468 Scientific Intelligence. 



SCIENTIFIC INTELLIGENCE. 



I. Physics and Chemistey. 



1. On the Law of Gaseous Flow. — Hirn has published an 

 extended memoir on the law of flow of elastic fluids, in which he 

 has given the results of experiments made to determine whether 

 a gas under a constant pressure flows into a reservoir where the 

 pressure, also constant, is less than its own, with a velocity 

 indefinitely increasing as the pressure in the reservoir decreases ; 

 or whether there exists a limiting velocity which is attained 

 when this second pressure is zero. Theoretically two kinds of 

 equations have been employed to express the law of flow. In 

 one of these, the work of expansion is neglected ; in the other, 

 more recent, it is included. Comparing together the volume- 

 equations of the two kinds, it appears that they both give a 

 maximum value for the volume of gas flowing under constant 

 pressure into a reservoir where the counter-pressure is variable. 

 As to the velocity-equations, the former indicates a continual 

 increase in velocity from zero to infinity with decreasing pres- 

 sures in the reservoir ; while the latter, or Weisbach's equation, 

 on the contrary, gives velocities converging toward an upper 

 limit, attained when the gas passes into an absolute vacuum. 

 Experiment then is to be invoked to decide, 1st, whether there 

 is such a maximum value for the volume of gas thus flowing, and 

 2d, whether the velocity of the flow tends toward a limiting 

 value as the counter-pressure diminishes. The apparatus con- 

 sisted of a weighted gasometer of known capacity, which forced 

 air through a suitable orifice into a reservoir kept exhausted by a 

 water-pump. Five forms of orifice were employed, two of which 

 were simply conical openings in a thin plate of brass 3 mm thick, 

 and the others conical ajutages, whose side formed angles of 13° 

 and 9° respectively with the axis, one of the two latter terminat- 

 ing in a cylindrical glass tube. The pressure in the gasometer 

 was maintained constant by the weights upon it, and the flow 

 was measured by noting electrically its rate of fall. The varying 

 pressure in the reservoir was also recoi'ded electrically. Repre- 

 senting graphically the results of the experiments, it appears 

 that the maximum value to which the volume-equations point 

 has no existence ; and that with regard to the velocity of flow, 

 the limiting value indicated by the equation of Weisbach, has 

 equally no foundation in fact. Hence the author concludes that 

 the true law of gaseous flow produced by difference of pressure, 

 is still unknown ; and concludes by calling attention to the 

 discrepancy between these results and those predicted by the 

 kinetic theory of gases. According to this theory, dry air under 

 a constant pressure would flow into a perfect vacuum with a 

 velocity which cannot exceed that of the gaseous molecules them- 

 selves at that temperature, which is about 485 meters per second. 

 But in the experiments above mentioned, even under a notable 



