320 Royal Society : — 



elliptic sector, areas o£ roulettes, the theory of Amsler's plani- 

 meter, Landen's theorem on the length of. a hyperbolic arc, Stei- 

 ner's theorem on the rectification of roulettes, and some others. 



It will be seen, from this brief account of the contents, that the 

 work is of a strictly elementary character. Such subjects as the 

 expansion of functions in trigonometrical series, elliptic functions, 

 and the Calculus of Variations are simply omitted ; while the trans- 

 formation of the independent variable and double integration are 

 but briefly noticed. "Within the limits which the author has assigned 

 to himself, however, the treatment is very full and satisfactory ; and 

 the work is well adapted to the wants of those for whom it is writ- 

 ten, viz. students in the Universities, very few of whom (as we 

 suppose) will find it necessary to enter into the subject beyond the 

 contents of the present volume. 



XXXVI. Proceedings of Learned Societies. 



ROYAL SOCIETY. 



[Continued from p. 237-] 



June 11, 1874. — Joseph Dalton Hooker, C.B., President, in the 



Chair. 

 nPHE following communications were read : — 

 -^ " Spectroscopic Notes. — No. III. On the Molecular Structure 

 of Vapours in connexion with their Densities." By J. Norman 

 Lockyer, F.E.S. 



1. I have recently attempted to bring the spectroscope to bear 

 upon the question whether vapours of elements below the highest 

 temperatures are truly homogeneous, and whether the vapours 

 of different chemical elements, at any one temperature, are all in a 

 similar molecular condition. In the present note, I beg to lay 

 before the Boyal Society the preliminary results of my researches. 



2. AVe start with the following facts : — 



I. All elements driven into vapour by the induced current 



give line-spectra. 



II. Most elements driven into vapour by the voltaic arc give 



us the same. 



III. Many metalloids when greatly heated, some at ordinary 

 temperatures, give us channelled-space spectra. 



IV. Elements in the solid state give us continuous spectra. 



3. If we grant that the spectra represent to us the vibrations 

 of different molecular aggregations (this question is discussed in 

 Note II.), spectroscopic observations should furnish us with facts 

 of some importance to the inquiry. 



4. To take the lowest ground. If, in the absence of all know- 

 ledge on the subject, it could be shown that all vapours at all stages 

 of temperature had spectra absolutely similar in character, then it 

 would be more likely that all vapours were truly homogeneous and 



