TRANSACTIONS OF SECTION A. 451 



fusion lies between 3'3 and 37 for many substances. Not many values of the 

 latent heat of dissociation have been obtained. In order to determine it, say, for 

 the separation of oxygen and hydrogen, we .should have to determine the amount of 

 work required to produce a sparli in a mixture of oxygen and hydrogen, and to 

 measure the exact amount of water or vapour of water combined by the spark, 

 as well as the range of temperature through which it had passed after its forma- 

 tion. Very few such determinations have been made. 



Our usual mode of producing heat is by the combination of the molecules of 

 different substances, and we are limited in the production of high temperatures, and 

 in the quantity of available heat necessary to dissociate any considerable quantity 

 of matter. If we heat vapours or gases, we may raise their temperatures up to a 

 point corresponding to the dissociation of their molecules, and we are limited in our 

 chemical actions to the temperatures which can be obtained by combining togetlier 

 the most refractory substances, as we are dependent on this combination for our 

 supply of heat. 



The combination of carbon and hydrogen -^nth oxygen will give us high tem- 

 peratures, so that by the oxyhydrogen blow-pipe most of the salts and oxides are 

 dissociated. The metalloids bromine, iodine, sulphur, potassium, &c., are the results 

 of the combination of two or more bodies bound together by internal forces much 

 stronger tlian the affinity of hydrogen or carbon for oxygen, for approximately 

 they obey the law of Dulong and Petit. 



For higher temperatures, in order to dissociate the most refractory substances, 

 we require the electric current, either a continuous current, as in the electric arc 

 from a battery, or a dynamo-machine, or, more intense still, the electrical discharges 

 from an electrical machine or from an induction coil. 



This electric current maj' be regarded as the most intense furnace for dissociating 

 large quantities of the most refractory substances, and the electric spark may be 

 regarded as something very much hotter than the oxyhydrogen blow-pipe, and 

 therefore of ser^^ce in reducing very small q uantities of substances which will yield 

 to no other treatment. The temperature of the electric arc is limited, and cannot 

 reach above the temperature of dissociation of the conductor, and in the case of the 

 constant current, which will not leap across the smallest space of air unless the 

 carbons have first been brought in contact, the current very soon cea.ses when the 

 point of fusion has l)een reached. Yet in the centre of the arc we have the gases of 

 those substances wliicli form tlie conductor ; and, as Professor Dewar has .shown, 

 we have the formation of acetylene and cyanogen and other compounds, and there- 

 fore must have attained the temperature necessary for their formation, i.e. the tem- 

 perature of their dissociation. The temperature of the induction spark, or, at least, 

 its dissociating power, is higher than that of the arc. We know that the spark 

 will pass across a space of air or a gaseous conductor, and we are limited by the 

 dissociation of the gaseous conductor, and get only very small quantities of the 

 dissociated substances, which immediately recombine, unless they are separated. If 

 the gases formed are of different densities they ^vill diffuse at different rates through 

 a porous diaphragm, and so may be obtained separated from one another. As the 

 molecules of bodies vifjrate they produce A'ibrations of the ether particles ; the period 

 of the oscillations depends on the molecules of the body, and these periodic -s ibra- 

 tions are taken up by their ether envelopes and by the lumiuiferous ether, and their 

 wave-length determined by means of the spectroscope. As the temperature is 

 increased, the amplitudes of oscillation of the molecules and of the ether increase, 

 and from the calculations of Lecoq de Ijoisbaudran, Stoney, Soret, and others, 

 it would appear that many of the lines in the spectra of bodies may be regarded 

 as harmonics of a fiuidamental ^•ibration. Thus Le3oq de Boisbaudran finds that 

 in the nitrogen spectrum the blue lines seen at a higli temperature correspond 

 to the double octave of certain vibration*, and that, at a lower temperature, red 

 and yellow lines are seen which correspond to a fifth of the same fundamental 

 vibration=!. 



Tlie bright line spectrum may })e regarded as arising from the vibratory motions 

 of the atoms. A widening of the lines may Im produced at a higher temperature 

 by the backward and forward motions of the molecules in the direction of the 



G G 2 



