﻿34 Mr. Carl Barus on 



. Were this the case, then the supersaturation at the lower 

 margin of the opaque field, expressed either isotherinally as 

 pressure or isopiestically as temperature, would lead easily to 

 the dimension of steam molecules. For the margin in question 

 is a locus at which the issuing steam condenses as a whole, or 

 in which the vapour contains within itself the conditions * of 

 condensation. 



1 6. In the absence of my other papers, the last remarks 

 need further elucidation. I have already shown t that the 

 opaque field at any temperature occurs cotemporaneously with 

 the breakdown of the stability of the steam-jet. In other 

 words, whenever at any temperature the velocity of the jet 

 is just sufficient to change steady into edging motion, then an 

 opaque field is observed in the colour-tube. One is therefore 

 amazed to find brilliant browns, oranges, and yellows appearing 

 after the jet-velocity or jet-pressure is even further increased, 

 and the natural inference is that with the occurrence of the 

 opaque field the degree of supersaturation reached is intense 

 enough to cause the steam to condense without nuclei. Con- 

 formably with this view of the case, condensation at pressures 

 below the opaque margin is induced by dust, whereas above 

 the opaque margin condensation is independent of dust. 

 Above and below the margin the colour-phenomena are thus 

 different in origin, and experiment bears this out to the extent 

 that the yellows in question, however produced, are not in- 

 fluenced by injections of active dust. 



Thus far all is plausible ; but the explanation given receives 

 a rude shock when further observation shows the height of 

 the asymptote to vary enormously (pressure, 20 cm. to 50 cm. 

 of mercury) for different nozzles. Even in one and the same 

 (brass) nozzle the height gradually decreases in the lapse of 

 time in proportion, no doubt, as the smoothness of the nozzle is 

 impaired by continued use (usually sulphur corrosion) . Hence 

 the hydraulic phenomenon — i. e. the breakdown of the stability 

 of the jet — is the primary occurrence, and the condensation 

 phenomenon follows upon this. It is still possible to patch up 

 the view above stated by supposing supersaturation to be 

 sacrificed with the breakdown of the jet, seeing that the 

 turbulent motion of the now noisy (intermittent) efflux of 

 steam is not such in which one would expect to retain the 

 supersaturated state. 



* Lord Kelvin's well-known equation (cf. Maxwell's < Heat,' chap, xx.) 

 is here made the basis of consideration. 



t Cf. Phil. Mag. xxxv. p. 315, 1893 ; cf. Am. Met. Journ. March 1893, 

 p. 505. 



