Decembeb 24, 1909] 



SCIENCE 



931 



water and hence more or less completely satu- 

 rated. Close pinch-cock at p firmly. Now by 

 compressing HB moderately and then releas- 

 ing suddenly a dense cloud, in general, will be 

 formed in B.^ Repeated compressions and ex- 

 pansions will bring down clouds of raiiidly 

 diminishing densities. The bulb B will be 



S=P 



Rg. I. 



freed of dust particles by about the tenth ex- 

 pansion — depending upon the dustiness of the 

 air originally drawn into the apparatus. 

 Having freed the condensing chamber of dust 

 particles, a dense cloud can again be formed 

 by compressing the bulb firmly and then re- 

 leasing. In this instance the expansion — i. e., 

 the ratio of the final to the initial volume of 

 the gas — is greater than the critical value 

 which for dust-free air is about 1.30. 



The apparatus is now ready for the perf orm- 

 ' J. J. Thomson's " Conduction of Electricity 

 through Gases," p. 107. 



ance of a number of interesting and striking 

 experiments. The formation of a single drop 

 in the expansion chamber is not an uncom- 

 mon sight. As is well known when the drops 

 are few they are of large size and fall rapidly, 

 while dense clouds formed in dust-laden air, 

 or in dust-free air exposed to an ionizing 

 agent, are composed of small drops, exhibit 

 color effects and often may be quite opaque. 

 This cloud settles slowly. The effect of dust 

 is shown in a marked way by drawiiig into B 

 a whiff of air laden with chalk dust. An ex- 

 ceedingly opaque cloud is obtained by present- 

 ing the nipple n to a burning match and draw- 

 ing in some of the particles of carbon. It re- 

 quires some twenty or thirty expansions to 

 free the bulb of these particles. Dense clouds 

 are formed by drawing in the gases through 

 which an electric discharge is passing. The 

 ionized air from the active side of a Koentgen 

 ray bulb gives a marked effect. These cloud 

 effects can be projected readily on a screen. 



TABLE I 



Critical Expansion in Dust-free Air 



For quantitative results the apparatus is 

 modified slightly. The trough in B may be 

 omitted. The tube T should be 20 to 25 cm. 

 long and graduated in cubic centimeters as 

 shown in Fig. 2. The hand bulb HB and the 

 tube T are filled with water to some conve- 

 nient zero on the graduation. To operate, com- 

 press HB sufficiently to give the desired ratio 

 on expansion. The free water column acts as 

 an index and its proximity to B insures satu- 

 ration. It is imperative that the walls of HB 

 be of double or treble strength, otherwise the 

 bulb on release will expand beyond the set 

 zero. For individual observation the bulb B 



