416 Lord Rayleigli. On the Dark Plane [Dec. 21, 



produce the smoke, but other substances, such as sulphur and phos- 

 phorus, have been tried. The experiment was not commenced until 

 the smoke was completely formed, and had come nearly to rest. In 

 some respects the most striking results were obtained from a copper 

 blade, about J-inch broad, formed by hammering flat one end of a 

 stout copper rod. The plane of the blade was horizontal, and its 

 length was in the line of sight. The unhammered end of the rod 

 projected from the box, and could be warmed with a spirit-lamp. 

 The dark plane was well developed. At a moderate distance above 

 the blade it is narrow, sometimes so narrow as almost to render neces- 

 sary a magnifying glass ; but below, where it attaches itself to the 

 blade, it widens out to the full width, as shown in the figure. "Whether 



bhe heated body be a thin blade or a cylindrical rod, the fluid passes 

 round the obstacle according to the electrical law of flow, the stream- 

 lines in the rear of the obstacle being: of the same form as in front of 

 it. This peculiarity of behaviour is due to the origin of the motion 

 being at the obstacle itself, especially at its hinder surface. If a 

 stream be formed by other means, and impinge upon the same 

 obstacle without a difference of temperature, the motion is of a 

 different character altogether, and eddies are formed in the shadow. 



The difference of temperature necessary to initiate these motions 

 with this dark plane accompaniment is insignificant. On July 20,. 

 1881, a glass rod, about J-inch in diameter, was employed. It was 

 heated in a spirit-lamp, and then inserted in the smoke-box. The 

 dark plane gradually became thinner as the rod cooled, but could be 

 lollowed with a magnifier for a long time. While it was still quite 

 distinct the experiment was stopped, and on opening the box the glass 

 rod was found to be scarcely warmer than the fingers. It was almost 

 impossible to believe that the smoky matter had been evaporated. 



In order to test the matter more closely, smoke was slowly forced 

 through a glass tube heated near the end pretty strongly by a spirit- 

 lamp, and then allowed to emerge into the concentrated sunshine. 

 No distinct attenuation of the smoke could be detected even under 

 this treatment. 



It is not necessary to dwell further upon these considerations, as the 

 question may be regarded as settled by a decisive experiment tried a 

 few days later. The glass rod before used was cooled in a mixture of 

 salt and ice, and after wiping was placed in the box. In a short time 



