On the analogies of light and heat / The Rev. Baden Powell.

  • Powell, Baden, 1796-1860.
Date:
[1852]
Catalogue details

Licence: Public Domain Mark

Credit: On the analogies of light and heat / The Rev. Baden Powell. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

    1/6 (page 173)
    Hence the distinction of at least two species of heating rays emanating at the same time from the same luminous source. From the neglect of this distinction much confusion has been kept up : and statements involving such confusion have been repeated from one elementary treatise to another. Again; notwithstanding that the experiments of Leslie and others on the absorption of heat from non-luminous sources, as well as those of Professor Bache on the radiation from surfaces, demonstrate that the effect has no relation whatever to colour, yet the contrary asser- tion has been often persisted in. Again “ dark heat” is often spoken of without recollecting that rays of the very same quality and properties exist in the compound radiation from luminous sources. The conclusions drawn from later experiments, (performed with all the advantages derived from the beautiful invention of the thermo- electric instrument of Nobili,) in many instances, are still vague, from want of attention to the distinction of different species of heat emanating at the same time from the same source. Melloni, in a most extensive and valuable series of experiments, taking as the sources of heat successively flame, incandescent metal, boiling mercury, and boiling water, and applying in each instance a long series of substances as screens, estimated the proportion of rays out of 100 stopped, which was very different for each screen and each source : evincing wide differences in “ diathermaneity,” while rock salt alone was almost totally “ diathermanous ” to rays from all sources alike. But we must still ask, what species of rays were those respectively stopped and transmitted ? To take the per centage simply is ambi- guous ; the body of rays is not homogeneous ; the property of trans- missibility should be viewed in combination with other properties of the specific rays, such as those evinced in their relations to the I texture or colour of the absorbing surface. Nor is the ambiguity removed, though the difference of source is specially referred to, if the heterogeneity of rays from the same source be overlooked. The mere classification of sources into lumi- nous and non-luminous will not suffice : still less a reference to their temperatures: it being perfectly well known that the temperature of 1 luminosity is very different for different substances.* Again Melloni has shewn that the diathermaneity is not pro- Iportional to transparency, by a classified series of transparent screens with the lamp. .. It must however be recollected that the term “ diathermaneity ” is applied indiscriminately to a heterogeneous body of rays: out of which some species of rays are entirely stopped, others entirely transmitted; and the great differences in “ diathermaneity ” for heat * References in detail to all the different researches here mentioned, will he found in the Author’s Two Reports on the state of our knowledge oi Radiant Heat in the British Association Reports, 1832 and 1840.