In the middle of the eighteenth century, when Lambert and Kant were recognizing system and design in the heavens, little progress had been made toward discovering the constitution of matter or revealing the laws of the hidden motions of things. Boyle had, indeed, made a beginning, not only by his study of the elasticity of the air, but by his distinction of the elements and compounds and his definition of chemistry as the science of the composition of substances. How little had been accomplished, however, is evident from the fact that in 1750 the so-called elements--earth, air, fire, water--which Bacon had marked for examination in 1620, were still unanalyzed, and that no advance had been made beyond his conception of the nature of heat, the majority, indeed, of the learned world holding that heat is a substance (variously identified with sulphur, carbon, or hydrogen) rather than a mode of motion. How scientific thought succeeded in bringing order out of confusion and chaos in the subsequent one hundred years, and especially at the beginning of the nineteenth century, can well be illustrated by these very matters, the study of combustion, of heat as a form of energy, of the constituents of the atmosphere, and of the chemistry of water and of the earth. Reference has already been made to Black's discovery of carbonic acid, and of the phenomena which he ascribed to latent heat. The first discovery (1754) was the result of the preparation of quicklime in the practice of medicine; the second (1761) involving experiments on the temperatures of melting ice, boiling water, and steam, stimulated Watt in his improvement of the steam engine. In 1766 Joseph Priestley began his study of airs, or gases. In the following year observation of work in a brewery roused his curiosity in reference to carbonic acid. In 1772 he experimented with nitric oxide. In the previous century Mayow had obtained nitric oxide by treating iron with nitric acid. He had then introduced this gas into ordinary air confined over water, and found that the mixture suffered a reduction of volume. Priestley applied this process to the analysis of common air, which he discovered to be complex and not simple. In 1774, by heating red oxide of mercury by means of a burning-glass, he obtained a gas which supported combustion better than common air. He inhaled it, and experienced a sense of exhilaration. "Who can tell," he writes, "but in time this pure air may become a fashionable article in luxury? Hitherto only two mice and myself have had the privilege of breathing it."