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18 Their common origin in the emission of an Auer burner is favourable to such a view, as is also the opacity of rock-salt and of water. But on the other hand, for Auer rays, the transparency of metals and other substances opaque to Rubens' rays constitute an apparently radical difference between the two sorts of radiations (note 7).

While investigating whether radiations analogous to those whose existence I recorded in the emission from an Auer burner (see p. 13) are not to be met also in other sources of light and heat, I established the following facts: the flame of an annular gas-burner emits such radiations; the chimney, however, should be removed, on account of the absorption of the rays by glass. A Bunsen burner scarcely