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James Clerk Maxwell
 
 
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Lettere al   Dr. HUGGINS   13 ottobre 1868    -2 maggio 1872
 
To Dr. HUGGINS, F.R.S.

                                                                    Ardhallow, Dunoon, Oct. 13/68.

 MY DEAR SIR—I sympathise with you in your great sorrow. Though my own mother was only eight years with   me, and my father became my companion in all things, I felt her loss for many years, and can in some degree   appreciate your happiness in having so long and so complete fellowship with your mother. I have little fear,   however, that the nearness to the other world which you must feel will in any way unfit you for the work on which  you have been engaged, for the higher powers of the intellect are strengthened by the exercise of the nobler   emotions.

                                                · · · · ·

     Your identification of the spectrum of comet 11 with that of carbon is very wonderful. The dynamical state of    comets’ tails is most perplexing, but the chemistry and activity of their heads leads to new questions. With respect   to the transparency of a heavenly body, I think it indicates scattered condition rather than gaseity. A cloud of large  blocks of stone is much more transparent than air of the sense average density. Such blocks in a nebula would  never be themselves seen, but perhaps if they were often to encounter each other, the results of the collision   would be incandescent gases, and might be the only visible part of the nebula.

     . . . Any opinion as to the form in which the energy of gravitation exists in space is of great importance, and   whoever can make his opinion probable will have made an enormous stride in physical speculation. The apparent    universality of gravitation, and the equality of its effects on matter of all kinds are most remarkable facts, hitherto    without exception; but they are purely experimental facts, liable to be corrected by a single observed exception.  We cannot conceive of matter with negative inertia or mass; but we see no way of accounting for the  propor-tionality of gravitation to mass by any legitimate method of demonstration. If we can see the tails of comets   fly off in the direction opposed to the sun with an accelerated velocity, and if we believe these tails to be matter   and not optical illusions or mere tracks of vibrating disturbance, then we must admit a force in that direction, and  we may establish that it is caused by the sun if it always depends upon his position and distance. I there-fore admit   that the proposition that the sun repels comets’ tails is capable of proof; but whether he does so by his ordinary   attrac-tive power being changed into repulsion by a change of state of the matter of the tail is another question.   Now, it seems ascertained by simple observations with telescopes that the coma is foraged by successive   explosions out of the nucleus, mostly on the side of the sun, and that the formation of the tail depends on the coma,   though the substance is invisible in the state of passing from the coma to the tail. Then, by your observations, the  nucleus and coma have light of their own, probably due to carbon in some gaseous form; but the tail’s light being  polarised in the plane of the sun is due to him. Hence the head is fire and the tail smoke. The head obeys
gravitation, which is exerted on it with precisely the same intensity as on all other known matter, solid or gaseous.
     The tail appears to be acted on in a contrary way. If the comet consisted of a mixture of gravitating and levitating    matter, and is analysed by the sun, then before the emission of the tail the acceleration due to gravitation should be   less than on a planet at the same distance; the more com-plete the discharge of tail the greater the intensity of   gravitation on the remaining head.

N.B.—To understand the dynamics of the tail, the motion in space of particular portions of it must be studied.
 

  
TO DR. HUGGINS, F.R. S.

                                                          11 Scroope Terrace, Cambridge, 2d May 1872.

MY DEAR SIR—Toby and I enclose our photographs with our best regards to you and Kepler. I had   intended to be in London tomorrow, but I am busy here. I hope the air-pump has recovered its cohesion. There  seemed to be a solution of continuity between the mercury and the glass.—Yours very truly,

                                                                                          J. C. M.