Intelligence and Miscellaneous Articles. 323 



A table of comparison of temperature by the air- thermometer 

 under varied conditions of temperature and pressure with the abso- 

 lute scale, is deduced from this formula. 



Expressions for the specific heats of any fluid in terms of the ab- 

 solute temperature, the density, and the pressure, derived from the 

 general dynamical theory, are worked out for the case of air accord- 

 ing to the empirical formula ; and tables of numerical results derived 

 exclusively from these expressions and the ratio of the specific heats 

 as determined by the theory of sound, are given. These tables 

 show the mechanical values of the specific heats of air at diflferent 

 constant pressures, and at different constant densities. Taking 

 1390 as the mechanical equivalent of the thermal unit as determined 

 by Mr. Joule's experiment on the friction of fluids, the authors 

 find, as the mean specific heat of air under constant pressure, 



•2390, from 0° to 100° Cent. 

 •2384, from 0° to 300° Cent. 



XLII. Intelligence and Miscellaneous Articles. 



NOTE ON THE PRODUCTION OF PYRO-ELECTRIC CURRENTS* 

 BY M. BECQUEREL. 



T^HE disengagement of electricity is produced by various means — 

 -*- friction, heat, light, the action of magnets, induction, molecular 

 and chemical actions, &c. I have endeavoured to produce the electric 

 force by combining the action of heat with that of the aflSinities. 

 My expectations have been realized ; I have succeeded in producing 

 currents which I shall call pyro-electric currents, by analogy with 

 the currents obtained in the ordinary batteries, and to distinguish 

 them from the thermo-electric currents, which are due to heat alone. 



These currents, which have a constant power as long as the tem- 

 perature does not vary very sensibly, are produced whenever solid 

 metallic or other substances, conductors of electricity, are in contact 

 with glass or any other vitreous substance in a state of igneous fusion, 

 or softened by heat ; but the greatest effect takes place only when 

 the substance is fused. 



In the memoir which I presented to the Academy on the 1st of May 

 last, on the disengagement of electricity in chemical actions*, I 

 showed that glass even at a slightly elevated temperature began to 

 conduct electrical currents, and that this property might be made 

 use of to study the disengagement of electricity produced by the 

 contact of platinum wires with flame. This conductibility begins to 

 be sensible at about 482° F. I have since tried to ascertain whether, 

 this conducting power increasing with the temperature, it would not 

 be possible when the glass is fused, or even before this takes place, 

 to substitute it for the acids and saline solutions in voltaic batteries. 

 I operated in the following manner: — 



First experiment, — If a rod of soft iron and a rod of copper be intro- 

 duced into a furnace filled with lighted coals, each of them being in 

 communication with the ends of the wire of an ordinary multiplier, 



* Phil. Mag. for July 1854. 

 Y2 



