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 of unstable dissociating compounds of the solvent with the substance dissolved." This, he said, was one of the several possible hypotheses. He also worked on the laws of the expansion of gases at low tensions, and as far back as the year 1861 he reached the idea of the "absolute boiling-point" (the critical temperature of Andrews) as that temperature at which a liquid cannot exist as a liquid, but forms a gas that cannot pass into the liquid state under any pressure whatever, and at which the cohesion and the latent heat of evaporation are both nil.

His great book, The Principles of Chemistry, in two large volumes, is a philosophical treatise, and its footnotes occupy the greater part of the work.

In 1877 he published another book on the Naphtha Production in America and the Caucasus. This led to his journey to the petroleum fields of America at the expense of the Russian Government; and up to the time of his death he was the leading expert in all conferences on the mineral oil question in Russia.

Mendeléeff was an indefatigable worker, even down to the day of his death, and living only for science. When, in 1887, there was a total eclipse of the sun, he ascended in a balloon alone to make observations. Alone he effected dangerous alterations in the balloon valves, and wrote an account of his observations.

Although elected a member of almost every scientific