On the Boundary of a Wave of Conducted Heat. 257 



have long ago been deduced by analogy on the ground of the 

 experience obtained on other substances. 



The result of this investigation might be regarded as a new 

 and brilliant confirmation of the universal validity of the law of 

 Dulong and Petit ; but it might be better to look upon what we 

 have found as a strong argument against the validity of that 

 law; for the law loses all physical and chemical value as soon as 

 its validity is essentially dependent on the temperature. 



As the great variability found in the specific heat of carbon 

 gives us excellent occasion to submit to closer investigation 

 several most important questions in the mechanical theory of 

 heat, in relation to true heat-capacity and internal work (for 

 which the above-given material of observation does not quite 

 suffice), I will undertake further investigations as soon as the 

 meteorological conditions make the employment of the ice calo- 

 rimeter again possible ; and before all I will endeavour to ascer- 

 tain the behaviour of the specific heat of the diamond at tempe- 

 ratures between —100° and 0°. 



Berlin, Laboratory of Geb.-Rath Helmholtz, 

 31st March, 187l\ 



XXXII. On a precise Method of tracing the Progress and of de- 

 termining the Boundary of a Wave of Conducted Heat, By 

 Alfred M. Mayer, Ph.D., Professor of Physics in the Ste- 

 vens Institute of Technology, Hohoken, N. J., U.S. America-^. 



IN 1870 Meusel experimented on the formation of double 

 iodides, and on the remarkable changes of colour produced 

 in these bodies by heatf. He prepared a double iodide of copper 

 and mercury by adding, to a warm solution of mercuric iodide in 

 potassium iodide, copper sulphate and then sulphurous acid; 

 the resulting precipitate is of a brilliant carmine red^ and (in 

 that experimented on by me) turns to a deep chocolate-brown 

 on heating to about 70^ C. In order forcibly to exhibit this 

 change of colour, Boettger moistened the iodide with weak gum- 

 water and painted it on paper : on heating the latter the change 

 of colour is produced ; and on cooling, the iodide regains its 

 former brilliancy. 



Dr. G. E. Barker had the kindness to present me with a card 

 so prepared ; and on experimenting with it I soon perceived the 

 valuable means it afforded of tracing the progress and of deter- 

 mining the boundary of a wave of conducted heat. To Dr. 



* From the American Journal of Science and Arts, vol. iv. July 18/2. 

 Communicated by the Author. 



t Ber. Berl. Chem. Ges. vol. iii. p. 123 (1870). Bull. Soc. Ch. (11.) 

 vol. xiii. p. 220 (1870). J. Pr. Ch. (II.) vol. ii. p. 136 (August 1870), 



Phil, Mag. S. 4. Vol. 44 No. 293. Oct. 187.2. S 



