90 Professor W. Thomson on Mechanical 



or impeded by the same circumstances. Mr Church states 

 that he has by this means determined the presence or absence 

 of this essential oil in various mixtures, where it certainly 

 could not have been detected by ordinary means of analysis. 



Thinking that the green exhibited in certain directions by 

 the purple murexide might be due to this cause, we examined 

 it hy means of a screen of yellow uranium glass, or of sulphate 

 of quinine solution. Neither of these, however, prevented in 

 any measure the exhibition of the green colour. A solution 

 of murexide in water, also, displays no sign of fluorescence. 



I regret that I have been prevented by other scientific en- 

 gagements from working out the above miscellaneous observa- 

 tions so fully as I could have desired ; yet I send you the 

 present notice of them, hoping it may interest some of your 

 readers, and may lead to a further examination of the sub- 

 stances by parties more accustomed than myself to optical 

 experiments. I remain, my dear Sir, yours, &c. 



John H. Gladstone. 



London, Nov. 13, 1854. 



On Mechanical Antecedents of Motion, Heat, and Light. 

 By William Thomson, Esq., Professor of Natural Philo- 

 sophy, University of Glasgow. Communicated to the British 

 Association, Section A, Monday, Sep. 28, 1854. [Author's 

 Abstract.] 



This communication was opened with some general expla- 

 nations regarding mechanical energy, and the terms which 

 have been introduced to designate the various forms under 

 which it is manifested. Any piece of matter, or any group of 

 bodies, however connected, which either is in motion, or can 

 get into motion without external assistance, has what is called 

 mechanical energy. The energy of motion may be called 

 either " dynamical energy,"" or " actual energy." The energy 

 of a material system at rest, in virtue of which it can get into 

 motion, is called " potential energy." The author showed the 

 use of these terms, and explained the ideas of a store of 

 energy, and conversions and transformations of energy, by 

 various illustrations. A stone at a height, or an elevated re- 



