122 



Prof. T. E. Thorpe on 



[June 15^ 



iu direction A B due to A. The waves received at B are approximately 

 plane and perpendicular to A B, so that the relation between the velocity 

 and pressure at B is that proper to a plane wave ; but it is otherwise in 

 the case of the sound received at A. Accordingly the reciprocal theorem 

 does not lead us to expect an equality between the pressures at A and B, 

 on which quantities the behaviour of the sensitive flames depends. On 

 the contrary, it would appear that the pressure at A corresponding to 

 the given velocity along A B should be much greater than in the case of 

 a plane wave, and then the relative advantage of the position A would 

 be explained. 



It will be seen that if the preceding arguments are correct, Prof. 

 Tyndall's experiment does not bear out the conclusions that he has based 

 upon it with respect to the observations of the Trench Commission at 

 Yillejuif and Montlhery. No acoustic clouds could explain the failui^e 

 of reciprocity then observed ; and the more probable hypothesis that the 

 effect was due to wind is not inconsistent with the observation that the 

 air (at the surface) was moving in the direction against which the sound 

 was best heard. 



Turther experiments on this subject are very desirable. 



VII. On Phosphorus Pentafluoride." By T. E. Thorpe, F.B.S., 

 Professor of Chemistry in the Yorkshire College of Scieuce. 

 Received June 6, 1876. 



Arsenic trifluoride acts dolently upon phosphorus pentachloride with 

 the formation of arsenic trichloride and the liberation of a hea"\y fuming 

 gas, which analysis shows to \)Q ijliosj^liorus pentcifluoi^ide. 



The production of the new gas may be represented by the equation 



5 AsFg + 3 PCI. = 5 ASCI3 + 3PE. . 



Direct experimeuts ha^ e shown that the amount of arsenic trichloride 

 actually produced in the reactions agrees closely with that demanded by 

 this equation. 



The accuracy of the formula was further controlled by determinations 

 of the density of the gas. Three experiments made by two independent 

 methods gave the numbers — 



1 62-98 



II 63-33 



III 63-39 



The number demanded by the formula PF. is 



63-0, 



hydrogen being the unit. 



