The Swedish investigator Scheele had, however, discovered this same constituent of the air before 1773. He thought that the atmosphere must consist of at least two gases, and he proved that carbonic acid results from combustion and respiration. In 1772 the great French scientist Lavoisier found that sulphur, when burned, gains weight instead of losing weight, and five years later he concluded that air consists of two gases, one capable of absorption by burning bodies, the other incapable of supporting combustion. He called the first "oxygen." In his _Elements of Chemistry_ Lavoisier gave a clear exposition of his system of chemistry and of the discoveries of other European chemists. After his studies the atmosphere was no longer regarded as mysterious and chaotic. It was known to consist largely of oxygen and nitrogen, and to contain in addition aqueous vapor, carbonic acid, and ammonia which might be brought to earth by rain. Cavendish obtained nitrogen from air by using nitric oxide to remove the oxygen, and found that air consists of about seventy-nine per cent nitrogen and about twenty-one per cent oxygen. He also by use of the electric spark caused the oxygen and nitrogen of the air to unite to form nitric acid. When the nitrogen was exhausted and the redundant oxygen removed, "only a small bubble of air remained unabsorbed." Similarly Cavendish had found that water results from the combination of oxygen and hydrogen. Watt had likewise held that water is not an element, but a compound of two elementary substances. Thus the great masses,--earth, air, fire, water,--assumed as simple by many philosophers from the earliest times, were resolving into their constituent parts. At the same time other problems were demanding solution. What are the laws of chemical combination? What is the relation of heat to other forms of energy? To the answering of these questions (as of those from which these grew) the great manufacturing centers contributed, and no city more potently than Manchester through Dalton and his pupil and follower Joule. John Dalton (1766-1844) was born in Cumberland, went to Kendal to teach school at the age of fifteen, and remained in the Lake District of England till 1793. In this region, where the annual rainfall exceeds forty inches, and in some localities is almost tropical, the young student's attention was early drawn to meteorology. His apparatus consisted of rude home-made rain-gauges, thermometers, and barometers. His interest in the heat, moisture, and constituents of the atmosphere continued throughout life, and Dalton made in all some 200,000 meteorological observations. We gain a clue to his motive in these studies from a letter written in his twenty-second year, in which he speaks of the advantages that might accrue to the husbandman, the mariner, and to mankind in general if we were able to predict the state of the weather with tolerable precision.