﻿Coloured Cloudy Condensation. 25 



Below about 9° 0., therefore, the field is opaque at all 

 pressures ; above 9°, the pressure at which blue changes into 

 opaque rapidly increases with increasing temperature ; and 

 the pressure at which brown-yellow changes to opaque de- 

 creases from an enormous value, and at even a more rapid 

 rate as temperature increases. Both loci ; curving at a 

 gradually retarded rate, eventually reach a common asym- 

 ptote at about 41 cm. (temperature being indefinitely high). 

 At the same time the colours which were very intense 

 at the lower temperature gradually become fainter, and the 

 opaque zone more translucent, until at about 40° of air-tem- 

 perature (depending on the size of the nozzle, § 15) the field 

 is clear and without colour. The escaping steam is gaseous, 

 and not visibly condensed. When temperature decreases 

 again from 40°, white-yellow is the first colour to appear, 

 showing that the particles here must be the smallest of the 

 whole series. At 35° the change from faint yellowish tones 

 to faint white-blues, when pressure is made to vary suitably 

 (see chart) from larger to smaller values, is quite marked. 

 There is no opaque demarcation, however, but rather a mix- 

 ture of colours, for the opaque field is hardly impervious to 

 light above 30°. Indeed one often notices a brownish field 

 surrounding the jet, on a violet-bluish ground. 



For all temperatures and pressures lying to the left of the 

 two curves the field is opaque, and it sends off a kind of cusp 

 to penetrate into higher temperatures. There is a character- 

 istic difference between the contours of the two margins ; for 

 whereas yellow-opaque after a sharp inflexion shoots up 

 almost vertically, blue-opaque shows a regular change of 

 curvature throughout. 



At about 13° in the chart I have inscribed the approxi- 

 mate positions of the successive interference-colours *, indi- 

 cating their positions by a cross. This cannot be nearly so 

 well done as the location of the opaque margin, and as the 

 colours are of smaller interest in the present paper, I will not 

 enter into the subject further. The contour of the successh e 

 colour-curves is easily surmised from the line for blue-opaque. 

 Similarly above the yellow-opaque line, a family of browns, 

 oranges, and yellows may be located. 



When, the dust-contents are increased, the margin of the 

 opaque field approaches the abscissa, and hence the colour 

 loci will be successively more crowded together. 



In the chart (Plate V. fig. 2) only a single air-tube (D, 

 fig. 1) was available. The air was heated to about 40° by 

 the circulating steam. This was then shut off and the 

 * See Barus, Am. Met. Journ. ix. p. 500 et seq., 1893. 



