Thus during the earlier part of his life, Lavoisier does not seem to have devoted himself in particular to any one branch of science. But about the year 1770 the announcement of the existence of more than one species of gaseous matter, arising out of the successive researches of Black, Scheele, Priestley, and Cavendish, had the effect of fixing his attention to the subject of pneumatic chemistry. The invaluable discoveries just alluded to had opened a new world to the inquirer into nature; and the labours of those distinguished experimentalists had conspired to commence a fresh era in science. Lavoisier was one of the first to appreciate at once the importance of the results they had arrived at, and the immense field of further research to which those results had opened the way. He perceived by a sort of instinct the glorious career which lay before him; and the influence which this new science thus, as it were, created, must have over every sort of physical research. Priestley possessed precisely those qualifications which are most available for striking out new and brilliant discoveries of facts; a boundless fertility of invention; a power of rapidly seizing remote analogies; and an equal readiness in framing and in abandoning hypotheses, which have no value, but as guides to experiment. Lavoisier, less eminent in these respects, possessed in a more peculiar degree the mental characteristics which enable their owner to advance to grand generalizations and philosophical theories upon the sure basis of facts. He possessed, in its fullest sense, the true spirit of inductive caution, and even geometrical rigour; and his observations, eminently precise and luminous, always pointed to more general views. In 1774, he published his ‘Opuscules Chimiques,’ in which, after a full and truly philosophical examination of the labours of preceding experimenters in the discovery of the gases and their characteristic properties, he proceeds to describe his own beautiful and fundamentally important researches, from which resulted the ‘True Theory of Combustion,’ which may be termed the very sun and centre of the whole modern system of chemistry. To the vague dreams of the alchemist had succeeded the remarkable theory of Hooke, who maintained that a certain ingredient of the atmospheric air (which also enters as an ingredient into several other bodies, especially nitre) was the _solvent_ which absorbed a portion of the combustible. This process was continued in proportion as more of the solvent was supplied. The solution took place with such rapidity, as to occasion those motions or pulsations in which Hooke believed heat and light to consist.