﻿26 Mr. Carl Barus on 



temperature and pressure at which the colours disappeared 

 noted on cooling. The mercury-thermometer is scarcely 

 sensitive enough for such observations, and the temperatures 

 of the diagram are probably too high. I have therefore 

 lumped all my observations between Feb. 10 and 23, 1893, 

 in this chart, seeing that the phenomenon as a whole is 

 well represented. 



7. In the following work, however, the apparatus, Plate V. 

 fig. 1, was used, with the phosphorus-tube closed up and 

 the phosphorus removed. Great care was taken to wait 

 for stationary temperatures, and about five (or more) steps 

 between 10° C. and 40° C. were selected for observation. 



The first set of experiments was made on Feb. 23, the 

 chart*, fig. 3, curve A being obtained in the morning, 

 and fig. 4 in the afternoon. The day was cold, with 

 snow covering the ground. The blue-opaque curve, A, fig. 3, 

 virtually reproduces fig. 2 ; but the curve, fig. 4, differs from 

 it inasmuch as the tangential angles in the latter case are 

 steeper, so that the locus is less curved and rises higher 

 than in figs. 2 or 3. In all cases yellow-opaque lies above 

 blue-opaque. I was at first inclined to refer this to differ- 

 ences of the vanishing standard, believing the two curves 

 to contain consistent observations, but differing from each 

 other for reasons purely subjective. Whether or not this is 

 the case can only be found by comparison with succeeding 

 series of observations, as will presently be seen. Taking the 

 observations at their face value, the indication is less dust for 

 the afternoon than for the morning. The curve P found for 

 artificially dusty air will be described below (§ 11). 



8. The next series of observations were made on Feb. 27 

 (cloudy), 28 (rain), and on March 2 (clear). There was but 

 little difference in the respective loci of the data except that 

 on the latter day the asymptote was somewhat below the 

 position for the other days (see chart, Plate V. fig. 5). 

 The common asymptote takes a mean position (pressure, 

 p = 43 cm.) between the corresponding values of figs. 2 

 and 3 (j? = 42 cm.) and fig. 4 Qt? = 48 cm.). 



9. On March 3, however, the asymptote rose again to the 

 value j> = 46 cm. The weather was cloudy, antedating 

 the storm of March 4, 1893. Two series of observations were 

 made. 



Finally, the results of March 6, 8, 10 agree in character 

 with fig. 5 ; while during the intermediate date, March 7, 

 the asymptote fell to the lower position p = 42 cm. These 



* The observations FF' in fig. 3 refer to filtered air and will be 

 described in § 14. 



