Dr. Tyndall on the Formation and Phenomena of Clouds. 157 



which have been already communicated in abstract to the Royal 

 Society, I had frequent occasion to observe the precipitation of such 

 clouds in the experimental tubes employed ; indeed several days at a 

 time have been devoted 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 conse- 

 quence of the transfer. Instead of a special vessel, the cylinders of 

 the air-pump itself were usually 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 par- 

 ticles of the clouds, which were produced by substances of very dif- 

 ferent refractive indices. 



Different clouds, moreover, possess very different degrees of sta- 

 bility ; some melt away rapidly, while others linger for minutes in 

 the experimental 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 precipitation 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- 



