﻿22 Mr. Carl Barus on 



is put into the basket. The dusty exhalation of a freshly cut 

 surface of phosphorus is almost nil at 0° C, but increases 

 with great rapidity when the temperature rises, § 26. At 

 any given temperature above 15° its dust supply is nearly 

 constant for a long period of time (hours) ; hence its avail- 

 ability in the present work, unless the weather is very cold. 

 Fortunately cold temperatures can usually be dispensed with 

 when artificially dusty air is examined. 



It is to be noted that all the tubes E, D, F must pass out 

 of the room. If, for instance, b opened into the room (which 

 would often be desirable for the reasons just mentioned) , then, 

 if the jet is not in action or only slightly in action, cold air 

 would pass into C and D, and out at b as well as at B into 

 the room. At low steam-pressure the flow of dusty air 

 would, therefore, necessarily be irregular. An advantage is 

 secured by making the common tube i C long, so that the 

 air may be well mixed before impinging upon the jet. At 

 best, however, air dusted in this way is an inferior substitute 

 for atmospheric air, and. the results show much more fluctua- 

 tion. Each of the valves, h,f, e, is provided with a suitable 

 dial and an index. The valve e must be free from leaks. It 

 is best, moreover, when atmospheric air is examined to re- 

 move the basket k altogether, and to close the hole with a 

 cork. The hole through which F discharges into E need not 

 be more than a few millimetres in diameter, and it is advisable 

 to carry it into the axis of E by aid of a glass tube. The 

 whole train of tubes is easily made of ordinary tinned drain- 

 pipe and suitable elbows. To summarize : the faint phos- 

 phorescent glow visible on phosphorus in the dark is a nearly 

 permanent dust-producer. This phosphorus-tainted air, dis- 

 charged through an -J-inch tube into the 2-inch tube of pure 

 air, usually produces persistent colour-effects at ordinary 

 temperatures. Thus the additional dilution * is less than yj^. 



5. A few remarks on the shortcomings of the apparatus 

 may be made here. It is clear in the first place that the 

 temperature of the air entering C will vary with the intensity 

 of the jet, i. e. the velocity of current, even if the other ad- 

 justments remain unaltered. For the more rapidly the air 

 passes through the drum D the less it is heated. This, 

 however, is no serious inconvenience since temperature is 

 measured at t. 



* It is best to provide the phosphorus tube with a glass stopcock in 

 place of the symbolic valve of fig. 1. On proper adjustment brilliant 

 colour-effects are obtained in this way for any reasonable length of time. 

 The line is quite permanent so long as the stopcock is left untouched, 

 but there is a change of colour whenever the degree of opening is 

 varied. 



