As early as 312 B. C. water was carried to Rome through an aqueduct ten miles long. It is estimated that the cost of construction of this aqueduct was about $12,700,000. Another was begun in 272 B. C., the length of which was forty-five miles. The water carried by this aqueduct was not used for drinking but for irrigating purposes and for flushing drains. Altogether there were at least nine aqueducts that supplied the ancient city of Rome. Of these nine, three are still used to supply modern Rome. One of these was finished by Agrippa in 27 B. C. We see by the foregoing that from time immemorial there has been a great expenditure of energy and money in producing a proper water supply for the human family. It also reveals the fact that the ancients knew something about engineering projects which some of us are likely to consider as modern accomplishments. Composition of Water Water is not, as was previously supposed, an elementary substance. In 1781 it was shown by Cavendish that it consisted of two parts of hydrogen and one part of oxygen (H_{2}O), and that it could be made synthetically by combining hydrogen and oxygen in this proportion, and that it could be separated analytically by various methods into its component parts. By volume, water is composed of two parts of hydrogen to one part of oxygen; by weight, one part of hydrogen to eight parts of oxygen. However, pure water does not exist in Nature since water is a universal solvent and in Nature it comes in contact with so many substances; therefore, it contains many substances in solution. Chemically pure water is found only in the chemist’s flask. Water is a liquid which is clear, colorless and odorless. To be palatable it should be cool, soft, well aerated and free from sediment and suspended impurities. One cannot judge the purity of water by any one of these qualities, for water that is palatable and gives no offense upon drinking might be polluted with that which would be detrimental to health. The palatability of water may be due to the carbonic acid present, which results from the decomposition of products contained in it. The carbon dioxide, which is present in rain water, is obtained from the air through which the rain passes. Carbon dioxide is also taken up by the water as it percolates through ground covered with vegetation. The presence of this gas increases the solvent powers of the water. Water may also contain metal in solution such as iron, arsenic and copper. The important foreign ingredients in water are those of organic origin, such as microscopic plants, vegetable fungi, detritus of vegetable life, minute insects, infusoria, ova of insects, minute parasites, and animal debris. Water usually contains millions of various micro-organisms, mostly harmless, although at times it may also contain so-called pathogenic germs. States of Aggregation