Gases under High Pressures. 179 



omission to consider what becomes of the jet after full pene- 

 tration into the receiver. The idea appears to have been 

 that the jet gradually widens in section as it leaves the 

 orifice and that in the absence of friction it would ultimately 

 attain the velocity corresponding to the entire fall of pressure. 

 The first to deal with this question seem to have been Mach 

 and Salcher *, but the most elaborate examination is that of 

 R. Emden f, who reproduces interesting pictures of the 

 effluent jet obtained by the simple shadow method of Dvorak J. 

 Light from the sun or from an electric spark, diverging from 

 a small aperture as source, falls perpendicularly upon the jet 

 and in virtue of differences of refraction depicts various 

 features upon a screen held at some distance behind. A per- 

 manent record can be obtained by photography. Emden 

 thus describes some of his results. When a jet of air, or 

 better of carbonic acid or coal-gas, issues from the nozzle 

 into the open under a pressure of a few millimetres, it is seen 

 to rise as a slender column of the same diameter to a height 

 of perhaps 30 or 40 cm. Sometimes the column disappears 

 without visible disturbance of the air ; more often it ends 

 in a small vortex column. When the pressure is raised, 

 the column shortens until finally the funnel-shaped vortex 

 attaches itself to the nozzle. At a pressure of about one-fifth 

 of an atmosphere there appears again a jet 2 or 3 cm. long. 

 As the pressure rises still further, the jet becomes longer and 

 more distinct and suddenly exhibits thin, bright, and fairly 

 equidistant disks to the number of perhaps 10 or 12, crossing 

 the jet perpendicularly. The first disks have exactly the 

 diameter of the nozzle, but they diminish as the jet attenuates. 

 Under still higher pressures the interval between the disks 

 increases, and at the same time the jet is seen to swell out 

 between them. These swellings further increase and oblique 

 markings develop which hardly admit of merely verbal 

 description. 



Attributing these periodic features to stationary sound 

 w r aves in the jet, Emden set himself to determine the wave- 

 length (X), that is the distance between consecutive disks, 

 and especially the pressure at which the weaves begin to 

 develop. He employed a variely of nozzles, and thus sums 

 up his principal results : — 



1. When air, carbonic acid, and hydrogen escape from 

 equal sufficiently high pressures, the length of the sound 



* Wied. Ann. Bd. xli. p. 144 (1890). 



t Wied. Ann. Bd. lxix. pp. 264, 426 (1899). 



t Wied. Ann. Bd. ix. p. 502 (1879). 



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