PHYSICS OF NINETEENTH CENT. RADIATION. 519 



sicists of the present century have stated that bodies of different tem- 

 peratures attracted each other. Other observers have spoken, on the 

 other hand, of a repulsive force between such bodies. 



Like many other new phenomena, those presented by vacuous spaces 

 became the subjects of possible investigation by reason of improve- 

 ments in instrumental appliances. It was not until means had been 

 devised for carrying the exhaustion of vessels to a far higher point than 

 could be attained by means of the ordinary air-pump, that the beautiful 

 phenomena produced by the electrical 

 discharge in Geissler's tubes (Chap. XX.) 

 could be studied. Geissler invented the 

 first arrangement by which vessels could 

 conveniently be exhausted by commu- 

 nication with a Torricellian vacuum. 

 Now, the amount of air remaining in the 

 receiver of a good air-pump of the ordi- 

 nary kind is capable of sustaining the 

 pressure of a column of mercury ^jth 

 to -^th inch high, after the pump has 

 been worked to the utmost of its power. 

 With Geissler's machine, however, a 

 vacuum corresponding with only o^th 

 inch may very easily be obtained. This 

 implies that only y-sVoth P art f tne 

 original air remains in the vessel. The 

 mercury apparatus was improved by 

 Sprengel, and his apparatus has with 

 slight modifications been always used 

 since, in order to obtain the highest 

 degree of rarefaction. Fig. 238 will illus- 

 trate the principle of the Sprengel Pump 

 by exhibiting the simplest form of the 

 apparatus. A B is a strong glass tube 

 with a bore not exceeding T ^th of an FIG. 238. THE SPRENGEL PUMP. 

 inch. The height of this from A to B 



must be at least 3 feet. Its lower end dips a very little below the 

 level of the mercury in the vessel c, which has a spout D, from which 

 the mercury falls into another vessel E. The top of the tube is con- 

 nected with the funnel F by a short india-rubber tube, which can be 

 compressed by the clamp G when required. At A the tube H A opens 

 obliquely, and at H this tube can be connected with the vessel to be 

 exhausted, I. When mercury is poured into the funnel F, it exhausts 

 air at A, and the whole length of A B may be observed to be filled with 

 lengths of mercury separated by air-spaces, all moving down in-to the 

 vessel B, whence the air and the mercury escape by the spout D. From 

 time to time the vessel E is replaced by another, while the collected 



