To Rudolf Clausius   1st June 1859

1^st June 1859

My dear Clausius,

Many thanks to you for the paper on the Potential function,¹ which though it is beyond me is still precious as a proof of your friendly feeling. The second copy has been duly presented to the Royal Society.²

I have been at work for some time upon a new subject which promises to give good results. Melloni, you know, proved the law of inverse squares to be true of radiant heat passing through air, and he inferred from this that “autre les limites d'observation, C'est à dire, pour une intervalle de cinq ou six metres, l'air n'exerce aucune absorption sensible sur le rayonnement des corps chauds.” This is the only experimental conclusion on this subject which we possess.³

But you know that M. Pouillet,⁴ and Hopkins⁵ have considered the influences of a diathermic envelope⁶ like our atmosphere to be such that while the rays of the sun can reach us freely, a great portion of them are stopped when they have been changed by the absorption of the earth into obscure heat. Pouillet you know also explains the greater warmth of the lower atmospheric strata by supposing that the radiant-heat emanating from the earth is there principally absorbed.⁷

Reflecting on all this I felt that our knowledge as to the action of gaseous bodies upon radiant heat was absolutely nil and I thought it very desirable to investigate the transmission of radiant heat through gases in general.

I had often verified the experiments of Melloni regarding the law of inverse squares;⁸ but I thought that our atmosphere’s hydrogen might act with greater energy than air, because all the components of this element are too opaque to radiant heat. I therefore had a tube constructed 4 feet long 3 inches wide, and by suitable means contrived to stop its end securely by plates of rocksalt. A stopcock was attached to the tube which could be screwed on to an airpump intended to exhaust the tube. There was also a passage devised for the admission of any gas, and also a second arrangement by which the vapour of any liquid might be admitted into the exhausted tube.

I first placed a source of heat at one side of this tube and a thermoelectric pile at the other. When the needle of the associated galvanometer had come to rest I exhausted the tube, and watched whether the removal of the air had any sensible influence on the radiation—it had not—the needle remained perfectly steady, and was equally so when the air was readmitted. The experiment was repeated with hydrogen and with the same result.

It was evident therefore that some other mode of experiments must be devised. I saw that the experiment just recorded has this disadvantage. When great heat was made use of the needle of the galvanometer [pushed] to the high degrees, and you know it requires a far greater amount of force to move the needle in this position than when it is near to zero. Then, on the contrary, a feeble source of heat was used, and the needle came to rest in a sensitive position, the fractional part of this, a small quantity absorbed by the gas in the tube might well escape observation.

My object then was to operate with quantities of heat so large that the percentage climbed would be a sensitive quantity, and to apply this quantity to the needle in its sensitive position. You will understand my way of proceeding in a moment. I took two thermo-electric piles;⁹ had a galvanometer¹⁰ constructed with two wires so that the currents from the two piles could be sent round the instrument in opposite directions. The heat falling in one pile had to pass through the tube, and hence the medium traversed by its [path] could be varied; while the medium traversed by the heat falling upon the other pile remained unchanged.

Imagine these two powerful currents looping round the galvanometer in opposite directions. They neutralize each other, and the needle stands at zero. I exhaust the tube, and find immediately that the current from the pile at this end of the tube now predominates, or in other words by the removal of the air a greater quantity of heat has been permitted to pass. When the tube is quite exhausted & the needle quite at rest in its new position let air be admitted. The needle then shows a diminution of the current which, when the tube was empty, was excited in the pile.

Effects which would surprise you have been obtained in this way; for example with air I can easily obtain a deflection from 40° on one side of zero to 40° on the other; the results moreover are perfectly constant, and the amount of absorption varies with the density of the air in the tube.

I have operated in the same manner with several other gases and vapours, and have obtained very remarkable results. It is quite probable that gases vary as much as solids & liquids in this department to radiant heat. Some of these bear the same relation to others that alum does to the better diathermic solids. As similar diversity reigns among the vapours. I believe that I shall be able to measure the absorption of plates of some gases not more than a millimetre in thickness.

The experiments have also proved that the selfsame gas operates differently at different qualities of heat. Yesterday I sent through the tube a powerful wave of heat which was not sensibly affected by the most energetic gas that I have as yet discovered. The same gas operating upon heat of a different quality causes the needle to move 80° on one side of zero to 80° on the other.

I am now in the middle of the investigation, and hope that when it is complete it will be found a good piece of work.

And now I have several favours to ask of you:—the first is that you would present my kind remembrances to M. Mousson, and ask him to be good enough to send me a copy of the little paper on the present state of the glacier question¹¹ which he gave to me in Zürich & which I have mislaid. It refers among other things to the application of Faraday’s experiment on regeneration to the receding of the crevasses;¹² leaving the question of structure untouched, because it was thought that it still remained to be solved.

I also want you to ascertain for me how I may obtain Rendu’s Essay upon glaciers.¹³ I would give a good deal for the perusal of this Essay if you should have a copy of it, and would lend it to me I should be careful to return it to you safely. It may be possible to obtain a copy of it among the booksellers of Zürich.

Thirdly, I want you to give me your opinion on that theory of the coldness of the upper strata of the atmosphere which refers it to the fact that the attenuated air has a much higher specific heat than that which is more dense. I should take it very kind of you if you would write to me soon on the three points each mentioned.

Believe me ever | most truly yours | John Tyndall

RI MS JT/1/T/176