190 



Capt. Abney. On Coloured Spectra. [June 19, 



XL " On the Production of Coloured Spectra by Light." By 

 Captain Abney, R.E., F.R.S. Received June 9, 1879. 



(Preliminary Note.) 



Last year I incidentally mentioned in a note to the Royal Society 

 ("Proceedings," vol. xxviii, p. 291), that the production of natural 

 colours by the agency of light, examples of which were shown by 

 Becquerel, was probably caused by the oxidation of silver compounds 

 employed. I have ventured to return to the subject, in order to show 

 that the colours are so produced and are not due to interference. 



I have sent, for the Society's inspection, pictures of the solar 

 spectrum on silver plates, and also on compounds of silver held in situ 

 by collodion. It will be observed that the spectrum has imprinted 

 itself in approximately its natural colours ; that on the silver plates it 

 is more brilliant than on the collodion film, but that in the latter the 

 colours are seen by transmitted as well as by reflected light. 



I reserve for the present the exact details of the production of these 

 pictures, but may say that they are produced by oxidation of silver 

 compounds when placed in the spectrum ; an exposure of two minutes 

 being amply sufficient with a wide slit to impress the colours. The 

 colouring matter seems to be due to a mixture of two different sizes of 

 molecules of the same chemical composition, one of which absorbs at 

 the blue end and the other at the red end of the spectrum, and the 

 sizes of these molecules are unalterable whilst exposed to the same 

 wave-lengths as those by which they were produced. I believe it 

 possible and probable that the colours may be preserved unchanged 

 when exposed to ordinary daylight. 



XII. " Relations between the Atomic Weights and certain Phy- 

 sical Properties (Melting and Boiling Points and Heats of 

 Formation) of Elements and Compounds." By Thomas 

 Carnelley,D. Sc., Assistant Lecturer on Chemistry in Owens 

 College, Manchester. Communicated by Professor H. E. 

 ROSCOE, F.R.S. Received June 10, 1879. 



(Abstract.) 



The object of the present paper is to trace the influence of the 

 atomic weights on the melting and boiling points and heats of forma- 

 tion of elements, and especially of their compounds. It is shown that, 

 as regards the elements, the melting points are a periodic function 



