William Crookes

On Attraction and Repulsion Resulting from Radiation

1. In a paper “On the Atomic Weight of Thallium,” presented to the Royal Society June 18, 1872, after describing a balance with which I was enabled to perform weighings of apparatus &c. in a vacuum, I noted a peculiarity in relation to the effect of heat in diminishing the apparent weight of bodies. I said, “That a hot body should appear to be lighter than a cold one has been considered as arising from the film of air or aqueous vapour condensed upon or adhering to the surface of the colder body, or from the upward currents of air caused by the expansion of the atmosphere in the vicinity of the heated body. But neither hypothesis can be held when the variation of the force of gravitation occurs in a vacuum as perfect as the mercurial gauge will register, and under other conditions which I am now supplying, and which I purpose embodying in a paper to be submitted to the Royal Society during a subsequent session”. With the vacuum-balance mentioned above I carried out many experiments, but was unable to obtain results which were at all concordant; and it was soon found necessary to investigate the phenomena with smaller and less complicated apparatus.

The Bakerian Lecture: On the Illumination of Lines of Molecular Pressure, and the Trajectory of Molecules

486. When the spark from a good induction coil traverses a glass tube containing a rarefied gas, certain phenomena are observed which vary greatly with the kind of gas and the degree of exhaustion. There is one appearance, however, which is constant in all the gases which I have examined, and within very wide limits of pressure, viz.: the well-known dark space round the negative pole. I have long been impressed with the idea that this dark space coating the pole was in some way related to the layer of molecular pressure causing movement in the radiometer, and the following experiments were instituted with the object of testing this hypothesis. A glass bulb (fig. 1) was furnished w ith platinum wire terminals sealed into the glass, ending outside in loops and. inside in aluminium poles; the positive pole being a wire and the negative pole a disk about 10 millims. diameter, bare in front and covered with mica at the back. The bulb being full of dry air and connected with the Sprengel pump, was exhausted. An induction coil capable of giving sparks 68 millims. long in air when actuated by 3 Grove’s cells, was connected with the terminals, the disk being always negative except when otherwise stated.

Contributions to Molecular Physics in High Vacua. Magnetic Deflection of Molecular Trajectory. Laws of Magnetic Rotation in High and Low Vacua. Phosphorogenic Properties of Molecular Discharge

586. The present paper is a continuation of the Bakerian Lecture “On the Illumination of Lines of Molecular Pressure and the Trajectory of Molecules read before the Royal Society, December 5, 1878. Phenomena there briefly referred to have since been more fully examined; new facts have been observed, and their theoretical bearings discussed; and numerous experiments suggested by Professor Stokes and others have been tried, with the result of acquiring much information which cannot fail to be of value in assisting to evolve a theory capable of embracing all the phenomena under discussion. 587. In par. 514 I described a piece of apparatus by means of which the molecular rays electrically projected from the negative pole at a high exhaustion were converged to a focus, the pole itself being hemi-cylindrical in shape. On referring to the coloured drawing illustrating the experiments it will be observed that the green phosphorescence of the glass (by means of which the presence of the molecular rays is manifested) does not take place close to the negative pole.

On Repulsion Resulting from Radiation. Part VI

The present paper is in continuation of one which I had the honour of reading before the Royal Society, December 11th, 1873, and which was published in the Philosophical Transactions, vol. clxiv. part 2, page 501. In that paper I described various pieces of apparatus, chiefly in the form of delicate balances suspended in glass tubes, by means of which I was enabled to show attraction or repulsion when radiation acted on a mass at one end of the beam, according as the glass tube contained air at the normal pressure, or was perfectly exhausted. At an intermediate internal pressure the action of radiation appeared nil. Towards the end of the paper 1 said (70), “I have arranged apparatus for obtaining the movements of repulsion and attraction in a horizontal instead of a vertical plane. Instead of supporting the beams on needle-points, so that they could only move up and down, I suspend them by the centre to a long fibre of cocoon-silk in such a manner that the movements would be in a horizontal plane. With apparatus of this kind, using very varied materials for the index, enclosing them in tubes and bulbs of different sizes, and experimenting in air and gases of different densities up to Sprengel and chemical vacua, I have carried out a large series of experiments, and have obtained results which, whilst they entirely corroborate those already described, carry the investigation some steps further in other directions. I have introduced two important improvements into the Sprengel pump which enable me to work with more convenience and accuracy. Instead of trusting to the comparison between the barometric gauge and the barometer to give the internal rarefaction of my apparatus, I have joined a mercurial siphon-gauge to one arm of the pump. This is useful for measuring very high rarefactions in experiments where a difference of pressure equal to a tenth of a millimetre of mercury is important. By its side is an indicator for still higher rarefactions; it is simply a small tube having platinum wires sealed in, and intended to be attached to an induction-coil. This is more convenient than the plan formerly adopted (51), of having a separate vacuumtube forming an integral part of each apparatus. At exhaustions beyond the indications of the siphon-gauge I can still get valuable indications of the nearness to a perfect vacuum by the electrical resistance of this tube. I have frequently carried exhaustions to such a point that an induction-spark will prefer to strike its full distance in air rather than pass across the 1/4 inch separating the points of the wires in the vacuum-tube. A pump having these pieces of apparatus attached to it was exhibited in action by the writer before the Physical Society, June 20th, 1874.

On Radiant Matter Spectroscopy. Part II. Samarium

90. In the concluding sentence of the Bakerian Lecture which I had the honour to deliver before the Royal Society, May 31, 1883, I said that the new method of Radiant Matter Spectroscopy there described had given me not only spectrum indications of the presence of yttrium as an almost invariable, though very minute, constituent of a large number of minerals, but had likewise revealed signs of another spectrum-yielding element. I stated that I had repeatedly seen indications of another very beautiful spectrum characterised by a strong red and a double orange band. That this second spectrum was not then new to me is shown by a paper sent to the Royal Society in 1881, in which I described a double orange band occurring in the phosphorescent spectrum of an earth less frequently met with than the “pale yellowish coloured earth” (since identified as yttria) which gave me the “red, orange, citron, and green bands.” 91. The method adopted to bring out the orange band is to treat the substance under examination with strong sulphuric acid, drive off excess of acid by heat, and finally to raise the temperature to dull redness. It is then put into a radiant matter tube, of the form shown in fig. 1, and the induction spark is passed through it after the exhaustion has been pushed to the required degree. The anhydrous sulphate thus left frequently shows the orange band in the radiant matter tube, though before this treatment the original substance shows nothing.

SPIRITUALISM VIEWED BY THE LIGHT OF MODERN SCIENCE

It is now many years since an authentic edition of these researches was published, and it is felt that a new edition would be welcomed by many people who have no access to the former one. At the time of his death, Sir William Crookes was preparing to issue a new edition, which might or might not have been recast and enlarged to include his later experiences. However this may have been, there is now no one competent to alter or add to what he has written. Nevertheless, the present volume is not a verbatim reprint of the original. It has been judged expedient to omit, as no longer relevant, certain correspondence challenging and vindicating his competency to make and record the crucial experiments with D. D. Home, and in addition the names of certain gentlemen who were at first referred to by initials, but whose names were subsequently given, have been inserted in the text. With these exceptions, everything is as first published by Sir (then Mr.) William Crookes.

The Bakerian Lecture: On Radiant Matter Spectroscopy: The Detection and Wide Distribution of Yttrium

In March, 1881, I sent to the Royal Society a preliminary notice of some results I had obtained when working on the molecular discharge in high vacua. When the spark from a good induction coil traverses a tube having a flat aluminium pole at each end, the appearance changes according to the degree of exhaustion. Supposing atmospheric air to be the gas under exhaustion, at a pressure of about 7 millims. a narrow black space is seen to separate the luminous glow and the aluminium pole connected with the negative pole of the induction coil. As the exhaustion proceeds this dark space increases in thickness, until, at a pressure of about 0.02 millim. (between 20 and 30M.), the dark space has swollen out till it nearly fills the tube. The luminous cloud showing the presence of residual gas has almost disappeared, and the molecular discharge from the negative pole begins to excite phosphorescence on the glass where it strikes the side. There is great difference in the degree of exhaustion at which various substances begin to phosphoresce. Some refuse to glow until the exhaustion is so great that the vacuum is nearly non-conducting, whilst others begin to become luminous when the gauge is 5 or 10 millims. below the barometric level. The majority of bodies, however, do not phosphoresce till they are well within the negative dark space. This phosphorogenic phenomenon is at its maximum at about 1M., and, unless otherwise stated, the experiments now about to be described were all tried at this high degree of exhaustion. Under the influence of this discharge, which I have ventured to call radiant matter, a large number of substances emit phosphorescent light, some faintly and others with great intensity. On examining the emitted light in the spectroscope most bodies give a faint continuous spectrum, with a more or less decided concentration in one part of the spectrum, the superficial colour of the phosphorescing substance being governed by this preponderating emission in one or other part of the spectrum.

The Bakerian Lecture: On Repulsion Resulting from Radiation. Part V

220. In this Part I propose to give the results of some researches which, during the past twelve months, have occupied much of my time. The researches include a quantitative examination of the repulsion exerted by a standard flame shining on pith and mica disks, coated with various powders, chemical precipitates, &c., and suspended in vacuo in a torsion apparatus. The character of the incident radiation has been varied by straining it through water, alum, or coloured media; the action of good and bad conductors of heat has been compared; and the influence which favourable presentation of the experimental surface, by curvature or obliquity, has upon its movement has been investigated, with the result of throwing much light on some of the debated problems in molecular physics to which, by general assent, the repulsion resulting from radiation is held to be due. In every step of this investigation, theory and observation have gone hand in hand, and at each point gained it has been my endeavour to permanently record such experimental proof in the convenient form of an instrument, so as to have it available for further examination. The reaction, along lines of greatest molecular pressure, between the experimental surface and the fixed case containing it, has been examined. Experimental proof has been obtained, not only of the existence of such a reaction, but of the direction in which it is chiefly exerted; and the apparatus devised during this inquiry, to put each step of the theory to an experimental test, has led to the construction of a modification of the radiometer named the otheoscope, in which the reacting surface is no longer the side of the glass case, but is specially made with a view of getting the greatest sensitiveness in the moving parts of the apparatus. Owing to the increased delicacy of the instruments now made, it has been possible to detect the existence of molecular pressure when radiation falls on a black surface, in air of normal density.

On the Viscosity of Gases at High Exhaustions.

By the viscosity or internal friction of a gas, is meant the resistance it offers to the gliding of one portion over another. In a paper read before the British Association in 1859, Maxwell presented the remarkable result that on theoretical grounds the coefficient of friction, or the viscosity, should be independent of the density of the gas, although at the same time he states that the only experiments he had met with on the subject did not seem to confirm his views.

Researches on the Atomic Weight of Thallium.

In June 1862, and in February 1863, I had the honour to lay before the loyal Society communications on the subject of the then newly discovered metal, thallium. In these I gave an account of its occurrence, distribution, and the method of extraction from the ore, together with its physical characteristics and chemical properties; also I discussed the position of thallium among elementary bodies, and gave a series of analytical notes. In the pages of the ‘Journal of the Chemical Society’ for April 1, 1864, I collated all the information then extant, both from my own researches and from those of others, introducing qualitative descriptions of an extended series of the salts of the metal. I propose in the present paper to lay before the Royal Society the details and results of experiments which have engrossed much of my spare time during the last eight years, and which consist of very laborious researches on the atomic weight of thallium.