﻿30 Mr. Carl Barns on 



near its mouth d, where the air-temperature (in winter) is near 

 the freezing-point, no effect is produced. Thus at 21°-22° 

 the blue-opaque margin was at 41-42 cm., showing that the 

 oxidation of phosphorus at zero is relatively negligible in 

 spite of the current of air. 



If, however, the same phosphorus be placed in the tube EG 

 at i, somewhere between the point of confluence and the colour- 

 tube, and where the temperature is say 20°, then it is actually 

 possible to obtain the yellow of the first order at steam-pressures 

 less than 1 cm. Thus at 19°, the yellow- opaque margin was 

 at 1*2 cm., and the colour persisted with increasing brilliancy 

 at all pressures above this. 



For temperatures greater than 20°, the tube is yellow at all 

 pressures until eventually above 35° all colour vanishes for 

 want of supersaturation. 



For temperatures below 20°, the tendency is to produce 

 opaque fields. Thus at 15° the tube is opaque at all pressures 

 above a few millimetres. 



The explanation of this somewhat puzzling behaviour is 

 this : at any given admissible temperature, the effect of 

 phosphorus dust is a change of the colour of the field in the 

 direction from blue through opaque to yellow in proportion 

 as more dust of the given kind is added. Again, the dust- 

 contents of the air passing over a given lump of phosphorus 

 decreases both with the rapidity of the current and with the 

 degree of cold. Hence at higher temperatures than 20° 

 brilliant brown-yellow fields are the usual occurrence when the 

 phosphorus lies in the air-tube i C. If withdrawn from the 

 air-tube and so circumstanced that its exhalation is diluted 

 with much air (tube F, fig. 1), then any colour may be pro- 

 duced, depending on the degree of dilution. On the other 

 hand, below 20° the oxidation takes place more and more 

 slowly, so that only -very gentle currents of air can carry off 

 enough dust to produce a yellow field. For strong currents 

 in C there is a double source of dilution, and opaque fields are 

 the rule. In other words, the air now approaches the state 

 AB in fig. 4, so far as dust-contents are concerned. 



I have entered into this subject at length because of its 

 important theoretical bearing, seeing that it is necessary to 

 disentangle a series of involved relations. 



13. In fig. 6 (diagram), the pair of curves ABC indicates 

 the margin of the opaque field for unusually pure atmospheric 

 air. Above the horizontal asymptote through B there is a 

 symmetrical disposition of browns, oranges, and yellows, the 

 order of colours decreasing upward. Below B the colours are 

 blues, greens, and hues of higher orders. The whole field to 



