From Edward Frankland

SOURCE: Tyndall, J., “On the Passage of Radiant Heat through Dry and Humid Air,” Phil. Mag., 26:172 (July 1863), pp. 44-54, on pp. 46-9.

Note to Michael L: Just detach the edited letter at the end but see MB’s note and the scan for where to put the big brackets in the results table (which we’re unable to work out how to do!)

From Edward Frankland 19/20 June 1863 2017

“MY DEAR TYNDALL,—At your request I have made a number of experiments on the comparative transcalency of common air, and of air deprived of its moisture by contact with monohydrated sulphuric acid. The apparatus which I used was that described by you in the Philosophical Transactions for December 1862*.¹ It was exclusively under my own control; and I arranged the details of manipulation in such a manner as appeared to me best calculated to eliminate all sources of error. My mode of operating was as follows:—The brass tube (AB, fig. 2 [see antè, p. 34]), open at both ends, formed the conduit for a portion of the thermal radiation from the source (S). These heat-rays, after passing through the tube, traversed several inches of intervening air-space before entering the cone of the thermo-electric pile, where they produced their effect, in opposition to that arising from another constant source of heat affecting the opposite face of the pile (the compensating cube). The differential action was indicated as usual by a delicate galvanometer. These arrangements being once for all made, I was able by means of an air-pump to introduce at pleasure into the tube either the ordinary air of the laboratory, dry air, or air rendered moist by passage over extensive surfaces of wet glass. At the commencement of the experiments the tube was of course full of the common air of the laboratory; the needle of the galvanometer marked 42°, and remained steady for a quarter of an hour within a degree of that point. I now interposed in the path of the rays entering the brass tube a sheet of tin-plate; the needle at once bounded from 42° up to 90°. It was thus evident that any obstruction to the passage of the rays of heat through the tube, or, in other words, any cooling of that face of the pile which was turned towards the tube, would be indicated by an increased deflection of the needle on the same side of zero, which I will call the - side, whilst a heating of the same face of the pile would be attended by a diminished deflection, or even by a passage of the needle to the opposite or + side of zero. The following are the results which I observed:—

Tube filled with Permanent deflection.

Common air ………… -42°

Air dried by contact with monohydrated sulphuric

acid and introduced gently into the open tube. +13

Common air which had spontaneously displaced

the foregoing dry air ……. -43

Common air gently drawn in by air-pump .. -43.3

Common air gently blown in from caoutchouc² bag. -45

Same air gently blown in from caoutchouc bag,

but dried by passage over sulphuric acid . +6

Air from same bag but not dried ….. -46.5

Air of laboratory ………. -42

Air dried and introduced as before ….. +14

Air dried as before with sulphuric acid, but after-

wards passed over fragments of glass moistened

with water ……….. -46

Common air gently drawn in by air-pump .. -42.5

“At the conclusion of the experiments I found that the deflection due to the total radiation was 86°.2.

“I also saw the following experiments made by yourself when the ends of the brass tube were closed by plates of rock-salt:—

Permanent deflection of needle (_°)

Tube filled with dry air …. +7

After exhaustion of tube …. -3

After admission of laboratory air . -42

“Rain was falling at the time these last determinations were made, and the air was very moist. On removing the plates of rock-salt from the tube they appeared to be quite dry; and after being breathed upon, the film of moisture soon disappeared and they recovered their previous lustre. I ought perhaps to mention that these experiments are not selected, they are the only ones I have made upon the subject, and they were performed in the sequence given above: after a very careful scrutiny I have been unable to detect any source of fallacy in them, and they therefore appear to me to prove conclusively that obscure radiant heat passes much more readily through dry than through moist air.

“In conclusion, I cannot but express my surprise and admiration at the precision and sharpness of the indications of your apparatus. Without having actually worked with it I should not have thought it possible to obtain these qualities in so high a degree in determinations of such extreme delicacy, and which are so well known to be exposed to numerous sources of derangement.

“Believe me, | “Yours very truly, | “E. Frankland.” | “Royal Institution, | June 19, 1863.”

“P.S. Since writing the foregoing letter, I have repeated the experiments there recorded without any source of heat at either end of the pile, in order to ascertain whether the introduction and withdrawal of dry air at all affected the galvanometer. The tube was first full of the common air of the laboratory, and the needle remained steadily at +12.5 for a quarter of an hour. A current of moistened air was now drawn through the tube for ten minutes in precisely the same manner as when the two sources of heat were employed, the needle being closely watched during the whole time. It oscillated between +12 and +13, but never passed these limits on either side. The current was not interrupted and the needle closely watched for five minutes: it remained perfectly steady at 12.5. A current of dried air was now conveyed through the tube for five minutes, the needle oscillated as before between 12° and 13°.

“Thus far I operated on the air exactly as in the experiments recorded in the foregoing letter. I then quadrupled the velocity of the current through the tube, introducing in the first place dry air: the needle in a first experiment moved 6 degrees in the direction of cold; but on repeating the experiment with both dry and moist air no effect whatever was produced. I now removed the tube and delivered a gentle current of dry air into the cone of the pile, immediately the needle moved 90° in the direction of cold; the current was continued uninterruptedly for ten minutes, during which time the needle gradually returned to nearly its original position. The current of dry air being now stopped, the needle moved 40° in the direction of heat, returning again gradually and slowly to its normal position. The same temporary deflection for heat was also produced in an exalted degree when the dry current was immediately succeeded by a moist one.

“These supplementary experiments lead me to the following conclusions:—

“1st. The gentle currents of air which were caused to flow through the tube in the experiments detailed in my letter did not in any way disturb the results of those experiments, neither would they have done so in any material degree even had their velocity been quadrupled.

“2nd. The impact of air dried than that previously in contact with the pile cools that face of the instrument with which it comes in contact, whilst the like impact of moister air produces the opposite effect.

“3rd. It is, however, impossible to confound the effects obtained in the above experiments on transcalency with those produced by the impact of dray and moist air upon the face of the pile, because in the first place the former are permanent, whilst the latter are essentially transitory; and in the second place the deflections due to the impact of dry or moist air against the face of the pile are always in the opposite direction to those obtained by the interposition of the same kind of air in the path of radiant heat. Thus, if the heat-rays falling upon one face of the pile be made to traverse dry air, the needle will move in the direction of heat, but if the apparatus be so arranged as to cause the dry air to impinge upon the face of the pile, the effect due to the greater transcalency of the dry air would be at first more or less neutralized, or even altogether overborne, by the cooling influence due to evaporation at the surface of the pile so brought into contact with dry air.—E. F.”

“Royal Institution, | June 20, 1863.”

J. Tyndall, ‘On the Passage of Radiant Heat through Dry and Humid Air’, Phil. Mag., 26 (1863), pp. 44–54, on pp. 46–9