130 Mr Bevan, Note on some Physical 



vacuum it distils practically instantaneously like any other vapour. 

 The method adopted was to attach a bulb of hardest Jena glass to 

 a mercury pump, having first placed some metallic sodium in the 

 stem of the bulb ; and then after exhaustion to melt the sodium 

 and shake a small globule into the bulb. After this introduction 

 of sodium into the bulb it was necessary to again exhaust with 

 the pump, as a good deal of hydrogen was given off from the 

 sodium and also from the action of sodium on sodium hydroxide 

 which is always present with the original sodium. If a good 

 vacuum was produced and then the sodium in the bulb heated no 

 definite cloud could be observed, but the sodium distilled at once 

 and settled on the cooler parts of the bulb. The distillation is in 

 fact so easy that the sodium can in a few minutes be distilled 

 from the stem into the bulb almost completely. It thus appears 

 that in vacuo the sodium vapour behaves as regards distillation 

 like any other vapour. 



It was found also that in an atmosphere of hydrogen the 

 definite cloud could not be observed if the hydrogen were per- 

 fectly dry and no sodium hydroxide were present. The absence 

 of oxygen thus seems to destroy this cloud-forming property. 

 At present I have not determined whether pure dry oxygen 

 facilitates the formation of the cloud, but this is a point which 

 I hope to decide later. If however into the bulb which has 

 been dried thoroughly and exhausted, sodium be distilled, and 

 then hydrogen dried with phosphorus pentoxide be admitted, 

 no definite surface to the sodium vapour can be observed on 

 heating. Near the heated sodium there can be seen with sodium 

 light a cloud of the vapour, but this cloud gradually decreases 

 in density and fades into the region of no absorption without 

 any definite boundary. In fact the sodium vapour behaves in 

 this case just as iodine vapour does when a small piece of iodine is 

 heated gently. 



It seems that when an ordinary piece of sodium is heated in 

 vacuo a large quantity of hydrogen is given off, some of it oc- 

 " eluded originally in the sodium and some of it formed by the 

 action of sodium on sodium hydroxide, and this gives rise to 

 an atmosphere sufficiently dense to enable the sodium vapour 

 cloud to form. 



These experiments are very easily performed in a Jena bulb, 

 and such bulbs can be used for a considerable time if they are 

 well heated before the introduction of the sodium. Without this 

 preliminary heating the sodium acts on the surface of the bulb, 

 owing apparently to a film of moisture on the surface, and causes 

 the glass to be coated with a white non-transparent film. At 

 a high temperature the glass appears to dissolve the sodium, 

 forming a brown transparent glass. 



