260 PHYSICS. 



flatter as tlie rarefication increases, exhibiting a close parallelism with 

 the i^henomeua of projectiles. Moreover, great heat is produced by the 

 impact of these molecules. A large hemispherical cup of aluminum 

 used as the negative pole gives a focus in which i)latinum may be melted. 

 The conditions in these high vacua are special. The free path of the 

 molecules is so long that the average molecule is allowed to obey its 

 own motions or laws without interference ; so that if this path is com- 

 parable to the dimensions of the vessel, the properties which constitute 

 gaseity are reduced to a minimum, and the matter becomes exalted to 

 an ultra gaseous state. The phenomena in these tubes reveal a new 

 world, a world where matter exists in a fourth state, where the corpus- 

 cular theory of light holds good, and where light does not always move 

 in a straight line. 



In a lecture on radiant matter, delivered at the Sheffield meeting 

 of the British Association, August 22, 1879, CrooUes discussed and 

 elegantly illustrated the principles given in the paper above quoted. 

 His vacua are thus grapliically described: "According to the best au- 

 thorities a bulb of the size of the one before you ( L3.5*=™ in diameter) 

 contains more than 1,000,000,000,000,000,000,000,000 (a quadrillion) of 

 molecules. Kow, when exhausted to the millionth of an atmosphere, 

 we shall still have a trillion molecules left in the bulb, a number quite 

 sufficient to justify me in speaking of the residue as ^natter. To suggest 

 some idea of this vast number, I take the exhausted bulb and perfor- 

 ate it by a spark from the induction coil. The sj^ark i> reduces a hole 

 of microscopical fineness, yet sufficient to allow molecules to penetrate 

 and to destroy the vacuum. The inrush of air impinges against the 

 vanes and sets them rotating after the manner of a wind-mill. Let us 

 suppose the molecules to be of such a size that at every second of time 

 a hundred millions could enter. How long, think you, would it take 

 for this small vessel to get full of air ? An hour, a day, a year, a century ? 

 Nay, almost an eternity! A time so enormous that imagination itself 

 cannot grasp the reality. Supposing this exhausted glass bulb, indued 

 with indestructibility, had been pierced at the birth of the solar system; 

 supposing it to have been present when the earth was without form and 

 void; supposing it to have borne witness to all the stupendous changes 

 evolved during the full cycles of geologic time— to have seen the first 

 living creature appear, and the last man disappear; supposing it to sur- 

 vive until the fulfillment of the mathematician's prediction that the sun, 

 the source of energy, four million centuries from its formation, will 

 ultimately become a burntout cinder; supposing all this— at the rate of 

 filling I have just described (100 milHou molecules a second) this little 

 bulb even then would scarcely have admitted its full quadrillion of 

 molecules. But what will you say if I tell you that all these molecules, 

 this quadrillion of molecules, will enter through the microscopic hole 

 before you leave this room? The hole being unaltered in size, the 

 number of molecules undiminished, this apparent paradox can only be 



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