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CCCCXCIV: FROM M. DUBOURG - Benjamin Franklin, The Works of Benjamin Franklin, Vol. VI Letters and Misc. Writings 1772-1775 
The Works of Benjamin Franklin, including the Private as well as the Official and Scientific Correspondence, together with the Unmutilated and Correct Version of the Autobiography, compiled and edited by John Bigelow (New York: G.P. Putnam’s Sons, 1904). The Federal Edition in 12 volumes. Vol. VI (Letters and Misc. Writings 1772-1775).
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FROM M. DUBOURG
Paris, 25 March, 1773.
If I have rightly understood your principles, the glass to be used in the Leyden experiment ought to combine these two qualities: first, it should be impermeable to the electric fluid; secondly, it should not be impermeable to the action of this fluid; or, to express the same 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 endeavor 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 earthly 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 diffierent 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 thicknesses 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, etc.,