110 On the Illumination of Lines of Molecular Pressure. [Dec. 5, 



strikes tlie target, with the phosphorescence on the glass screen 

 accompanying molecular impacts. 



Focus of Heat of Molecular Impact. 



The author finally describes -an apparatus in which he shows that 

 great heat is evolved when the concentrated focus of rajs from a 

 nearly hemisj)herical aluminium cup is deflected sideways by a 

 magnet to the walls of the glass tube. By using a somewhat larger 

 hemisphere and allowing the negative focus to fall on a strip of 

 platinum foil, the heat rises to the melting point of platinum. 



An Ultra- gaseous State of Matter. 



The paper concludes with some theoretical speculations on the state 

 in which the matter exists in these highly exhausted vessels. The 

 modern idea of the gaseous state is based upon the supposition that a 

 given space contains millions of millions of molecules in rapid move- 

 ment in all directions, each having millions of encounters in a second. 

 In such a case, the length of the mean free path of the molecules is ex- 

 ceedingly small as compared with the dimensions of the vessel, and 

 the properties which constitute the ordinary gaseous state of matter, 

 which depend upon constant collisions, are observed. But by great 

 rarefaction the free path is made so long that the hits in a given 

 time may be disregarded in comparison to the misses, in which 

 case the average molecule is allowed to obey its own motions or laws 

 without interference ; and if the mean free path is comparable 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, in which the very decided but hitherto masked pro- 

 perties now under investigation come into play. 



Bays of Molecular Light. 



In speaking of a ray of molecular light, the author has been guided 

 more by a desire for conciseness of expression than by a wish to advance 

 a novel theory. But he believes that the comparison, under these 

 special circumstances, is strictly correct, and that he is as well entitled 

 to speak of a ray of molecular or emissive light when its presence is 

 detected only by the light evolved when it falls on a suitable screen, 

 as he is to speak of a sunbeam in a darkened room as a ray of vibratory 

 or ordinary light when its presence is to be seen only by interposing 

 an opaque body in its path. In each case the invisible line of force 

 is spoken of as a ray of light, and if custom has sanctioned this as 

 applied to the undulatory theory, ii cannot be wrong to apply the 

 expression to emissive light. The term emissive light must, however, 

 be restricted to the rays between the negative pole and the luminous 



