TO JOHN PERKINS.

Concerning the Small-pox in Philadelphia.

SIR,

Philadelphia, 13 August, 1752.

I received your favor of the 3d instant. Some time last winter I procured from one of our physicians an account of the number of persons inoculated during the five visitations of the small-pox we have had in twentytwo years; which account I sent to Mr. W. V., of your town, and have no copy. If I remember right, the number exceeded eight hundred, and the death's were but four. I suppose Mr. V. will show you the account, if he ever received it. Those four were all that our doctors allow to have died of the small-pox by inoculation, though I think there were two more of the inoculated who died of the distemper; but the eruptions appearing soon after the operation, it is supposed they had taken the infection before in the common way.

I shall be glad to see what Dr. Douglass may write on the subject. I have a French piece printed at Paris, 1724, entitled, Observations sur la Saignée du Pied, et sur la Purgation, au Commencement de la Petite Vérole, et Raisons de doubte contre l'Inoculation. A letter of the Doctor's is mentioned in it. If he or you have it not, and desire to see it, I will send it. Please to favor me with the particulars of your purging method, to prevent the secondary fever.

I am indebted for your preceding letter, but business sometimes obliges one to postpone philosophical amusements. Whatever I have wrote of that kind are really, as they are entitled, but Conjectures and Suppositions; which ought always to give place, when careful observation militates against them. I own I have too strong

a penchant to the building of hypotheses; they indulge my natural indolence. I wish I had more of your patience and accuracy in making observations, on which alone true philosophy can be founded. And, I assure you, nothing can be more obliging to me, than your kind communication of those you make, however they may disagree with my preconceived notions.

I am sorry to hear, that the number of your inhabitants decreases. I some time since wrote a small paper of Thoughts on the Peopling of Countries,* which, if I can find, I will send you, to obtain your sentiments. The favorable opinion you express of my writings may, you see, occasion you more trouble than you expected from,

Sir, yours, &c.

B. FRANKLIN.

TO CADWALLADER COLDEN.

Remarks on Mr. Colden's Theory of Light. lation of Electrical Papers into French.

DEAR SIR,

Trans

Philadelphia, 14 September, 1752.

When I had read your favor of May the 20th, I resolved to read and consider more carefully Sir Isaac Newton's Optics, which I have not looked at these many years. I delayed answering, till I should have an opportunity of doing this, but one thing or other has hitherto hindered. In the winter I may possibly have more leisure..

In the mean time I would just mention, that the interposition of a hill between a bell and the ear does

See this paper in Vol. II. p. 311.

interrupt a great part of the sound, though not all; and we cannot be certain that an opaque body placed between the eye and a luminous object intercepts all the light, since, as you observe, it does not follow that where we see no light there is therefore none existing. What you say of the separation of the distinct parts of light, which, once separated, remain always the same, has more weight with me, and indeed seems conclusive; at least, I see at present nothing to object.

I congratulate you on the prospect you have, of passing the remainder of life in philosophical retirement. I wish for the same, but it seems too distant. I might then more punctually perform my part in the correspondence you honor me with; than which I have none more instructive or agreeable.

Send me, if you please, the translation of your piece into High Dutch. I understand a little of the German language, and will peruse and return it. At present I cannot guess the meaning of the passage you mention. Unless perhaps, as your twentieth section speaks of "a power that neither resists nor moves, and exerts no kind of action of itself, without the concurrence of some other power; so that in the absence of other powers it must be in a perfect inaction," &c., it may be some kind of Dutch wit, and intended to joke that quietism, which in Germany is supposed to be very prevalent in Pennsylvania, many of their Quietists having removed hither.

I see by Cave's Magazine for May, that they have translated my electrical papers into French, and printed them in Paris. I hope our friend Collinson will procure and send me a copy of the translation. Such things should be done by men skilled in the subject, as well as in the language, otherwise great mistakes are easily made, and the clearest matters rendered obscure and unintelligible.

I am sorry you could not see Mr. Kinnersley's Lectures; they would have pleased you. I send you Mr. Wilson's book, which I just received from London, and think it contains the best directions for the use of the machine, that are extant. When you have done with it, please to return it to, Dear Sir,

Your most humble servant,

B. FRANKLIN.

FROM CADWALLADER COLDEN TO B. FRANKLIN.

Cause of the Cohesion of the Particles of Matter. — Concerning a Translation of a German Writer's Remarks on Mr. Colden's Philosophical Principles.

DEAR SIR,

Coldenham, 24 October, 1752.

I now send back to you Wilson's Electricity, for the use of which I am much obliged. My youngest son, the only one I have with me, hopes to be able to make electrical experiments tolerably well. Mr. Wilson, I think, is on the true scent of the cause of electricity, though it be plain he is not sufficiently informed of the nature of that elastic fluid, which he calls ether, to be able sufficiently to account for the phenomena. In my opinion, some more perfect knowledge of the air, than we have is likewise necessary, and the cause of the cohesion of the parts of bodies, which last has been lately the subject of my meditations. No tolerable account of this, so far as I know, has been given by any philosopher; and, if this can be accounted for from my principles, it will go a great way to confirm them. I hope to let you see something on this head.

At present, I shall only mention one theorem, viz. that the parts or particles of bodies are at a great distance from each other in the ratio of their bulk. To prove this, it is supposed that the solid particles of inert matter are impenetrable by light, or that light cannot be in the same space with matter. Then, since light freely passes through pellucid bodies in straight lines, and in all directions, with a very small diminution of its quantity, the distance between the particles of pellucid bodies must be very great in proportion to their bulk, if the prodigious number of those particles be considered, which must be in a pellucid body of any bulk. This may be easily conceived, by placing a number of points in any order, and considering how they must stop the progress of light in many directions. Again, the number of particles in the same quantity of space in several bodies is as their specific gravity. Now, considering the difference of specific gravity of pellucid bodies, and of any other opaque body, and that it does not arise to a great degree, the parts of all bodies must be at a great distance from each other in proportion to their bulk. Sir Isaac Newton accounts for the cohesion of the parts of bodies, from the stronger attraction in little bodies than in great bodies; but, if this were the cause, the parts of bodies must move together into mutual contact, if some other power do not keep them separated. What I call ether is essentially different from Mr. Wilson's ether, and from that elastic fluid, which I think produces electrical phenomena. Sir Isaac Newton was far from having clear conceptions of what I call ether; though he perceived from the phenomena that some such medium must exist between the several bodies in the universe, and within them between their component parts.

I have at last got the Remarks on the first causes