41- 



NATURE 



[Sept. 2, iSSo 



of an electrolyte as is given out again by the combination of the 

 substances composing it. 



We are making rapid strides towards the exact determination 

 of those relations between the various modes of motion or forms 

 of energy which were so ably shadowed forth, and their exis- 

 tence established long ago, by Sir William Grove in his " Corre- 

 lation of the Physical Forces," where, in stating the conclusion 

 of his comparison of the mutual interchange of physical forces, 

 he distinctly lays down the principles of energy in this state- 

 ment : "Each force is definitely and equivalently convertible 

 into any other ; and where experiment does not give the full 

 equivalent, it is because the initial force has been dissipated, not 

 lost, by conversion into other unrecognised forces. The equiva- 

 lent i? the limit never practically reached." 



The laws of Faraday, that ( I ) when a compound is electrolysed 

 the mass of the substance decomposed is proportional to the 

 quantity of electricity w-hich has produced the change, and that 

 (2) the same current decomposes equivalent quantities of different 

 substances, i.e., quantities of their elements In the ratio of their 

 combining numbers, have given rise to several determinations 

 of the relation between chemical affinity and electromotive force. 

 In a paper lately communicated to the Physical Society, Dr. 

 Wright has discussed these several determinations, and has 

 given an account of a neu- determination by himself. The 

 data at present extant show that when one gramme of hydrogen 

 unites with 7"9S grammes of oxygen, there are about 34,100 

 units of heat given out, making the latent heat of dissociation of 

 one gramme of water equal to 3,797 units. The results obtained 

 are compared with the heat given out by the combustion of hy- 

 drogen and oxygen, and the value of the mechanical equivalent 

 of heat is deduced from these determinations. 



The value obtained by Dr. Wright, which depends on the 

 value of Clark's standard cell, and therefore depends upon 

 the value of the ohm, agrees fairly well with Joule's deter- 

 mination from the heat produced by an electric current in 

 a wire, but is greater than Joule's value as obtained from his 

 water-friction experiments. This may be accounted for by sup- 

 posing an error in the value of the ohm or B.A. unit, making 

 it too large by v^ or 2 per cent. Kohlrausch has also made 

 comparisons of copies of the B.A. unit with standard coils, and 

 comes to the conclusion that the B.A. unit is i'96 per cent, too 

 large. On the other hand, Prof. Rowland, in America, has 

 made a new determination, and finds that according to his cal- 

 culations the B.A. unit is nearly i per cent, too small. These 

 differences in the values obtained by different methods clearly 

 point to the necessity for one or more new determinations of the 

 unit, and I would venture to suggest that a determination should 

 be made under the authority of this Association, by a committee 

 appointed to carry out the work. And it is not sufficient that 

 this determination should be made once for all, for there is 

 reason to think that the resistance of standard coils alters with 

 time, even when the material has been carefully selected. It 

 has been found that coils of platinum silver which were correct 

 copies of the standard ohm have become so altered, and have 

 their temperature coefficients so changed, that there are doubts 

 as to the constancy of the standards themselves. Pieces of 

 platinum-silver alloy cut from the same rod have been found to 

 have different temperature coefiicients. The value "03 1 for 1° C. 

 is given by Matthiessen for this alloy, yet two pieces of wire 

 drawn from the same rod have given, one •021 per cent, and the 

 other -04 per cent, for 1° C. Possibly this irregularity in the 

 platinum-silver alloys may be due to something analogous to the 

 segregation which Mr. Roberts has found to take place in copper- 

 silver alloys in their molten state, and which Matthiessen in 

 i860 regarded as mechanical mix tures of allotropic modifications 

 of the .alloy. 



A recommendation has been made that apparatus for determining 

 the ohm should be set up ia London, and that periodically deter- 

 minations be made to test the electrical constancy of the metals and 

 alloys used in making coils. A committee should be authorised 

 to test coils and issue certificates of their accuracy, just as is 

 done by the Kew Committee with regard to meteorological 

 instruments. The direct relation bet«een Heat and Chemical 

 work has been established, and the principles of Conservation 

 of Energy been shown to be true in Chemistry by the experiments 

 of Berthelot and of Thomsen, so that we may say that when a 

 system of bodies passes through any succe.>sion of chemical 

 changes, thelieat evolved or absorbed when no external mecha- 

 nical effect is produced depends solely upon the initial and final 

 tales of the system of bodies, whatever be the nature or the 



order of the transformations. The extension of this principle to 

 the interaction of the molecules and atoms of bodies on one 

 another is of vast importance in relation to our knowledge of the 

 constitution of matter, for it enable; us to state that each 

 chemical compound has a distinct level or potential which miy 

 be called its own, and that when a compound gives up one of its 

 elements to another body, the heat evolved in the reaction is the 

 difference between the heat of formation of the first compound 

 and that of the resulting product. 



We have become accustomed to regard matter as made up 

 of molecules, and those molecules to be made up of atoms 

 separated from one another by distances which are great in 

 cjmpari on with the size of the atom, which we may regard as 

 the smallest piece of m.atter that we can have any conception of. 

 Each atom may be supposed to be surrounded by an envelope 

 of ether which accompanies it in all its movements. The density 

 of the ether increases rapidly as an atom is approached, and it 

 would seem that there must be some force of attraction between 

 the atom and its ether envelope. All the atoms have motions of 

 translations in all possible directions, and according to the theories 

 of Maxwell and Boltzmann, and the experiments of Kundt, 

 Warburg, and others on the specific heat of vapours, in onc- 

 iiioii: molecules in the gaseous state there is no motion of 

 rotation. According to the theory of Piclet, the liquid state, 

 being the first condensation from the gaseous state, must consist 

 of at least two gaseous atoms combined. These two atoms are 

 bound to one another through their ether envelopes. Then the 

 solid state results from the condensation of a liquid, and so a 

 solid molecule must consi-t of at least two liquid molecules, i.e. at 

 least four gaseous molecules, each surrounded by an atmosphere of 

 ether. M. Pictet imagines these atoms to be centres of attrac- 

 tion ; hence in the solid with four such centres the least displace- 

 ment brings into action couples tending to prevent the molecule 

 from twisting as soon as external fjrces act upon it. All the 

 molecules constituting a solid will be rigidly set with regard to 

 one another, for the least displacement sets in action a couple or 

 an opposing force in the molecules on one another. 



Let us now fallow the sketch which M. Pictet has given of 

 changes which we may consider it to undergo when we expend 

 energy upon it. Suppose a solid body is at absolute zero of 

 temperature, which may be regarded as the state in which tlie 

 molecules of a body are in stable equilibrium and at rest, the 

 application of heat gives a vibratory motion to the molecules ol 

 tlie solid, which increases with the temperature, the mean 

 amplitude of vibration being a measure of the temperature. 

 We may regard the sum of all the molecular forces as the specific 

 heat of the body, and the product of the sum of all the molecular 

 forces by the mean amplitude of the oscillations ; i.e. the product 

 of the specific heat and the temperature will be the quantity of 

 heat or the energy of motion of the body. As more and more 

 heat is applied, the amplitude of vibration of the molecules 

 increases until it is too great for the molecular forces, or forces of 

 cohesion, and the melting point of the solid is reached. Besides 

 their vibratory motion, the molecules are now capable of motions 

 of translation from place to place among one another. To 

 reduce the solid to the liquid state, i.e. to make the amplitude 

 of vibration of the molecules sufficient to prevent them from 

 coming within the sphere of the forces of cohesion, requires a 

 quantity of heat which does not appear as temperature or 

 molecular motion, and hence it is termed the latent heat of fusion. 

 The temperature rem.iins constant until the melting is complete, 

 the heat being spent in bursting the bonds of the solid. Then a 

 further application of heat increases the amplitude of vibration, 

 or raises the temperature of the liquid at a rale depending on its 

 specific heat until the succession of blows of the molecules over- 

 comes the external pressure and the boiling-point is reached An 

 additional quantity of heat is applied which is spent in changing 

 the body to a gas, i.e., to a state of higher potential, in which 

 the motion of translation of the molecules is enonnously increased. 

 When this state is attained, the temperature of the gas again 

 begins to increase, as heat is applied, until we arrive at a certain 

 [iijint, when dissociation begins, and the molecules of the sepa 

 rate substances of which the Indv is composed have so large an 

 amplitude of vibration that the bond which unites them can no 

 longer bring them again into their former positions. The 

 potential of the substances is again raised by a quantity 

 which is proportional to its chemical affinity. Again, we 

 may increase the amplitude of vibration, i.e., the temperature 

 of the molecules, and imagine the possibility of higher and 

 higher degrees of dissociation. 



