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The new lines, accordingly, filled places in a manner already prepared for them, and were thus unmistakably associated with the hydrogen-spectrum. This is now known to be represented in prominences by twenty-seven lines, forming a kind of harmonic progression, only four of which are visibly darkened in the Fraunhofer spectrum of the sun. The chemistry of "cloud-prominences" is simple.
It made, nevertheless, capricious appearances at South Kensington during that autumn, and again October 21, 1894, while in September, 1892, Bélopolsky could obtain no trace of it on orthochromatic plates exposed with the 30-inch Pulkowa refractor. No fluctuations in the hydrogen-spectrum were betrayed by them; but subordinate lines of unknown origin showed alternate fading and vivification.
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