The absorptive power of the odour of musk was 72 or 74 times that of the air that conveyed it into the experimental tube; the quantity that produced it was quite inappreciable, yet the perfume was so persistent that the pieces of the apparatus through which it had passed had to be boiled in a solution of soda before they were fit for other experiments.
The absorption of many gases and vapours having been determined, their radiation was measured by a very simple arrangement. The thermo-electric pile was raised on a stand with a screen of polished tin in front of it. A heated copper ball in a perforated ring on a low stand was placed behind the screen; all direct radiation from the ball was thus cut off, but the heated air rising in a column above the screen radiated its heat on the pile and deflected the needle of the goniometer 60° when the ball was red-hot; but the radiation of the hot air was neutralized by another source of radiant heat on the opposite side of the pile which kept the needle steadily at zero. Then a purified gas or vapour conveyed by a pipe into the perforated ring which held the ball rose mixed with the heated air above the screen, but the radiation of the gas or vapour alone was shown by the deflection of the needle, because that of the air was compensated. With this apparatus Professor Tyndall proved that the amount of the absorption of each gas and vapour is exactly equal to the amount of its radiation. He has shown that this result is a necessary consequence of the dynamical nature of heat. For as no atom or molecule is capable of existing in vibrating ether without accepting a portion of the motion, the very same quality whatever it may be that enables it to do so, must enable it to impart its motion to still ether when plunged into it. ‘Hence from the existence of absorption we may on theoretic grounds infallibly infer a capacity for radiation; from the existence of radiation we may with equal certainty infer a capacity for absorption, and each of them must be regarded as the measure of the other.’ This reasoning, founded simply on the mechanical relations of the ether and the atoms immersed in it, is completely verified by experiment.
Hitherto the absorption and radiation of heat by the same thickness of different gases and vapours have been compared with each other, but in a recent series of experiments Mr. Tyndall has compared the action of different thicknesses of the same gas or vapour on radiant heat. The experiments extend from a thickness of 0·01 of an inch to that of 49·4 inches. The instrument employed for ascertaining the action of the smaller thickness was a horizontal hollow cylinder closed at one end by a plate of rock-salt. A second cylinder was fitted into this with its end also closed by a plate of rock-salt. This cylinder moved within the other like a piston, so that the two plates of rock-salt could be brought into flat contact with one another, or could be separated to any required distance, and the distance between the plates was measured by a vernier. At one end of the cylinder there was a source of constant heat, and the differential goniometer already described at the other. With this apparatus Mr. Tyndall found that olefiant gas maintains its great superiority over the other gases in absorptive power at all thicknesses. A layer of that gas not more than 0·01 of an inch thick intercepted about one per cent. of the total radiation. This great absorption corresponded to a deflection of 11° of the needle of the goniometer, and such was the delicacy of the apparatus that it would be possible to measure the action of a layer of this gas of less thickness than a sheet of writing paper. A layer of olefiant gas two inches thick intercepts nearly 30 per cent. of the entire radiation. A shell of olefiant gas two inches thick surrounding our globe would offer no appreciable hindrance to the solar rays in coming to the earth, but it would intercept, and in great part return, 30 per cent. of the terrestrial radiation; under such a canopy the surface of the earth would probably be raised to a stifling temperature.
The apparatus for measuring the action of the greater thicknesses of gas was a hollow brass cylinder 49·4 inches long, closed at both ends by plates of rock-salt, and divided internally into two compartments or chambers by a third plate of rock-salt movable in the interior; the source of heat being at one end and the differential goniometer at the other.
Carbonic oxide and carbonic acid are pervious to a vast majority of the rays of radiant heat. When the cylinder was filled with carbonic oxide gas and so divided, by moving the internal plate of rock-salt, that a stratum of the gas 8 inches long was next to the source of heat, and that 41·4 inches long farthest from it, the 8 inches of gas intercepted 6 per cent. of the whole radiation. But when the plate of rock-salt was moved till the column 41·4 inches long was next to the source of heat, and that of 8 inches farthest from that source, or behind the long one, the absorption of the 8 inches was sensibly zero. In like manner eight inches of carbonic acid gas when in front of a column of 41·4 inches of the same gas absorbed 61⁄4 per cent. of the whole radiation, while placed behind that column the effect was nearly zero. The reason is that when the 8 inch stratum is in front, it stops the main portion of the rays which give it its thermal colours,[[7]] while placed behind these same rays have been almost wholly withdrawn, and to the remaining 94 per cent. of the radiation the gases are sensibly permeable.
It is inferred from an extension of this reasoning that the sum of the absorptions of the two chambers taken separately must always be greater than the absorption effected by a single column of the gas of a length equal to the sum of the two chambers; this conclusion is illustrated in a striking manner by the experiments. It is also found that when the mean of the sums of the absorptions is divided by the absorption of the sum, the quotient is sensibly the same for all gases. It may farther be inferred that the sum of the absorptions must diminish and approximate to the absorption of the sum as the two chambers become more unequal in length, and that the sum of the absorptions of the two chambers is a maximum when the medial plate of rock-salt divides the long tube into two equal parts.
When air enters an exhausted tube it is heated dynamically by the collision of its particles on the sides of the tube as it rushes in to fill the vacuum; and when the tube is exhausted again by the air pump, chilling is produced by the application of a portion of the heat of the air to generate vis viva. This dynamic principle occurred in some of the experiments, and was dexterously adopted and applied to the solution of a striking and unprecedented problem: ‘To determine the radiation and absorption of gases and vapours without any source of heat external to the gaseous body itself.’
The two external sources of heat being therefore dispensed with in the absorptive apparatus, the thermo-electric pile was presented to the cold glass tube which was exhausted, and the needle of the goniometer stood at zero. Nitrous acid on entering the exhausted tube became heated and radiated its heat upon the adjacent face of the pile which deflected the needle of the goniometer through 28° in the direction that indicates absorption. As the heat of the gas became gradually exhausted, the needle returned slowly to zero. The pump was now worked, the rarefied gas in the tube was chilled, and the adjacent face of the pile gradually poured its heat on the chilled tube till the temperature of the pile was so much lowered, that the needle was deflected 20° on the negative side of zero, that is on the side denoting radiation.
When olefiant gas entered the exhausted tube, the needle showed an absorption of 67°, and when the gas was pumped out again, the needle showed a radiation amounting to 41°. When the gas was then pumped out, very dry atmospheric air was introduced into the tube,—the needle pointed to 59° indicating absorption; and when it was pumped out again the needle swung to nearly 40° on the other side of zero, indicating radiation. Remembering that the radiation and absorption of dry air only produce a deflection of 1°, it is evident that the preceding great deflection of the needle is entirely owing to the action of the small residue of olefiant gas that remained in the exhausted tube. In order to ascertain how much the quantity of a gas or vapour might be reduced before its action became insensible, the vapour of boracic ether, which has the greatest absorptive energy, was chosen.
The mercurial gauge for measuring the pressure or tension of the vapour already mentioned remained attached to the apparatus. When one-tenth of an inch of the vapour of boracic ether was admitted into the exhausted tube, the barometer stood at 30 inches: hence the tension of the vapour within the tube was the 1⁄300th part of an atmosphere. Dynamically heated by dry air the radiation of the vapour produced a deflection of 56°. Again the tube was exhausted to 0·2 of an inch and the quantity of vapour was thereby reduced to 1⁄150th of its first amount; the needle was allowed to come to zero, and the residue of the vapour produced a deflection of 42°. The pump was again worked till a vacuum of 0·2 of an inch was obtained, this residue containing of course the 1⁄150th of the quantity of ether present in the tube; and on dynamically heating the residue, its radiation produced a deflection of 20°.