If I have understood your principles correctly, the glass that is to be used in the Leyde experiment must fulfill these two conditions: first, it must be impenetrable to electric fluid; second, it must not be impenetrable to the energy of this fluid; or, to express the same idea in different terms, the electric fluid must not be able to pass from one surface to the other, but its flow in one direction on one surface of the glass must be able to instigate a flow in the other direction on the opposite surface.

Glass normally fulfills both of these conditions, but not every kind of glass; there are even types of glass that the electric fluid can pass through almost as easily as it goes through metals. It is a natural property of certain types of glass, and an accidental property of others. It would appear astonishing that no physicist has yet thought of researching the causes of all these differences, if ordinary physics were sufficient for conducting the research, but here physics needs the help of chemistry, which doubtless would not refuse to apply its powers to the clarification of such an interesting point.

I would not propose that chemists analyze different types of glass that can or cannot be permeated by electricity; I would propose rather that they seek to imitate the different types, which would be far easier for them to do.

Pure vitrifiable earth is doubtless the sole ingredient of rock crystal, which can be considered a genuine natural glass; but art has not yet arrived at the point where it is able to procure us such a simple type of glass, and what is more, we have very little hope of ever being able to attain such a level of perfection.

No type of earth is known to us which is so easily vitrified that some additional melting agent is not needed in order to facilitate the process of vitrification. Now, three principal kinds of melting agents can be distinguished, which are saline, metallic and earthy; for there are different types of earth, and although each of them on its own is fire-proof, together they serve as melting agents for one another, just as there are several different types of salt, and several different types of metal that can serve as melting agents for various kinds of vitrifiable earth, and that can be combined in different proportions with these same kinds of earth. One must not be any more surprised to find types of glass which are more or less able to be permeated by electricity, than one would be surprised to find types which can or cannot be permeated by light. Since there are some kinds of glass that are transparent and some that are opaque, or differently colored, why wouldn’t there be kinds of glass which were conductors or non-conductors of electricity?

This is not a problem that a chemist would find difficult to resolve, but it would nonetheless be the object of a rather lengthy enterprise to compile a comparative series of various kinds of glass that possessed one or the other of these qualities in all the varying degrees. The very places where the different kinds of glass would be ranged in the series, from your greenish American glass to the white glass of London, would indicate at first glance the mixture of ingredients of which each kind of glass was composed.

On the other hand, because part of these ingredients may evaporate, due to the intensity of the heat to which the glass substance is exposed during its first or second baking, and because the heat does not have the same intensity in every part of the furnace, it is hardly surprising that you have discovered considerable differences among glass globes of the same manufacture, as you point out on page 181.

Apart from the natural qualities of various kinds of glass (qualities that result from the specific composition of each kind), there can also result very great differences in the diversity of the thickness of mass of each piece of glass, if only for the reason that the heat could not have been exactly the same within the different layers of a very thick piece of glass, nor could the speed of the cooling process have been anywhere near the same. I am not even taking into consideration the fact that it seems nearly impossible for the action of moving electric fluid to be transmitted effectively from one surface of a body to the other if the body is too massive.

In conclusion, it is also easy to understand that a considerable degree of heat, by rarefying the substance of a thick piece of glass, could make the glass able to be penetrated by the electric fluid; and that this degree of heat must nevertheless be relative to the thickness of the glass; and that Monsieur Kinnersley therefore needed a heat of only 210 degrees (which is the degree at which water boils on the Fahrenheit thermometer), in order to render the extremely thin glass of a Florentine decanter permeable by electric shock, while Monsieur Cavendish needed a heat of 400 degrees in order to render a slightly thicker glass able to be permeated by the current.

What makes me wish that some chemist would be so good as to enlighten us on all these points is the fact that it would be impossible for anyone to be overly concerned about sparing lovers of physics unnecessary expenditure. For that is something which can make some of them give up physics altogether, and can somewhat cool the zeal of many others. I am, etc.