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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.,
LONDON, 30 March, 1773. DEAR SIR:—You punish my delay of writing to you very properly by not writing to me. It is long since I have had the pleasure of hearing from you. But it is my fault, and I must for my own sake write to you oftener, though I have little to say, or you will quite forget me.
I thank you for your advice to send an English copy of my writings to the Academy, and shall do it as soon as the new edition now in hand here is finished.
I am glad you see some weight in the experiments I sent you concerning pointed rods. Mr. Wilson is grown angry that his advice was not followed in making them blunt for the public magazines of gunpowder, and has published a pamphlet reflecting on the Royal Society, the committee, and myself, with some asperity, and endeavoring to alarm the city with the supposed danger of pointed rods drawing the lightning into them and blowing them up. I find it is expected from me that I make some answer to it, and I shall do so, though I have an extreme aversion to public altercation on philosophic points, and have never yet disputed with any one who thought fit to attack my opinions. I am obliged to you for the experiment of the point and ring.
There is no being sure of any thing before it happens; but, considering the weight of your reputation, I think there is little reason to doubt the success of your friends' endeavors to procure from
our Society here the honor of adding you to their number at the next election. In the meantime will you for my sake confer the same kind of honor on our young Society at Philadelphia. When I found that our first volume of American Transactions was favorably received in Europe, and had procured us some reputation, I took the liberty of nominating you for a member, and you were accordingly chosen at a full meeting in Philadelphia on the 15th of January last. I sent a copy of that volume to the Academy of Sciences at Paris when it first came out, but I do not remember to have heard that they ever received it. I think it was Mr. Magalhaens' who undertook to convey it. If it miscarried I will send another, and by the first opportunity one for yourself.
Two ships are now fitting out here by the Admiralty, at the request of the Royal Society, to make a voyage to the north pole or to go as near to it as the ice will permit. If they return safe we shall probably obtain some new geographical knowledge and some addition to natural history. With the greatest esteem and respect, I am, etc.,
* John Hyacinth de Magalhaens, a Portuguese by birth, who resided a large part of his life in England. His name frequently occurs in Franklin's letters. He is said to have been "an able linguist, and well versed in chemistry and natural philosophy," and to have published respectable treatises on mineralogy and some other branches of science. He was a member of the Royal Society. This is the same person (whose name is sometimes printed Magellan) that gave to the American Philosophical Society a donation of two hundred guineas, which was to be invested in a secure fund, and the interest disposed of annually in premiums to the author of the best discovery, or most useful invention, relating to navigation, astronomy, or natural philosophy.SPARKS
TO THOMAS CUSHING
LONDON, 3 April, 1773. SIR:— My last was of the 9th past, since which nothing material has occurred relating to the colonies. The Assembly's answer to Governor Hutchinson's speech is not yet come over, but I find that even his friends here are apprehensive of some ill consequences from his forcing the Assembly into that dispute; and begin to say it was not prudently done, though they believe it meant well. I enclose for you two newspapers, in which it is mentioned. Lord Dartmouth the other day expressed his wish to me, that some means could be fallen upon to heal the breach. I took the freedom to tell him that he could do much in it, if he would exert himself. I think I see signs of relenting in some others. The Bishop of St. Asaph's sermon before the Society for Propagating the Gospel is much talked of, for its catholic spirit and favorable sentiments relating to the colonies. I will endeavor to get a copy to send you. With great esteem and respect, I have the honor to be, sir, your most obedient and most humble servant,
TO WILLIAM FRANKLIN
LONDON, 6 April, 1773. DEAR SON :- I received yours of February 2d, with the papers of information that accompany it.