Abstract :
[en] Spring, W. Recueil des Travaux Chimiques des Pays-Bas (1905), 24, 297-304; SciFinder (Chemical Abstracts Service: Columbus, OH); https://scifinder.cas.org (accessed July 8, 2010).
Compare de Bruyn and Wolff, Abstr., 1904, ii, 470. The fluorescence of a solution of fluorescein or of eosin in optically transparent water (Abstr., 1899, ii, 537) becomes invisible in daylight when the amount of the fluorescent substance present falls to 0.000,000,01 gram per c.c., and is just visible at the apex of a powerful beam of electric light when the concentration is 0.000,000,000,000,001 gram per c.c. It is found that the area of liquid illuminated by the apex of the cone of light must be at least 1 sq. mm. in order to render the fluorescence visible, and consequently, assuming that 1 cubic millimetre of the liquid contains at least one molecule of fluorescein (mol. wt. 408) for example, then the weight of an atom of hydrogen would be 2.5 × 10-21 grams. This value, which represents only the higher limit among possible values, is much smaller than those arrived at from other considerations by Clerk Maxwell, Kelvin, de Heen, Annaheim, and others.
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