ScpL 2, 1880] 



NATURE 



413 



If temperature means the amplitude of vibration of the 

 molecules, then we might expect that only tho=e bodies which 

 have their temperatures increased by the saaie amount when 

 equal amounts of heat are applied to them can possibly com- 

 bine with one another ; and fo the fact that the increase of 

 temperature bears a fixed ratio to the increase of heat may be 

 the cause in virtue of which bodies can combine with one 

 one another. Were other bodies to begin to combine together 

 at any definite temperature, they would immediately be torn to 

 piece; again when the temperature is even tlightly raised, be- 

 cause the amplitudes of vibration of their molecules no longer 

 remain the same. This idea of temperature is supported by the 

 fact that a combining molecule of each substance requires the 

 same amount of heat to raise its temperature by the same number 

 of degree?, the atomic weights being proportional to the masses 

 of the combining molecules. The celebrated discovery of Fara- 

 day, that in a voltameter the work done by an electric current 

 always decomposes equivalent quantities of different substances, 

 combined with the fact that in the whole range of the physical 

 forces work done is equivalent to the application of heat, is quite 

 in accordance with the view that no molecule can combine with 

 another which has not its amplitude of vibration altered by the 

 same amount when equal quantities of heat are applied to both. 

 As soon as we get any divergence from this state of equal 

 motions for equal increments of heat, then we should expect that 

 a further dissociation of molecules would take place, and that 

 only those which are capable of moving togetlier can remain 

 still associated. 



Just as in the change of state of a body from the solid to 

 the liquid, or from the liquid to the gas, a great amount of 

 heat is spent in increasing the motion of translation of the 

 molecules without altering the temperature, so a great amount 

 of heat is spent in producing dissociation without increasing the 

 temperature of the dissociated substance-, since the principle of 

 conservaiion of energy has been shown by M. Eenhelot to hold 

 for the dissociation of bodies. We may conveniently make use 

 of the term latent heat of dissociation for, the heat required to 

 dissociate a unit of mass of a substance. 



We may thus sum up the laws of physical and chemical 

 changes ; — 



1. All the physical phenomena of change of state consist in 

 the subdivision of the body into molecules or particles identical 

 with one another. 



2. The reconstitution of a body into a liquid or a solid being 

 independent of the relative position of the molecules, only depends 

 on the pressure and temperature. 



3. Dissociation separates bodies into their elements, which are 

 of different kinds, and the temperature remains constant during 

 dissociation. 



4. The reunion of dissociated bodies depends on the relative 

 position of the elements, and so depends on the grouping of the 

 molecules. The atomic weight being the ma=s of a molecule as 

 compared with hydrogen, the specific volume, i.e., the atomic 

 weight divided by the density, is the volume or mean free path of 

 a molecule. 



Building up his theor>' of heat on these pi inciples, M. Pictet 

 arrives at a definite relation between the atomic weight of a body, 

 its density, its melting-point, and its coefficient of expansion, 

 which may be stated thus — 



The volume of a solid body will be increased as the tempera- 

 ture rises by an amount which is proportional to the number of 

 molecules in it, and inversely as its specific heat. At a certain 

 temperature peculiar to each body, the amplitude of the heat 

 oscillation is sufficient to melt the solid, and we are led to admit 

 that for all bodies the intermolecular distance corresponding to 

 fusion ought to be the same. The higher the point of fusion of 

 a body, the shorter, on this theory, must be its heat vibrations. 

 The product of the length of nuing (the heat oscillations) by the 

 temperature of fusion ought to be a constant number for all solid 

 bodies. 



A comparison of the values of the various quantities involved 

 in these statements show s a very satisfactory agreement between 

 theory and experiment, from which it appears that for many 

 different substances the product of the length of swing by the 

 temperature of fusion lies between 3'3 and 37 for most sub- 

 stances. Not many values of the latent heat of dissociation 

 have been made. In order to determine it, say, for the separa- 

 tion of oxygen and hydrogen, we should have to determine the 

 amount of work required to produce a spark 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 

 formation. 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 together the most refractory 

 substances, as we are dependent on this combination for our 

 supply of heat. 



The combination of carbon and hydrogen with oxygen will 

 give us high temperatures, so that by the oxyhdrogen 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 than 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 may 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 service in reducing very small quantities 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 ca-e 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 ceases when the point of fusion has been 

 reached. Yet in the centre of the arc we have the gases of 

 those substances which form the conductor ; and, as Prof. 

 Dewar has shown, we have the formation of acetylene and 

 cyanogen and other compounds, and therefore must have 

 attained the temperature necessary for their formation, i.e. the 

 temperature of their dissociation. Tlie 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 will difTuse at different rates through 

 a porous diaphragm, and so may be obtained separated from 

 one another. As the molecules of bodies vibrate they produce 

 vibrations of the ether particles, the period of the oscillations 

 depends on the molecules of the body, and these periodic vibrations 

 are taken up by their ether envelopes end by the luminiferous ether, 

 and their wave-length determined by r^eans of the spectroscope. 

 The bright-line spectrum may be regarded as arising from the 

 vibratory motions of the atoms. As the temperature is in- 

 creased, the amplitudes of oscillation of the molecules and of 

 the ether increase, and from the calculations of Lecoq de 

 Boisbaudran, Stoney, Soret, and others, it would appear that 

 many of the lines in the spectra of bodies may be regarded as 

 harmonics of a fundamental vibration. Thus I-ccoq de Bois- 

 baudran findi that in the nitrogen spectrum the blue lines seen 

 at a high temperature correspond to the double octave of certain 

 vibrations, and that, at a lower temperature, red and yellow 

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

 vibrations. 



The bright-line spectrum may be regarded as arising from the 

 vibratory motions of the atoms. A widening of the lines may 

 be produced at a higher temperature by the backward and 

 forward motions of the molecules in the direction of the observer. 

 A widening of the lines may also be produced by increase of 

 pressure, because it diminishes the free path of the molecules, 

 and the disturbances of the ether arising from collisions become 

 more important than vibrations arising from the regular vibrations 

 of the atoms. 



Band spectra, or chanelled space spectra, more readily occur 

 in the case of bodies « hich are not very readily subject to 

 chemical actions, or, according to Professors Liveing and Dewar, 

 in the case of cooler vapours near the point of liquefaction. 



