272 



Lord Kelvin. On the 



[Feb. 13, 



that vacuum is not essential for the generation of the Rontgen light 

 might seem to be demonstrated by an experiment by Lord Blythswood, 

 wh ch he described at a meeting of the Glasgow Philosophical Society 

 last. Wednesday (February 5). As a result he exhibited a glass photo- 

 graphic dry plate with splendidly clear marking which had been pro- 

 duced on it when placed inside its dark slide, wrapped round many 

 times in black velvet cloth, and held in front of the space between the 

 main electrodes of his powerful Wimshurst electrical machine, but not 

 in the direct line of the discharge. He also exhibited photographic 

 results obtained from the same arrangement with only the differ- 

 ence that the dark slide, wrapped in black velvet, was held in the 

 direct line of the discharge. In this case the photographic result 

 was due, perhaps wholly, and certainly in part, to electric sparks or 

 brushes inside the enclosing box, which was, as usual, made of 

 mahogany with metal hinges and interior metal mountings. It is not 

 improbable that the results of the first experiments described by Lord 

 Blythswood may also be wholly due to sparking within the wooden 

 case. I have suggested to him to repeat his experiments with a 

 thoroughly well closed aluminium box, instead of the ordinary photo- 

 graphic dark slide which he used, and without any black cloth 

 wrapped round outside. The complete metallic enclosure will be a 

 perfect guarantee against any sparks or brashes inside. 



If the arrangement which I now suggest, with no sparks or brushes 

 between A A and the roof, gives a satisfactory photographic result, or 

 if it shows a visible glow on phosphorescent material placed any- 

 where in the space between AA and the roof above it, or above the 

 aluminium roof, it would prove the truth of Rontgen's hypothesis. 

 But failure to obtain any such results w 7 ould not disprove this hypo- 

 thesis. The electric action, even with the place of the spark so close 

 to the field of the action sought for as it is at D, in the suggested 

 arrangement, may not be sudden enough or violent enough to produce 

 enough of longitudinal waves, or of condensational-rarefactional 

 vibrations, to act sensibly on a photographic plate, or to produce a 

 visible glow on a phosphorescent substance. 



(Extract from ' Nature,' referred to above.) 

 " Velocity of Propaaation of Electrostatic Force. 



"Dr. Bottomley's note published in ' Nature,' of January 23, quotes 

 an extract from my Baltimore Lectures of October, 1884, in which 

 this subject is spoken of, with an illustration consisting of two metal 

 spheres at a great distance asunder, having periodically varying oppo- 

 site electrifications maintained in them by a wire connecting them 

 through on alternate current dynamo. 



** For an illustration absolutely freed from connecting wire and all 



