of their putrefaction, a time intervened in which the flesh might be only tender, and only sufficiently so to be served at table. Add to this, that persons, who have eaten of fowls killed by our feeble imitation of lightning (electricity), and dressed immediately, have asserted, that the flesh was remarkably tender.

The little utility of this practice has perhaps prevented its being much adopted. For, though it sometimes happens, that a company unexpectedly arriving at a country-house, or an unusual conflux of travellers to an inn, may render it necessary to kill a number of animals for immediate use; yet, as travellers have commonly a good appetite, little attention has been paid to the trifling inconvenience of having their meat a little tough. As this kind of death is nevertheless more sudden, and consequently less severe, than any other, if this should operate as a motive with compassionate persons to employ it for animals sacrificed for their Use, they may conduct the process thus.

Having prepared a battery of six large glass jars (each from twenty to twenty-four pints) as for the Leyden experiment, and having established a communication, as usual, from the interior surface of each with the prime conductor, and having given them a full charge (which, with a good machine, may be executed in a few minutes, and may be estimated by an electrometer), a chain which communicates with the exterior of the jars must be wrapped round the thighs of the fowl; after which the operator, holding it by the wings, turned back and made to touch behind, must raise it so high that the head may receive the first shock from the prime conductor. The animal dies instantly. Let the

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