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PROFESSOR TYNDALL ON THE ACTION OF FREE MOLE COLES ON 
journals/' 5 ' I was induced to defer the detailed publication of the experiments. The 
investigation itself had taught me the difficulties and dangers which beset it. These 
had reference both to the methods of experiment and to the purity of the substances 
employed. To secure the perfect constancy of the sources of heat, and the perfect 
steadiness of the galvanometer, when the flux of heat was powerful, involved a 
lengthened discipline. With neither gases nor vapours, moreover, was it easy to 
obtain uniform results. When generated in different ways, the action of the same 
gas would sometimes prove itself so discordant as to suggest to me the possible 
existence of novel allotropic conditions to account for such variations of behaviour. 
Two samples, moreover, of nominally the same liquid, would furnish vapours yielding 
results far too divergent to be tolerated. The drying apparatus also contributed its 
quota of disturbance. These anomalies were finally traced to the fact that an 
incredibly small amount of impurity derived from the stronger gases or vapours 
sufficed to disguise and falsify the action of the weaker ones. All this had to be 
learnt; and when learnt, I thought it desirable, for the sake of accuracy, not to 
publish the results which had been gained with so much labour, but to go once 
more, with improved appliances, over the same ground. This I did; though it 
involved the total abandonment of seven weeks’ uninterrupted experimental work 
in 1859, of seven weeks’ similar work in 1860, and of many fragmentary efforts. On 
the 10th of January, 1861, the memoir containing an account of the investigation was 
handed in to the Royal Society.! 
The first point of importance established in. 1859 and developed in the memoir just 
mentioned was that already referred to; namely, the fact of absorption, and large 
differences of absorption. The second point—destined, I think, to throw light on the 
deeper problems of molecular physics—was the proof that while elementary gases 
offered a scarcely sensible impediment to radiant heat, equally transparent compound 
gases exhibited, in many cases, an energy of absorption comparable to that of the most 
athermanous solids and liquids. Determining, for example, the action of a mechanical 
mixture of two elementary gases, it was proved that without altering either the quan¬ 
tity of matter, or its perfect transparency to light, the absorption of invisible heat 
might be increased many hundred-fold by the passage of the constituents of the 
mixture into a state of chemical combination. 
* Proceedings of the Royal Society, May 26, 1859; Proceedings of the Royal Institution, June 10, 
1859; Bibliotheque Universelle, July, 1859; Cosmos, vol. 15, p. 321; Nuovo Cimento, vol. 10, p. 196; 
Comptes Rendus, 1859; and in other journals. 
t Section 3 of the Bakerian Lecture for 1861 reveals some of the difficulties which beset the earlier 
stages of these inquiries. To secure strength of radiation and steadiness of the needle I passed from 
source to source, obtaining my temperatures in turn from water, oil, fusible metal, sheets of copper 
heated by regulated flames, and from other things. Approximate results were readily obtainable; but 
I aimed at a degree of accuracy which would render any material retractation afterwards unnecessary. 
Soundness of work I thought preferable to rapidity of publication. 
