28 Dr. C. K. Akin on Calcescence. 



14 of the previous paper. At C is a right-angled crest ; and 

 the full line traversing C is that for which b = 1 . The dotted 

 continuations of the same line above C satisfy the same equation, 

 but do not belong to any possible wave-surface. The full lines 

 below C are wave-lines for which b has a series of values greater 

 than 1, viz. lyg-, lyy, ly 3 2 j & c - » an( ^ the dotted curves above 

 C fulfil the same equations, but are not possible wave-lines. 

 The dotted lines to the right and left of C are stream-lines 

 for which b has values less than 1, such as \~, T |, -fy, &c, and 

 are not continuous wave-lines. 

 Glasgow University, 

 November 19, 1864. 



R 



VII. On Calcescence. By Dr. C. K. Akin*. 



\ADTESCENCE. — The radiescent state of substances is 

 known to originate in three different modes, which may 

 be distinguished by the terms Spontaneous Radiation, Pro- 

 duction, and Reproduction of Rays. As spontaneous radiation 

 may be designated all those phenomena of ray-emission to which, 

 apparently, no immediate cause can be assigned, and which, in 

 the last instance, are probably owing to certain velocities im- 

 pressed on the molecules of matter from all beginning, together 

 with certain intermolecular relations, corresponding in some 

 degree to the primitive tangential tendency and attractive forces 

 which sustain the motions of the solar and planetary systems. 

 Under production of rays may be comprised all those pheno- 

 mena of radiation which are engendered by the agency of finite 

 movement, affinity, cohesion, or electricity ; whilst under repro- 

 duction those instances of radiation may be classed which arise 

 from the incidence or communication of rays derived from dis- 

 tant or contiguous sources. 



2. Renovation. — The reproduction of rays derived from distant 

 sources, to which it is wished generally to call attention in this 

 paper, may take place under circumstances of a double nature, 

 and is termed accordingly Diffusion (or Reflexion) and Renova- 

 tion — the term diffusion being common and well known, and 

 that of 'renovation introduced here with a view to avoid future 

 ambiguity. The important distinction which underlies these 

 terms may be best understood from the language employed by 

 Dr. Young for its elucidation (or rather for some more general 



* Being the abstract of two papers read before the Mathematical and 

 Physical Section of the British Association at Newcastle, in August and 

 September 1863. Reprinted, with additions and verbal corrections, from 

 No. 39 of the ' Reader' (September 26, 1863). N.B. Paragraphs newly 

 added will be enclosed within brackets. 



