78 Dr. S. H. Milner on a 



near enough), the vessel of G30 c. c. holds '9 x 10~^ grs. of 

 mercury. This is spread into a film o£ 11 sq. cms. Thus the 

 thickness of the film is 6x 10-^. It was estimated that this 

 film transmitted about I of the light of the i)araffin lamp. 

 The transmitted light had a neutral tint. 



The coefficient of absorption of solid mercury calculated 

 from these data comes out 3 x lO'^. 



In order to make a fair comparison between the vapour 

 and the solid, the greater density of the latter must be 

 allowed for. By dividing the coefficients of absorption by the 

 respective densities, we get numbers by which the absorp- 

 tions of equal masses of material in the two states can be 

 compared. 



-r^ ,, ,T coefficient of absorption . , . 



For the vapour the ^ r- is less than 'Ob. 



^ density 



For the solid^ this constant has a value of something like 



2 X 10^ 



' Thus, if we suppose the intensity of a beam of light to be 



diminished in a given ratio by passing through a film of solid 



mercury, then a stratum of mercury vapour containing not 



less (and perhaps much more) than 2 x 10' times the mass of 



mercury per sq. cm. would be required to produce the same 



effect. 



XI. Note on a JSei^: Form of Vacuum Stopcock. By S. K. 

 MiLNER, D.Sc.j Lecturer in Physics, University Coller/e, 

 Sheffield *. 



AN obvious defect of the ordinary form of mercury-sealed 

 stopcock is that with it, although the interior of the 

 apparatus to which it is connected is sealed ofi:' by mercury 

 from the atmosphere, there is no mercury sealing between 

 the two portions of the apparatus joined by the stopcock ; 

 yet this is, in the usual form of stopcock, the route by which 

 a leak is most likely to take place. The only way of removing 

 this defect with which I am acquainted is the somewhat 

 inconvenient one of using a movable reservoir by which 

 mercury can be let into or out of one of the connecting- tubes 

 to the stopcock. To obviate the necessity for this addition 

 in a case where special airtightness of the tap was desired, I 

 have recently made use of a form of stopcock which interposes 

 a mercury sealing to the entrance of air into the apparatus by 

 every route along which a leak might take place. 



* Commuuicated bv the Author. 



