318 On the Formation and Phenomena of Clouds. [Mar. 4, 



solely to the generation and examination of clouds formed by the sudden 

 dilatation of the air in the experimental tubes. 



The clouds were generated in two ways : one mode consisted in opening 

 the passage between the filled experimental tube and the air-pump, and 

 then simply dilating the air by working the pump. In the other, the 

 experimental tube was connected with a vessel of suitable size, the passage 

 between which and the experimental tube could be closed by a stopcock. 

 This vessel was first exhausted ; on turning the cock the air rushed from 

 the experimental tube into the vessel, the precipitation of a cloud within 

 the tube being a consequence of the transfer. Instead of a special vessel, 

 the cylinders of the air-pump itself were usually s employed for this 

 purpose. 



It was found possible, by shutting off the residue of air and vapour after 

 each act of precipitation, and again exhausting the cylinders of the pump, 

 to obtain with some substances, and without refilling the experimental 

 tube, fifteen or twenty clouds in succession. 



The clouds thus precipitated differed from each other in luminous 

 energy, some shedding forth a mild white light, others flashing out with 

 sudden and surprising brilliancy. This difference of action is, of course, to 

 be referred to the different reflective energies of the particles of the clouds, 

 which were produced by substances of very different refractive indices. 



Different clouds, moreover, possess very different degrees of stability ; 

 some melt away rapidly, while others linger for minutes in the experi- 

 mental tube, resting upon its bottom as they dissolve like a heap of snow. 

 The particles of other clouds are trailed through the experimental tube as 

 if they were moving through a viscous medium. 



Nothing can exceed the splendour of the diffraction-phenomena exhibited 

 by some of these clouds ; the colours are best seen by looking along the 

 experimental tube from a point above it, the face being turned towards the 

 source of illumination. The differential motions introduced by friction 

 against the interior surface of the tube often cause the colours to arrange 

 themselves in distinct layers. 



The difference in texture exhibited by different clouds caused me to look 

 a little more closely than I had previously done into the mechanism of 

 cloud-formation. A certain expansion is necessary to bring down the 

 cloud ; the moment before precipitation the mass of cooling air and vapour 

 may be regarded as divided into a number of polyhedra, the particles along 

 the bounding surfaces of which move in opposite directions when precipita- 

 tion actually sets in. Every cloud-particle has consumed a polyhedron of 

 vapour in its formation ; and it is manifest that the size of the particle 

 must depend, not only on the size of the vapour polyhedron, but also on 

 the relation of the density of the vapour to that of its liquid. If the 

 vapour were light, and the liquid heavy, other things being equal, the 

 cloud-particle would be smaller than if the vapour were heavy and the 

 liquid light. There would evidently be more shrinkage in the one case 

 than in the other : these considerations were found valid througohut the 



