Lines of Molecular Pressure. 63 



curved at low exhaustions, aud gets flatter as the exhaustion in- 

 creases. A flatter trajectory corresponds to a higher velocity. 

 This may arise from one of two conditions : either the initial im- 

 pulse given by the negative pole is stronger, or the resisting medium 

 is rarer. The latter is probably the true one. The molecules 

 which produce the green phosphorescence must be looked upon as 

 in a state differing from those arrested by frequent collisions. 

 The latter impede the velocity of the free molecules and allow 

 longer time for magnetism to act on them ; for, although the de- 

 flecting force of magnetism might be expected to increase with the 

 velocity of the molecules, Prof. Stokes has pointed out that it 

 would have to increase as the. square of the velocity, in order that 

 the deflection should be as great at high as at low velocities. 



Comparing the free molecules to cannon-balls, the magnetic 

 pnll to the earth's gravitation, and the electrical excitation of the 

 negative pole to the explosion of the powder in the gun, the tra- 

 jectory will be flat when no gravitation acts, and curved when 

 under the influence of gravitation. It is also much curved when 

 the ball passes through a dense resisting medium ; it is less curved 

 when the resisting medium gets rarer ; and, as already shown, 

 intensifying the induction spark, equivalent to increasing the 

 charge of powder, gives greater initial velocity, and therefore 

 flattens the trajectory. The parallelism is still closer if we com- 

 pare the evolution of light seen when the shot strikes the target, 

 with the phosphorescence on the glass screen from .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 rays from a nearly hemispherical alumi- 

 nium cnp is deflected sideways, to the walls of the glass tube, by a 

 magnet. By using a somewhat larger hemisphere, and allowing 

 the negative focus to fall on a strip of platinum-foi], 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 movement 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 

 exceedingly 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 with the misses, in 

 w T hich case the average molecule is allowed to obey its own motions 

 or laws without interference ; and if the mean free path is com- 

 parable to the dimensions of the vessel, the properties which con- 

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

 exalted to an ultra-gaseous state, in which the very decided but 



