of Ammonia, Sulphur dioxide, and Carbon dioxide. 373 



caused to vibrate, emitting a strong shrill note, by stroking 

 it at one end with a rag moistened with water at the temper- 

 ature of the air in the room. The weight of the tube was 

 adjusted to resonance by attaching with marine glue circular 

 rings of sheet lead, hammered into a hemispherical shape 

 over a hard wood form, so as to fit exactly the ends of the 

 tube. Exact resonance was attained by varying the tem- 

 perature of the room slightly by opening the window until 

 the dust figures were sharp, the tube being allowed sufficient 

 time to take up the temperature of the surrounding air. 



Fig. 



BC D 



The air tube was 150 cm. long and 4*25 cm. internal 

 diameter. It was fitted with a cork piston movable by a 

 glass rod, and was firmly clamped in a horizontal position 

 with its axis along that of the gas tube. 



By shaking the gas tube and then tapping, the powder 

 was caused to settle in the form of a thin uniform line 

 extending the whole length of the tube. Even with every 

 precaution to avoid moisture, the silica adheres slightty to 

 the glass, but this does not interfere with the measurements. 

 The gas tube was now clamped with the silica at the bottom, 

 then by loosening the clamp slightly the tube was rotated 

 through about 45°. Silica was introduced into the air tube 

 by spreading the powder on a ruler, which, on tilting, left 

 a line of powder on the glass. The tube was rotated as 

 described. Fresh silica was used in the air tube for each, 

 experiment. 



The temperature was recorded by a thermometer placed 

 alongside the tubes. 



After a few trials, resonance was obtained in both tubes. 

 The position of the nodes was marked by strips of gummed 

 paper, and the distances were then measured by a calibrated 

 meter rule. 



The diagram of the dust figures shown in fig. 2 and the 

 Table I. show how the mean wave-lengths were calculated. 



