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17 may mention tinfoil, sheets of copper and brass 0.2 mm. thick, a sheet of aluminium 0.4 mm. thick, a steel lamina 0.05 mm. thick, a silver leaf 0.1 mm. thick, a paper booklet, containing twenty-one gold leaves, a glass sheet, 0.1 mm. thick, a sheet of mica of 0.15 mm., a plate of Iceland spar of 0.4 mm., a block of paraffin of 1 cm., a beech board 1 cm., a plate of ebonite of 1 mm., etc. Fluor spar is but slightly transparent with a thickness of 5 mms., similarly sulphur 2 mms. thick, and glass 1 mm. thick. These results I only give as a first indication, for when they were obtained, the co-existence of four different species of radiations, which may have very different properties, was not taken into account (note 5).

It will be highly interesting to investigate whether other sources, and in particular the sun, do not emit analogous radiations to those we are dealing with in the present communication, and also whether the latter produces any calorific action (note 6).

Now, ought these radiations in reality to be considered as akin to the large wave-length radiations discovered by Professor Rubens?