110 ANNUAL OF SCIENTIFIC DISCOVERY. 



known that when a receiver filled with ordinary undried air is ex- 

 hausted, a cloudiness, due to the precipitation of the aqueous 

 vapor dilVused in the air, is produced by the first lew strokes of 

 the pump. It is, as mii^ht be expected, possible to produce 

 clouds in this way with the vapors of otlu-r liquids than water. 

 In the course of the experiments on tlie chemical action of light 

 which have been already communicated in abstract to the Royal 

 Society, I liad frequent occasion to obsen'c 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 dilat- 

 ing the air by working the pump. In the other, the experi- 

 mental 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 e^xhausted ; on turn- 

 ing the cock the air rushed from the experimental tube into the 

 vessel, the precipitation of a cloud within the tube being a con- 

 sequence of the transfer. Instead of a special vessel, the cyl- 

 inders of the air-pump itself were usually employed for this 

 purpose. It was found possiljle, by shutting off the residue of 

 air and vapor after each act of precipitation, and again exhaust- 

 ing the cylinders of tlie pump, to obtain with some substances, 

 and without refilling the e.\i)erimental tube, 1-5 or 20 clouds in 

 succession. The clouds thus precipitated diilercd from each 

 other in luminous energy, some shedding forth a mild white 

 light, others fiashing out with sudden and suri)rismg brilliancy. 

 This difference of action is, of course, to be referred to the dif- 

 ferent reflective energies of the particles of the clouds, which 

 were produced by substances of very different refractive indices. 

 Difl'erent clouds, moreover, possess veiy different degrees of 

 stability ; some melt away rapidly, while others linger for min- 

 utes 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 splendor of 

 the diffraction phenomena exhibited by some of these clouds; 

 the colors are best seen by looking along the expi'rimental tube 

 from a point above it, the face I)eing turned towards the source 

 of illumination. The differential motions introduced by friction 

 against the interior surface of the tube often cause the colors to 

 arrange themselves in distinct layers. The difi'crence in texture 

 exhibited by dift'erent clouds caused me to look a little more 

 closely than I had previously done into the mechanism of cloud- 

 formation. A certain expansion is necessaiy to bring down the 

 cloud; the moment before precipitation the mass of cooling air 

 and vapor may be regarded as divided into a number of polyhe- 

 dra, 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 vapor in its forma- 



