﻿20 Mr. Carl Barus on 



estimation in terms of the colour-intensity of the blue, and the 

 apparatus is graduated by comparing it with the direct dust- 

 counter. Based as this apparatus is on colour discriminations, 

 it is not adapted to give more than a few steps of dust- 

 contents, and Mr. Aitken chiefly recommends it for qualita- 

 tive purposes, such, for instance, as may present themselves 

 in sanitary work. 



2. During the course of my experiments on the thermal 

 distribution in steam-jets, I had frequent occasion to note the 

 actuating steam-pressure at which the intense blue-violet 

 field of my colour-tube merges into opaque, eventually to 

 reappear (pressure increasing) as an orange-brown field of 

 the first order. It struck me that here was a sufficiently 

 sharp criterion for fixing a value of pressure depending in 

 the given apparatus only on the temperature and the dust- 

 contents of the inflowing air. In other words, for a given 

 kind of air, and at a given temperature, there are two well- 

 defined pressures at which colour (blue and yellow) vanishes 

 into blackness. If the kind of air remains the same while its 

 temperature varies, the paired values of pressure will also 

 vary markedly, so that the margins of the opaque field may 

 be mapped out in a diagram in which pressure is expressed 

 in its dependence on temperature. It is the chief purpose 

 of the present paper to show the character of this diagram, 

 and to indicate the manner in which the positions of the loci 

 vary, when the dust-contents of the inflowing air are also 

 varied. Incidentally I will endeavour to ascertain the more 

 immediate cause of the opaque field, and to see whether the 

 water molecules may not themselves become nuclei of con- 

 densation, §§ 15, 16. 



3. Apparatus. — Full details of the necessary apparatus is 

 given in PI. V. fig. 1, where the colour-tube is shown at AA, 

 and the method of varying the temperature and dustiness of 

 the inflowing air is shown at E, D, F. The colour-tube is 

 identical in form vdth the apparatus described in an earlier 

 paper *. 1 need only call to mind here that the steam issues 

 at the jet j } from a nozzle about "16 cm. in diameter, and 

 that the tube AA is about 50 to 60 cm. long, and, in common 

 with the air-hole C, about 5 cm. in diameter. The glass 

 plates g and a are kept clear by moistening with a solution 

 of caustic potash, and the mirror M reflects skylight through 

 the tube. Mixed steam and air escape at B, and provision is 

 made (not shown) for screening off extraneous light from g, 

 the window through which the colour observations are made. 



* Bams, Amer. Meteorolog. Journal, ix. p. 488 (1893). 



