86 Mr. W. Crookes on Attraction and 



The stopper de, and the glass plates c and b, are well fastened 

 with a cement of resin 8 parts and bee's-wax 3 parts*. 



The advantage of a glass-thread suspension is that the beam 

 always comes back to its original position. Before you is an 

 instrument of this description, perfectly exhausted and fitted up 

 with pith plates at each extremity. A ray of light from the 

 electric lamp is thrown on to the mirror c, and thence reflected 

 on to the opposite wall. The approach of a finger to either extre- 

 mity of the beam causes the luminous index to travel several 

 feet, showing repulsion. A piece of ice brought near causes the 

 spot of light to travel as much in the opposite direction. 



Here is another form of the apparatus (fig. 5). The letters 

 and description are the same as in fig. 4, the apparatus, 

 however, being double. The pieces f, g on the end of one 

 beam consist of platinum-foil exposing a square centimetre of 

 surface, whilst the extremities f',g' on the other beam consist of 

 pith plates of the same size. It has already been explained that 

 the neutral point of rarefaction for platinum is much higher than 

 for pith ; consequently at a pressure intermediate between these 

 two neutral points, radiation ought to cause the platinum to 

 be attracted and the pith to be repelled. This is seen to be 

 the case. A wide beam of radiant heat thrown in the centre of 

 the tube on to the plates g,f causes g to be attracted and/' to 

 be repelled, as shown by the light reflected from the mirrors 

 c, c'. The atmospheric pressure in the apparatus is equal to 

 about 40 millions, of mercury. 

 -V* The position of the neutral point not only depends on the 

 density of the body acted on by radiation, as in the above case, 

 but also on the relation of surface to mass. Thus a square cen- 

 timetre of thin platinum-foil on the extremity of the beam 

 requires a lower exhaustion for neutrality than a thicker piece 

 exposing the same surface. Also a flat disk of platinum has a 

 lower neutral point than the same weight of platinum in the 

 form of a sphere. 



Intensity of radiation likewise affects the neutral point. With 



* This is the best cement I have used for standing a vacuum : for a 

 few hours it is perfect. But at the highest exhaustions it seems to leak in 

 the course of a day or two. India-rubber joints are of no use in these ex- 

 periments, as, when the vacuum is near upon perfect, they allow oxygenized 

 air to pass through as readily as the pump will remove it. Whenever 

 possible the glass tubes should be united by fusion ; and where this is im- 

 practicable mercury joints should be used. The best way to make these is 

 to have a well-made perforated conical stopper, cut from plain india-rubber, 

 fitting into the wide funnel-tube of the joint and carrying the narrow tube. 

 Before fitting the tubes in the india rubber this is heated in a spirit-lamp 

 until its surface is decomposed and very sticky; it is then fitted into its 

 place ; mercury is poured over, and oil of vitriol on the top of that. When 

 well made, this joint seems perfect. 



