176 _ REPORT—1862. 4 
thermo-electric action in which unequally heated air or gas forms part of 
the circuit. The currents obtained when a hot and cold platinum wire are 
dipped into dilute sulphuric acid and other liquids are well known; and 
finally (in 1858), Mr. Wild published a laborious research, in which he seems 
to prove the development of thermo-electric currents not only at the junction 
between metals and various solutions, but also between two different solutions. 
Thus, although none of the above observers seem to have tested the oxides, 
there seems little reason to doubt that they may be classed with other elec- 
trolytes, and may give rise to currents in the same manner. On the other 
hand, I cannot yet consider it definitively proved that any of the currents 
obtained from electrolytes are due to a true thermo-electrie action—that is 
to say, to an absorption of heat only, especially as Mr. Wild could find no 
trace of the Peltier heating and cooling effect at the junctions of his solutions. 
Further research, showing the source of the power developed, is most de- 
sirable. 
While consulting the literature connected with this subject, I found that 
Gaugain had to some extent preceded me in the discovery of the loose-con- 
tact currents, in a paper published in the ‘ Comptes Rendus’ in 1853. He 
comes to the same conclusion as I had done independently, that they were 
due to the unequally heated film of foreign matter, and places oxide of iron 
below platinum, and oxide of copper above gold and zinc, but below iron, 
instead of very much above it as I find. He does not appear to have ob- 
served the exceedingly high electromotive force to be obtained from these 
bodies, no doubt owing to the use of a short galvanometer coil of thick wires, 
such as is commonly used for thermo-electric researches. He introduces a 
carburet of iron, of which I find no trace, with more positive properties than 
oxide of copper, to explain some of his results. He gives very few data on 
which to found his theory, but simply mentions his conclusions, and appears 
to have made no direct experiment whatever with the oxides. Owing to 
these circumstances his experiments seem to have attracted little attention. 
I have endeavoured to contrive a convenient apparatus by which to study the 
properties of the oxides, but have not hitherto met with much success, owing 
to the great difficulty in maintaining a constant difference of temperature 
between the surfaces of the very thin film, which can alone be used with 
success. Next year I hope to obtain further results in elucidation of these 
quasi thermo-electric currents from electrolytes. 
I now wish to add a few remarks on the currents which occur when true 
metallic contact is made between a hot and cold end of a wire of one metal. 
The existence of these currents was placed beyond all doubt by Magnus’s 
careful experiments, but their connexion with other thermo-electric phenomena 
has hitherto remained entirely without explanation. Wild has suggested 
that they might be due to a thermo-electric couple formed with hot air or gas 
at the moment of junction; but experiments which I have made show this 
explanation to be founded on a mistaken conception of the duration of the 
current, which is by no means instantaneous, but lasts at least five minutes 
with copper or with iron wires, very gradually decreasing in intensity from 
a maximum to zero. 
Another explanation, viz. that the deflection is due to a sort of discharge 
of a statical effect produced by the unequal distribution of heat, is also nega- 
tived by the same consideration, as well as by the fact that a tension of suffi- 
cient magnitude to produce such a charge could not possibly have escaped 
observation by direct measurement. 
Professor W. Thomson has shown conclusively, in his ‘ Dynamic Theory of 
