494 



NA TURE 



\ April 22, 1875 



into Kuskokvim Bay, Behring Sea, as believing that the 

 mammoth, whose tusks they constantly find came from 

 the east, and were destroyed by the spells of their 

 shaman. 



In the kitchen-middens of the deserted Esquimaux 

 villages of Jacobshafn, West Greenland, Dr. Ober^ dis- 

 covered bones of the Walrus and Cystophoia cristata, 

 which no longer ventures into this ice-blockaded fjord ; 

 and also of the bear Ursiis maritimns, which is now 

 rarely seen south of the Waigat, associated with arrow- 

 heads, stone flakes, and scrapers, of clear quartz crystals 

 and green jasper {anginal; of the Greenlanders), found in 

 the basalt of Pisco, 



Charles E. De Range 



(To be continued^ 



ON ATTRACTION AND REPULSION RESULT- 

 lAG FROM RADIATION 



A' 



T the Royal Society conversazione the ether evening 



^ the most interesting object exhibited was, beyond 

 all doubt, the radiometer of Mr. Crookes. Mr. Crookes' 

 discovery is of so much importance that our readers will 

 be glad to have an abstract of a paper on the subject, 

 recently read by Mr. Crookes at the Royal Society. It 

 was the second part of a paper which the author sent to 

 that Society in August 1S73. 



Mr. Crookes commences by describing improvements 

 which he has made in the Sprengel pump, and in various 

 accessories which are necessary when working at the 

 highest rarefactions. He describes different new forms 

 which enable the phenomena of repulsion by radia- 

 tion to be observed and illustrated. A bulb three inches 

 diameter is blown at the end of a glass tube eighteen 

 inches long. In this bulb a fine glass stem with a sphere 

 or disc of pith, &c., at each end is suspended by means 

 of a cocoon fibre. The whole is attached to the Sprengel 

 pump in such a way that it can be perfectly exhausted, 

 and then hermetically sealed. Besides pith, the terminals 

 may be made of cork, ivory, metal, or other substance. 

 During exhaustion several precautions have to be taken, 

 and to get the greatest delicacy in an apparatus of this 

 kind, there is required large surface with a minimum of 

 weight. An apparatus constructed with the proper pre- 

 cautions is so sensitive to heat that a touch with the 

 finger on a part of the globe near one extremity of the 

 pith will drive the index round over go°, whilst it follows 

 a piece of ice as a needle follows a magnet. With a large 

 bulb very well exhausted and containing a suspended bar 

 of pith, a somewhat striking effect is produced when a 

 lighted candle is placed about two inches from the globe. 

 The pith-bar commences to oscillate to and fro, the swing 

 gradually increasing in amplitude until the dead centre 

 is passed over, when several complete revolutions are 

 made. The torsion of the suspending fibre now offers 

 resistance to the revolutions, and the bar commences to 

 turn in the opposite direction. This movement is kept 

 up with great energy and regularity as long as the candle 

 burns. 



Mr. Crookes discusses the action of ice, or a cold sub- 

 stance, on the suspended index. Cold being simply 

 negative heat, it is not at first sight obvious how it can 

 produce the opposite effect to heat. The author, how- 

 ever, explains this by the law of exchanges, and shows that 

 attraction by a cold body is really repulsion by radiation 

 falling on the opposite side. According to the same law, 

 it is not difficult to foresee what will be the action of two 

 bodies, each free to move, if they are brought near to 

 each other in space, and if they differ in temperature 

 either from each other or from the limiting walls of the 

 space. The author gives four typical cases, with experi- 

 ments, which prove his reasoning to be correct. 



Experiments Are described with the object of ascer- 



taining whether the attraction by heat, which, commen- 

 cing at the neutral point, increases with the density of the 

 enclosed air, will be continued in the same ratio if the 

 apparatus is filled with air above the atmospheric pres- 

 sure. This is found to be the case. Various experiments 

 are described with bulb-apparatus, in which the bulb is 

 surrounded with a shell containing various adiathermous 

 liquids, and also with a shell of vacuum. In all cases 

 radiation passed through, producing the normal action of 

 attraction in air and repulsion in a vacuum. 



Mr. Crookes next describes a form of apparatus by 

 which measurable results are attainable. It consists of a 

 long glass tube, with a wider piece at the end. In it is 

 suspended a lump of magnesium by a very fine platinum 

 wire, the distance between the point of suspension and 

 the centre of gravity of the magnesium bob being 39'i4 

 inches. Near the magnesium is a platinum spiral, capa- 

 ble of being ignited by a voltaic battery. Observations 

 of the movement of the pendulum were made with a tele- 

 scope v-iith micrometer eyepiece. With this apparatus a 

 large series of experiments are described, starting from 

 air of normal density, and working at intermediate pres- 

 sures up to the best attainable vacuum. 



With this apparatus it was found that a candle-flame 

 brought within a few inches of the magnesium weight, or 

 its image focussed on the weight, and alternately obscured 

 and exposed by a piece of card at intervals of one second, 

 will soon set the pendulum in vibration when the vacuum 

 is very good. A ray of sunlight allowed to fall once on 

 the pendulum will immediately set it swinging. 



The form of apparatus is next described, which the 

 author has finally adopted, as combining the greatest 

 delicacy with facility of obtaining accurate observations, 

 and therefore of getting quantitative as well as qualitative 

 results. It consists of a glass apparatus in the shape of 

 an inverted T, and containing a horizontal glass beam 

 suspended by a very fine glass thread. At the extremities 

 of the beam are attached the substances to be experi- 

 mented on, and at the centre of the beam is a small 

 mirror from which a ray of light is reflected on to a gra- 

 duated scale. The advantage which a glass thread pos- 

 sesses over a cocoon fibre is that the index always comes 

 accurately back to zero. In order to keep the luminous 

 index at zero, except when experiments are being tried, 

 extreme precautions must be taken to keep all extraneous 

 radiation from acting on the torsion-balance. The whole 

 apparatus is closely packed all round with a layer of 

 cotton-wool about six inches thick, and outside this is 

 arranged a double row of Winchester quart bottles filled 

 with water, spaces only being left for the radiation to fall 

 on the balance, and for the index ray of light to get to 

 the mirror. 



However much the results may vary when the vacuum 

 is imperfect, with an apparatus of this kind they always 

 agree amongst themselves when the residual gas is re- 

 duced to the minimum possible ; and it is of no conse- 

 quence what this residual gas is. Thus, starting with the 

 apparatus full of various vapours and gases, such as air, 

 carbonic acid, water, iodine, hydrogen, ammonia, &c., at 

 the highest rarefaction there is not found any difference 

 in the results which can be traced to the residual gas. A 

 hydrogen vacuum appears the same as a water or .-in 

 iodine vacuum. 



With this apparatus the effect of exposing torsion- 

 balance to a contmuous radiation is described, and the 

 results are shown graphically. The effect of a short (11 '3 

 seconds) exposure to radiation is next described, and the 

 results are given in the form of a Table. 



In another Table are given the results of experiments 

 in which a constant source of radiation was allowed to act 

 upon one end of the torsion-beam at a distance of 140 or 

 280 millims., various substances being interposed. The 

 sensitiveness ol this apparatus to heat-rays appears to be 

 greater than that of an ordinary thermo-multiplier. Thus 



