88% Mr. Joule ow Specific Heat. 



that the laws of propagation just obtained for circular vibra- 

 tions are equally true for elliptical and rectilineal vibrations ; 

 for, as is well known, we may produce elliptically or plane 

 polariaed light by superposing two rays of circularly polarized 

 light. 



The second consequence is this, that 



^ = C, - C3W2 + C^n^ &c. 



^ = n (G2 - C4«2 + c,n^ &c.), 



where Cj C^ Cg &c. are constants independent of ?i and 



of the amplitude of vibration. I shall reserve the proof of this 

 for my next communication, in which I shall also attempt an 

 explanation of the rotation of the plane of polarization pro- 

 duced by certain liquids, the peculiar absorbing power of tour- 

 maline, and some other phaenomena. 

 Balham, Surrey, September 24, 1844. 



LlII. On Specific Heat. By J. P. Joule, Esq.^ 



rp^HE well-known law which applies to the specific heat of 

 A many simple bodies, while interesting in itself as one of 

 the doctrines of physical science, is of great importance to 

 theoretical chemistry as a criterion for the determination of 

 atomic weights. Dulong and Petit, the philosophers who first 

 announced that the specific heat of simple boilies is inversely 

 proportional to their atomic weights, proved their proposition 

 by experiment in the case of several solid (chiefly metallic) 

 bodies. Subsequently several attempts have been made to 

 discover the law of the specific heat of gases, liquids and com- 

 pound bodies. With regard to gases, Haycrattfj and subse- 

 quently De la Rive and Marcet J, have attempted to prove that 

 under the same pressure and volume all gases have the same 

 specific heat. Unfortunately, the practical difficulty of ascer- 

 taining the specific heat of aeriform fluids is so great, that consi- 

 derable uncertainty exists in the results obtained by the most 

 skilful experimenters, and hence we find that the law of Hay- 

 craft, De la Rive and Marcet has failed to be confirmed by the 



* Communicated by the Author, having been read before the Chemical 

 Section of the British Association at York, September 27, 1844. 



t Trans, of the Royal Society of Edinburgh, vol. x. part 1. p. 215. (or 

 Phil. Mag. S, 1. vol. Ixiv. p. 200.) 



X Armales de (Jhimie, 1827, tome xxxv. p. 27. 



