produced by Cooling Gases to Low Temperatures, 535 

 Carbon Dioxide. 



This gas was prepared from hydrochloric acid and marble 

 and dried by passing through tubes of calcium chloride and 

 phosphorus pentoxide. The results obtained with C0 2 are 

 illustrated by those given in the following table : — 



Pressure in 

 centims. 



Temperature 

 of cooling. 



Effect obtained : ^ , 

 ■> a ^ u \ Kemarks. 

 in cloud chamber. 



35 

 35 

 Pressure could not 1 

 be kept constant. J 



-73° C. 



-32 



-87 



5 to 10 drops. 



Thin shower. 



~ -, -, , -i f Some of the gas 

 Coloured cloud. ^ aolidified> g 



80 



80 



80 



Pressure could not 1 



be kept constant J 



-67 

 -70 

 -72 



-73 



No effect. 



10 to 15 drops. 



Grood shower. Some gas solidified 



Coloured cloud. Ditto. 



117 



Pressure could not 1 

 be kept constant. J 



-66 



-68 



5 or 6 drops. 



Very heavy rain Gas solidified. 



We shall now consider certain possible explanations of the 

 production of these nuclei and discuss their validity. 



(1) By some direct action upon the walls of the cooled tube. — 

 This was disproved at the outset of the investigation by 

 direct experiment, the gas in the tester being air. A double 

 " tester " was made consisting of two similar glass U-tubes 

 arranged side by side "in parallel" and symmetrically placed 

 with respect to the tube B and tap D (see figure). One of 

 the U-tubes contained a number of glass flakes obtained by 

 breaking up a bulb of very thin glass. There was thus 

 in this " tester " a much larger extent of glass surface, the 

 volume of the air inside not being appreciably diminished. 

 Cooling tests (by means of liquid air) were performed upon 

 the " testers " alternately, and it was found that the effects 

 obtained were practically the same ; if anything, the " tester M 

 containing the flakes of glass gave a slightly smaller effect. 

 This disproves the explanation that the effect is due to some 

 action at the glass surface. There is also indirect evidence 

 against this view. For, as we have shown, the effects depend 

 in all cases upon the pressure and nature of the gas inside the 

 " tester." Contrast for example the results for C0 2 and 

 Hydrogen. 



