48 PROF. OSBORNE REYNOLDS ON THE MANNER IN 



tained a substance which, after purification, crystallized from 

 alcohol in small needles strongly resembling, but certainly 

 not identical with alizarine. It is probably an isOmeride 

 of purpurine. Isoanthraflavine, Avhen treated in the same 

 way, yields a body which has all the properties of Mr. Per- 

 kin's anthrapurpurine. 



VIII. On the Manner in which Raindrops and Hailstones 

 are formed. By Professor Osborne Reynolds, M.A. 



Read October 31st, 1876. 



When the particles of water or ice which constitute a 

 cloud or fog are all of the same size, and the air in Avhich 

 they are sustained is at rest or is moving uniformly in one 

 direction, then these particles can have no motion relatively 

 to each other. The weight of the particles Avill cause them 

 to descend through the air with velocities wliicli depend on 

 their diameters ; and since they are all of tlie same size, 

 they will all move with the same velocity. 



Under these circumstances, therefore, the particles will 

 not traverse the spaces which separate them, and there can 

 be no aggregation so as to form raindrops or hailstones. 



If, however, from circumstances to be presently consi- 

 dered, some of the particles of the cloud or fog attain a larger 

 size than others, these will descend faster than the others, 

 and will consequently overtake those immediately beneath 

 them ; with these they may combine so as to form still 



