100 



Prof. Tyndall on a New Series of 



[Recess, 



ment of its particles. The cord finally disappeared, while the funnels 

 melted into two ghost-like films, shaped like parasols. The films were 

 barely visible, being of an exceedingly delicate blue tint ; they seemed 

 woven of blue air. To compare them with cobweb or with gauze would 

 be to liken them to something infinitely grosser than themselves. 



In a second trial the result was very much the same. A cloud which 

 soon assumed the parasol shape was formed in front, and five inches lower 

 down another cloud was formed, in which the funnels already referred 

 to were considerably sharpened. It was connected as before by a filament 

 with the cloud in front, and it ended in a spear-point which extended 12 

 inches further down the tube. 



After many changes, the film in front assumed the shape of a bell, to the 

 convex surface of which a hollow cylinder about 2 inches long attached 

 itself. After some time this cylinder broke away from the bell and formed 

 itself into an iridescent ring, which, without apparent connexion with 

 anything else, rotated on its axis in the middle of the tube. The inner 

 diameter of this ring was nearly an inch in length, and its outer diameter 

 nearly an inch and a half. 



The whole cloud composed of these heterogeneous parts was animated 

 throughout by a motion of rotation. The rapidity of the rotation could be 

 augmented by intensifying the beam. The disks, funnels, strata, and con- 

 volutions of the cloud exhibited at times diffraction colours, which changed 

 colour with every motion of the observer's eye. 



Moisture appeared to be favourable to the production of these ap- 

 pearances ; and it hence became a question how far they were really pro- 

 duced by the light : hydrobromic acid, even from its solution, fumes when 

 it comes into contact with the aqueous vapour of the air ; its residence 

 in water does not appear to satisfy its appetite for the liquid. The same 

 effect, as everybody knows, is observed in the solution of hydrochloric 

 acid. Might not, then, those wonderfully shaped clouds be produced by 

 an action of this kind, the presence of the light being an unnecessary 

 accident ? 



The hydrobromic acid was permitted to enter the experimental tube 

 and remain in diffuse daylight for five minutes. On darkening the room 

 and sending the electric beam through it, the tube was optically empty. 

 Two minutes' action of the light caused the clouds to appear, and they 

 afterwards went through the same variety of changes as before. 



No matter how long the hydrobromic acid was allowed to remain in the 

 tube, no action occurred until the luminous beam was brought into play. 

 The tube filled with the mixture of air, aqueous vapour, and hydrobromic 

 acid was permitted to remain for fifteen minutes in the dark. On send- 

 ing the beam through the tube it was found optically empty ; but two 

 minutes' action of the light developed the clouds as before. 



Permitting the beam to pass through a layer of water before entering 



