Fluorescence and Photo-Chemistry . 757 



radicle, though, as we have seen, we could not get them of 

 carbon atoms. We know of many compounds which contain 

 chains containing very large numbers of CH 2 . 



Summary, 



A theory of the structure of solids is given based on the 

 views as to the nature of the structure of the atom and 

 the mechanism of chemical combination which I have given 

 in previous papers in the Philosophical Magazine. Since 

 the atom of a monovalent element has one disposable electron, 

 that of a divalent element two, and so on, there must in 

 a crystal of a monovalent element be such an arrangement 

 of electrons and atoms that for each atom there is one 

 electron, in a crystal of a divalent atom there must be two 

 electrons for each atom, and so on.. It is shown that this 

 condition determines the crystallographic forms in which 

 the various elements can crystallize, and leads to a con- 

 ception of the structure of crystalline solids which allows us 

 to calculate without further assumptions the values of 

 certain physical constants such as the bulk modulus, the 

 value of the critical frequency, and the dielectric constants. 

 The values of these are calculated for elements of different 

 valencies, and are found to agree closely with those found 

 by experiment. 



LXXXVIII. Fluorescence and Plioto- Chemistry. 

 By B... W. Wood, Johns Hopkins University *. 



[Plate XVI.] 



THOUGH it has been known for some time that many 

 fluorescent solutions are bleached by the action of 

 light, Perrin appears to have been the first to definitely 

 associate fluorescence with chemical change and to regard 

 the light* emitted by fluorescent substances as due to the 

 " flashes " of exploding molecules. Perrin showed that 

 thin films of a solution of fluorescein were bleached at 

 the spot on which the ligh't was focussed by the sub-stage 

 parabolic reflector used for the examination of colloidal 

 particles under the microscope. He also examined other 

 fluorescent substances and found that in all cases they were 

 bleached with greater or less rapidity by the action of 

 light. 



* Communicated by the Author. 



