81 Mr. W. Crookes on Attraction and 



gives place to repulsion, which gets stronger and stronger as the 

 vacuum approaches perfection. 



In order to illustrate more strikingly the influence exerted by 

 a trace of residual air, an apparatus (fig. 2) is here shown in 

 which the source of heat (a platinum spiral, a, rendered incan- 

 descent by electricity) is inside the glass tube instead of out- 

 side it as before. A mass of magnesium, b, turned conical, is 

 suspended in a glass tube, cde, by a fine platinum wire of such 

 a length as to vibrate seconds. The upper end of the platinum 

 wire is sealed into the glass at e, and passes through to the outside 

 for the purpose of electrical experiments. The platinum spiral 

 is arranged so that when the pendulum hangs free the magne- 

 sium mass is about \ inch from it. In air the red-hot spiral 

 produces decided attraction on the magnesium ; and by properly 

 timing the contacts with the battery, a considerable swing can 

 be accumulated. On perfectly exhausting the apparatus, how- 

 ever, the incandescent spiral is found to energetically repel, 

 and a very few contacts and breaks properly timed are suffi- 

 cient to get up the full swing the pendulum is capable of. 



A simpler form of the apparatus for exhibiting the phenomena 

 of attraction in air and repulsion in a vacuum consists of a long 

 glass tube a b (fig. 3) with a globe c at one end. A light index 

 of glass with pith-balls at the ends d } e is suspended in this globe 

 by means of a cocoon fibre. When the apparatus is full of air 

 at ordinary pressure, a ray of heat or light falling on one of the 

 pith-balls gives a movement indicating attraction. 



"When the apparatus is exhausted until the barometric gauge 

 shows a depression of 12 millims. below the barometer, neither 

 attraction nor repulsion results when radiant light or heat falls 

 on the pith. When the vacuum is as good as the pump will 

 produce, strong repulsion is shown when radiation is allowed to 

 fall on one end of the index. The heat of the hand, or even of 

 the body several feet off, is quite sufficient. The action is in 

 proportion to the surface acted on rather than to the mass. 



The barometric position of the neutral point dividing attrac- 

 tion from repulsion varies with the density of the mass on which 

 radiation falls, on the ratio of its mass to its surface, and in a 

 less degree on the intensity of radiation. In the case of pith it 

 is seen to lie at about 12 millims. below a barometric vacuum, 

 whilst with a heavy metal it is within a tenth of a millim. of a 

 vacuum. Experiments to try to determine the law governing 

 the position of the neutral point are now in progress. 



Ice, or a cold substance, produces the opposite effects to heat. 

 Thus a bar of pith suspended in a vacuum is energetically re- 

 pelled by the warm hand, whilst it is as strongly attracted by a 

 piece of ice. Cold being simply negative heat, it is not easy at 



