﻿Coloured Cloudy Condensation. 23 



Similarly the amount of dust introduced into the air will 

 (probably) depend on the rate at which the current passes 

 the basket at k. Hence at great jet intensities the air will 

 be less dusty than for small intensities. I have found no 

 easy way by which this discrepancy can be evaluated ; and 

 my experiments with artificially dusted air are intended 

 rather to show the character of the dust variation than to 

 map out precise loci, §§ 11-13. Fortunately the dust effect 

 is so striking that there is no possibility of misinterpretation. 

 Experiments which I made by introducing dust with jet 

 pumps and aspirators showed few advantages. To vary the 

 dust-contents uniformly at all jet-pressures, the mouths c, d 

 of the air-tubes must be introduced into a large artificially 

 dusted room, instead of the atmosphere. But this method 

 also presents grave difficulties. The final resort seems to be 

 to examine the atmosphere at different times and in different 

 places, or to construct apparatus for the rapid filtration of 

 air, §§ 14, 15. 



One serious theoretical question may be referred to here. 

 It is necessary that at all steam-pressures the amount of air 

 entering C should be nearly proportioned to the amount of 

 issuing steam. No doubt this is nearly the case ; for not 

 only do the air- and steam-currents increase and decrease to- 

 gether, but the air is admitted in excess of the quantity neces- 

 sary to produce condensation at the supersaturated parts of 

 the jet, and it is to this condensation that the colour indica- 

 tions apply. If the valve e be closed, and the valves /and h 

 all but closed, the pressures at which the margin of the opaque 

 zone appears from blue increase ; but the temperature regis- 

 tered at t also increases at even a greater relative rate, so that 

 the apparent effect is, curiously enough, rather an excess than 

 a deficiency of dust (§ 13). 



I infer from this that in the work below the air is always 

 admitted in quantity sufficient to produce its maximum dust 

 effect. To test this question preliminarily, I replaced the 

 0*16 cm. nozzle by another 0*09 cm. in diameter, and thus 

 (ccet. par.) only discharging one third as much steam as the 

 former. The new results virtually coincided with the old 

 (§ 15) ; and hence, though the relations below were obtained 

 from a given apparatus, they are probably true generally * so 



* It will be expedient to consider the small differential effect of varia- 

 tions of the barometer, and the tendency of the pressure corresponding 

 to " blue-opaque " to fall with the time of efflux, &c., in the Bulletin 

 cited. Brass nozzles corrode in the course of time, so that it is advisable 

 to make them of platinum. With some unpolished nozzles the " yellow- 

 blue " asymptote may fall even as low as 20 centim. of mercury at 30° air- 

 temperature. 



