42 1 Prof. B. Wiedemann on the Behaviour of Gases 



thermochemical decompositions. A determination of the 

 quantity of heat necessary for the decomposition of the mole- 

 Gales of the elements must be important for the science of 

 chemistry. If, for example, we have the reaction 



HH + C1C1 = 2HC1, 



the thermic effects are of three kinds: — first, heat is absorbed 

 in separating one atom of hydrogen from the other ; a further 

 quantity of heat is absorbed in separating one atom of chlo- 

 rine from the other ; and, lastly, twice as much heat is pro- 

 duced as corresponds to the formation of one molecule of 

 hydrochloric acid. 



Until now only the total result has been measured. It is 

 now possible, by spectroscopic methods, to determine the first 

 two portions by themselves, and so to determine the true heat 

 of combination. 



If we assume, as is in the main confirmed by the preceding 

 investigations, that the spectra are conditioned by the quantity 

 of electricity given to each molecule and the energy absorbed 

 by it, the known dependence of spectra on the pressure may 

 be explained as follows: — 



When the pressure is great in a discharge -tube, the number 

 of gas-molecules is large also. As at the same time the num- 

 ber of discharges for an equal supply of electricity is compa- 

 ratively small, the energy given up by each separate discharge 

 must be relatively very large. At the same time this may be 

 confined to a few molecules, in consequence of the formation 

 of a small spark. A very great quantity of energy is given 

 to each molecule ; it decomposes into its atoms, and we have 

 a line spectrum. 



If the pressure falls, the number of discharges increases, 

 the energy given up decreases, and, moreover, all portions of 

 the gas are put into motion: hence the number moved is 

 larger, a smaller quantity of energy is given to each molecule; 

 it is no longer sufficient for the decomposition of molecules 

 into atoms, and the spectrum is a band spectrum. 



If now we include air-sparks in the circuit, the quantity of 

 electricity transmitted in each discharge is increased, and con- 

 sequently the energy given up ; hence the band spectrum is 

 converted into the line spectrum. At very low pressures 

 complications occur which cannot be dealt with till we possess 

 a complete theory of the discharge. 



If we find in the experiments (p. 407) that the line spectrum 

 appears together with the band spectrum, first weak and then 

 iu creasing in strength, this is in entire accordance with the 

 conclusions which follow from the kinetic theory of gases. 



