The principal measurements of this specimen are the following: The specimens above described are from the lower Eocene of New Mexico. Yale College, New Haven, Oct. 23, 1876. THE AMERICAN JOURNAL OF SCIENCE AND ARTS. [THIRD SERIES.] ART. XLIII.--Experiments on the nature of the force involved in Cookes' Radiometer; by Prof. O. N. ROOD, of Columbia College. IT is impossible for a physicist to regard the little instrument devised by Mr. Crookes with other than a feeling of unusual interest, based partly on the performance of the apparatus itself, and partly on possible applications which immediately suggest themselves. The explanation of these curious phenomena offered by Ronalds, and afterward more in detail by Stoney,* together with the confirmatory experiment of Schuster, led me to devise two new methods for still farther testing the theory thus advanced, and to make at the same time an examination of the phenomena when the suspended discs were under the ordinary atmospheric pressure. The explanation offered by Stoney is based on the mechanical theory of gases, and reaches the final result that a reaction takes place between the blackened sides of the movable vanes and the glass envelope, so that there is a tendency for them to recede from each other. I first arranged an experiment so that I could at will destroy the possibility of this reaction taking place, without interfering with the other necessary conditions. Description of the apparatus.-Two discs of thin aluminium foil, A and B, fig. 1, the same in size, were prepared, and * On Crookes' Radiometer; G. Johnstone Stoney, Phil. Mag., March and April, 1876. AM. JOUR. SCI., THIRD SERIES-VOL. XII, No. 72.-DEC., 1876. blackened, each on one side, with lampblack to which a minute portion of spirit varnish had been added. Each disc was folded A 1. 2. B B so as to be double, the two leaves not being in contact. Disc B carried in front of it a plate of mica equal in size with itself, and distant from it about 5 mm. The system was arranged so as to be capable of suspension by a single fiber of silk, and was provided with a small directing magnet. Fig. 2 gives a view of the arrangement from above. The discs thus arranged were enclosed in a clear glass flask, which was exhausted to a pressure of 25 of a millimeter and sealed up. The flask with its contents was then placed on a graduated circle and centered. The deviations of the disc were observed by a compound microscope of low power, which was capable of independent rotation about the axis of the circle. It was provided with an index with which tenths of degrees could readily be estimated, the circle itself being divided into half degrees. The small magnet connected with the discs was rendered nearly astatic by an external magnet: it consumed thirtytwo seconds in making a single oscillation. Experiments.-The light of a luminous gas flame at a distance of twelve inches was allowed to fall on the blackened disc not provided with a plate of mica, its companion being protected by a triple screen of sheet brass from the action of the flame. Under these circumstances the exposed disc moved away from the light after it had come to rest ten readings were made; below is the result obtained by two such experiments: 3°.06 3°-23 mean deviation away from light. Next, the vane provided with the mica shield was exposed, the other being screened. After a slight agitation it came to rest, and ten readings were made as before; the results of two experiments are given below: 0°.26 away from light. 0°.10 mean. It will be seen that the interposition of the mica plate attached to the vane actually did prevent any reaction from taking place between the disc and the walls of the flask, so that prac tically the disc remained immovable. In the next experiment the brass screen was entirely removed, and the light allowed to fall on both discs simultaneously, a plate of mica, identical in substance with that attached to the vane, being placed outside of the flask and opposite the unprotected vane, so that both vanes received the same amount of radiation, the only difference being that one of them was deprived of its direct communication with the walls of the flask. Under these circumstances the disc without the attached mica screen moved instantly away from the light, as was shown by two careful experiments: 20.34 away from light. 2o.38 mean. These results I regard as confirmatory of the theory advanced by Stoney, and as fatal to the idea that the motion is directly dependent on the impact of light or heat, for both discs received the same amount of heat and light. According to the theory just referred to in these experiments repulsion took place between the blackened disc and the mica attached to it, but being firmly fastened together, no motion of either could result. Assuming this repulsion to exist I devised an apparatus for making it evident and for measuring its amount. Description of Apparatus.-A disc of aluminium, folded double and blackened on one side, was provided with a small magnet, and suspended by a single fiber of silk in the interior of a glass flask. In front of this was similarly suspended a disc of glass such as is used for covering microscopic objects; it also was provided with a small magnet and both discs were properly counterpoised. These discs were each seventeen millimeters long, and fourteen millimeters high; the weight of each was 0.3 gram, and the minute magnets attached to them were of nearly equal strength. The distance of the points of suspension apart. was five millimeters. Figure 3 furnishes a view of the system seen from above. A is the aluminium disc, G that of glass, P and P' are the points of suspension, C and C' the counterpoises, which being on different levels readily passed each other; and NS and N' S' are the controll A 3. ing magnets. The exhaustion of the flask was carried to 0.24 of a millimeter. Experiments.--It will be observed that the magnets were so arranged as to tend to cause the discs to touch each other, but in ordinary daylight, or even in feeble daylight, the repulsion was so strong as to cause the discs to assume a parallel position, or even to diverge several degrees. They could not be brought into contact even by covering the flask with white writing paper, but paper thickly painted with lamp black always caused contact in a few minutes. When this apparatus was placed in a darkened room, and suddenly exposed to a luminous gas flame sixteen inches distant, the discs instantly diverged, right and left, with sensibly equal velocities, and after some slight oscillation came to rest. Below are given the final deviations in two trials; five readings being taken in each experiment. Taking the mean of the two experiments, and calling the deviation of the aluminium 100, that of the glass is 65-7. The results just given are sufficiently concordant, and could have been obtained repeatedly without greater variation; still, owing to slight defects of workmanship in the apparatus, I do not lay any great stress on them, further than to prove that both discs are deflected, and that the deflection of the glass disc toward the light is somewhat less than that of the aluminium away from it. In another experiment with a similar apparatus, it was found that if the deviation of the aluminium was taken as 100, that of the glass was 74.8. The result of this experiment, is, as it seems to me, absolutely fatal to any theory which assumes that the repulsion in a Crookes' radiometer is due to the direct impact of heat or light, and I think also it cannot well be explained by assuming the existence of ordinary convection currents. Experiments under the full pressure of the atmosphere.—Being anxious to make some experiments under the ordinary atmospheric pressure, I arranged a blackened disc of aluminium foil, composed of two lamina, counterpoised, provided with a small magnet, and suspended by a single fiber of silk in a glass flask. The time of oscillation of the system I could not determine, owing to friction against the air, but judge that it was between thirty and forty seconds. This apparatus was centered on the graduated circle, and the disc observed as before with a compound microscope. It was soon ascertained that the observer exercised, apparently, an attraction on the suspended disc, even when seated nearly in its prolongation; it followed and pointed toward the observer sitting at a distance of little less than a meter with considerable promptitude, being thus deflected from |