Mr. W. J. Lewis's Crystallographic Notes. 453 



investigation as to how the special motion assigned to the line 

 of spheres can properly represent the character of the motion 

 of the molecules of a gas in its normal state is my own, no 

 special investigation of this kind being contained in Mr. 

 Waterston's paper. To Mr. Waterston, however, is mainly 

 due the initiative in this subject. As I should be sorry to 

 claim any originality that I did not possess, I would respect- 

 fully direct the attention of readers to the portion of Mr. 

 Waterston's paper bearing on this subject. 



PS. (2). Professor Clerk Maxwell, to whom this paper was 

 communicated, and who has taken a kindly interest in the sub- 

 ject, has worked out mathematically the velocity for a wave or 

 impulse propagated by a system of particles moving among 

 each other according to the conditions of equilibrium inves- 

 tigated in the first part of this paper — the diameter of the 

 particles being assumed so small as to be negligible compared 

 with their mean distance, and the particles being further as- 

 sumed spherical, so that there is no movement of rotation 

 developed at the encounters (which would involve loss of 

 velocity). 



Under these premises, the velocity of the wave was found 



to be ~- (or 0*745) into the mean velocity of the particles. 



In most gases the velocity of sound is slightly less than 

 this. This is referable to the movements of rotation deve- 

 loped at the encounters of the molecules (which calculably 

 would delay the wave to a certain extent). In vapour of 

 mercury, according to the determinations of Kundt and War- 



\/5 

 burg, the velocity of sound is exactly —^- into the molecular 



velocity. 



London, May 1877. 



LX. Crystallographic Notes. By W. J. Lewis, M.A., Fel- 

 low of Oriel College , Oxford, and Assistant in the Mineral 

 Department, British Museum*. 



[Plate IV.] 



Barium Nitrate. 



LAST autumn my friend Mr. T. Davies, of the British 

 Museum, kindly brought me a fairly large crystal with 

 a very large number of faces on it. It had been found at the 



* Communicated by the Oystallological Society. Read April 12, 1877. 



