332 Mr. J. Thomson on certain curiotts Motions 



such as is often met with, and is sometimes not easily avoided. 

 It should then be Hghtly dusted over with some fine powder not 

 apt to be quickly wet : Lycopodium powder will serve the pur- 

 pose. Then the tube filled with spu-it is to be dipped with its 

 open point into the surface of the water, and instantly a nearly 

 circular patch round the point of the tube Vidll be seen occupied 

 with liquid rushing outwards and completely divested of the 

 covering of powder, while on the part outside of that patch there 

 will be seen, by the motions of the powder, one, two, three, or 

 many radial streams flowing outwards from the middle, and other 

 return streams or eddies flowing backwards to the margin of the 

 patch, on arriving at which each particle seems suddenly as if 

 driven outwards wdth a rapid impulse. The margin of the cen- 

 tral patch is usually to be seen formed like as of leaves of a plant 

 growing out all round, and some superimposed on others, and 

 all in rapid motion. The nature and causes of these forms of 

 the margin, and of the eddies outside of the margin, I have not 

 as yet been able satisfactorily to explain. 



Another experiment may be made which is quite in accordance 

 with the explanations already given, and which, being due to con- 

 densation of alcohol on a surface of water, is interesting when 

 viewed in comparison with that in which the motions were shown 

 to be produced by evaporation : — If a silver spoon, perfectly 

 wetted with water so that a thin film adheres to it, be held over 

 an open cup or vessel containing strong alcohol, the surface of 

 the liquid will become greatly agitated with numerous motions, 

 which are to be attributed to the unequal and vai-ying condensa- 

 tion of the vapour of the alcohol at difierent parts of the surface 

 of the film, according as the vapour is wafted about in fumes by 

 the air. 



While engaged in the investigation of the phsenomena which 

 I have now described, my attention has been turned to some 

 other very interesting phjenomena previously observed by Mr. 

 Cornelius Varley, and described by him in the fiftieth volume of 

 the Transactions of the Society of Arts. He observed with the 

 aid of the microscope numerous motions of extremely curious 

 and wonderful characters in fluids undergoing evaporation. 

 Although I have not yet had it in my power to examine into all 

 the phsenomena he has discovered relative to these motions, yet 

 I think that many of them, or all, are to be explained according 

 to the principles I have now proposed. 



I have not had access to the Transactions of the Society of 

 Arts to read Mr. Varley's paper in full, but I quote the follow- 

 ing abstract of his results from Queckett's 'Treatise on the 

 Microscope,' 1st ed. p. 413: — "The plan recommended is as 

 follows : take an animalcule-cage of moderate size, and upon the 



