STEAM JET. 99 



It will be seen that for each temperature of the inflowing air there 

 is a definite steam pressure at which the field of the tube just becomes 

 opaque, and condensation within the jet, therefore, begins to be tumul- 

 tuous. The edge of the opaque field is sharply marked, and above 15 it 

 is possible to pass through it in a march of continually increasing steam 

 pressures; for the opaque field vanishes with a kind of cusp, dependent 

 for its position, naturally, on the apparatus used. 



What the figure imperfectly suggests, however, is the occurrence of 

 similar loci of axial color, which run parallel to the edge of the opaque 

 zone, on the clear side of it. The colors follow the spectrum series 

 reversed, v, b, g, y, o, r, growing continually fainter and vanishing into 

 daylight. 



The field becomes at once opaque if a strong nucleator like phosphorus 

 is placed near C, fig. 50; or any color may be obtained in this way by 

 carefully regulating the additions of nuclei, as shown elsewhere. Mere 

 smokes, like salammoniac, are ineffective; in fact the field made opaque 

 by phosphorus may be cleared by such smoke, added in reasonable 

 quantity. 



Ordinary non-filtered air is practically ineffective. This touches the 

 first point to be made. I have shown elsewhere that the fog chamber 

 and the steam jet mutually supplement each other; the former respond- 

 ing measurably to nuclei reckoned in thousands per cubic centimeter, 

 the latter to nuclei reckoned in millions per cubic centimeter, to speak 

 roughly. In other words, the whole sequence of coronas, of which there 

 are many periods visible in an apparatus of reasonable size, has been 

 passed through when the occurrence of axial color begins, the latter end 

 with particles so fine as to be optically ineffective. 



It is for this reason that ordinary non-filtered air, which produces such 

 remarkable effects in the fog chamber, is almost without effect on the 

 steam jet. A faint scarcely discernible pink tinge is all that is seen, 

 and it is therefore possible to omit the filtration of air altogether. The 

 use of dust-free air will not change the conditions, which lie wholly 

 below the scope of the steam jet. This is even the case when the air 

 is artificially dusted by less powerful nucleators like weak X-rays or 

 weak radium, the additions being as a rule relatively insignificant. 



As the increase of steam pressure can only increase the supersatu- 

 ration, it follows that the tumultuous precipitation of the steam jet 

 characterizing the opaque zone must take place on the nuclei of dust-free 

 air, for at this stage of the phenomenon the corona-producing dusts 

 are ineffective. 



Much attention has been given by Professor Wood in this country, 

 and by others abroad, to the occurrence of optical resonance in con- 



