246 Royal Society: — Mr. W. Crookes on Attraction 



the same ratio if the apparatus is filled with air above the atmo- 

 spheric pressure. This is fouud to be the case. 



Various experimeuts 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. 



The author next describes a form of apparatus by which mea- 

 surable results are attainable. It consists of a long glass tube, 

 with a wider piece at the end. In it is suspended a lump of mag- 

 nesium by a very fine platinum wire, the distance between the point 

 of suspension and the centre of gravity of the magnesium bob 

 being 39*14 inches. Near the" magnesium is a platinum spiral, 

 capable of being ignited by a voltaic battery. Observations of 

 the movement of the pendulum are made with a telescope with 

 micrometer eyepiece. With this apparatus a large series of experi- 

 ments are described, starting from air of normal density, and 

 working at intermediate pressures up to the best attainable vacuum. 

 The results are given in two tables. 



With this apparatus it was found that a candle-flame brought 

 within a few inches of the magnesium weight, or its image focused 

 on the weight and alternately obscured and exposed by a piece of 

 card at intervals of one second, will soon set the pendulum in vi- 

 bration 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 hori- 

 zontal 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 graduated scale. The ad- 

 vantage which a glass thread possesses 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 

 inches thick ; and outside this is arranged a double row of Win- 

 chester 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 im- 

 perfect, with an apparatus of this kind they always agree among 

 themselves when the residual gas is reduced to the minimum pos- 

 sible ; and it is of no consequence 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 



