448 REPORT — 1880. 



We are making rapid strides towards the exact determination of tliose relations 

 between the various modes of motion or forms of energy wliicli were so ably 

 shadowed forth, and their existence established long ago, by Sir William Grove in 

 his ' Correlation of the Physical Forces,' where, in stating the conclusion of his com- 

 parison of the mutual interchange of physical forces, he distinctly lays down the 

 principles of energy in this statement : ' 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 

 unrecocnised forces. The equivalent is the limit never practically reached.' 



The laws of Faraday, that (1) when a compound is electrolysed the mass 

 of the substance decomposed is proportional to the quantity of electricity which has 

 produced the change, and that (2) the same current decomposes equivalent quanti- 

 ties of different substances, i.e. quantities of their elements in the ratio of their 

 combinino- numbers, have given rise to several determinations of the relation between 

 chemical afhnity and electromotive force. In a paper lately communicated to the 

 Physical Society, Dr. Wright has discussed these several determinations, and has 

 given an accovmt of a new determination by himself. The data at present extant 

 show that when 1 gramme of hydrogen unites with 7'98 grammes of oxygen there 

 are about 34,100 units of heat given out, making the latent heat of dissociation of 

 1 o-ramme of water equal to 3797 units. The results obtained are compared with 

 the heat given out by the combustion of hydrogen and oxygen, and the value of 

 the mechanical equivalent of heat is deduced from these determinations. 



The value of this mechanical equivalent obtained by Dr. AVright, which depends on 

 the value of Clark's standard cell, and therefore depends on the value of the ohm, 

 a'^rees fairly well with Joule's determination from the heat produced by an electric 

 current in a wire, but is greater than Joule's value as obtained fi'om his water-friction 

 experiments. This may be accounted for by supposing an error in the value of the 

 ohm or B.A. miit, making it too large by 1'5 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 1'96 per cent, too large. On the other hand, Pro- 

 fessor Rowland, in America, has made a new determination, and finds that accord- 

 ino- to his calculations the B.A. unit is nearly 1 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 shoidd 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 shoidd 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 care- 

 fully selected. It has been fomid 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-coefficients. The value '031 for 1° 0. is given 

 by Matthiessen for this alloy, yet two pieces of wire drawn from the same rod 

 have o-iveu, one •021 per cent, and the other -04 per cent, for 1° C. Possibly this 

 irreo-ularity in the platinum-silver alloys may be due to something analogous to 

 the secreo-ation which Mr. Roberts has found to take place in copper-silver alloys 

 in their molten state, and which Matthiessen in 1860 regarded as mechanical 

 mixtures of allotropic modifications of the alloy. 



A recommendation has lieen made that apparatus for determining the ohm 

 should be set up in London, and that periodically determinations be made to test 

 the electrical constancy of the metals and alloys used in making coils. A com- 

 mittee should be authorised to test coils and issue certificates of their accuracy, 

 iust as is done by the Kew Committee with regard to meteorological instruments. 

 The direct relation between Heat and Chemical work has lieen established, and the 

 principles of Conservation of Energy been shown to Ije 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 succession of chemical changes, the heat evolved or 

 absorbed when no external mechanical effect is produced depends solely upon the 



