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observations of his own are embodied in this work, which had never 

 appeared in any scientific journal. A fifth edition was commenced in 1855, 

 but left unfinished. In this year he commenced his important labours on 

 the density of the vapour of bromine, sulphur, phosphorus, arsenic, and 

 mercury, nitrous acid, nitric acid, sulphuric acid, &c., and on the relation 

 of the density of vapours to their chemical equivalents. In the same year 

 he commenced his researches on benzoyl, which suggested to him a simple 

 theory of the constitution of those organic combinations in which compound 

 radicals are assumed to exist. His experiments on the formation of ether 

 led him to the doctrine of chemical combinations and decompositions by 

 contact, whereby dormant affinities in mixtures, or compounds held together 

 by feeble affinities, become active by mere contact with a substance 

 chemically inactive. These labours in the domain of organic chemistry 

 wholly occupied him for nearly twelve years. At the conclusion of 

 this period he turned his attention to geology. Indeed, ever since 

 he had engaged in researches on the artificial production of minerals, 

 he used to theorize on the formation of rocks, and on the existence 

 of mineral springs and volcanos. In his earlier travels, while his main 

 object was the examination of chemical manufactures and smelting- 

 furnaces, his attention was also directed to the geology of the countries 

 through which he passed. He frequently devoted the concluding lectures 

 of each half-year's course to a sketch of the geological structure of the 

 earth, and the changes which its surface had undergone. Year after year 

 he made systematic journeys in the Eifel, with the intention of publishing 

 a complete description of the extinct volcanos of that district, and 

 connecting it with a theory of volcanic action. And, as the study of this 

 region made a comparison with the volcanos of other countries desirable, 

 he visited in succession the principal volcanic districts of Italy, France, and 

 Germany. But, notwithstanding all this preparation, the description of 

 the Eifel was never printed, with the exception of some pages distributed 

 among the hearers of lectures of a popular character given by him in the 

 winter of 1838 and 1839. In these he states the views of the nature of 

 volcanic processes which he then entertained. They appear to have been 

 founded on a very careful study of volcanic phenomena. He supposes the 

 explosive action to be caused by the vapour of water. The only hypothesis, 

 however, by which the presence of water in an active volcano could at that 

 time be accounted for, was beset by serious difficulties. These have since 

 been removed by the beautiful experiment made by Daubree, which shows 

 that when one side of a stratum of porous rock is heated, water in contact 

 with the opposite side makes its way through it, in the direction of the 

 heated part, notwithstanding the high pressure of the vapour generated on 

 that side. 



During the autumnal vacation of 1861 he made his last geological 

 excursion in the Eifel ; in December of that year he began to suffer from 

 disease of the heart, the complaint increased in severity in the summer of 



