In Connecticut also the colors of many public water-supplies have been recorded in the reports of the State Board of Health on the platinum color-standard. The waters of the Middle States, with rare exceptions, are almost free from color. In the Northwest waters are obtained often with very high colors, even considerably higher than the New England waters, and some of the Southern swamps also yield highly colored waters. REMOVAL OF COLOR. Peaty coloring-matter is almost perfectly in solution, and only a portion of it is capable of being removed by any form of simple filtration. In order to remove the coloring-matter it is necessary to change it chemically, or to bring it into contact with some substance capable of absorbing it. For this reason sand filtration with ordinary sands, having no absorptive power for color, commonly removes only from one fourth to one third of the color of the raw water. MEASUREMENT OF TURBIDITY. The amount of mud or turbidity in a water is often expressed as the weight of the suspended matters in a given weight of the water. Most of the data relating to turbidities of waters are stated in this way, because this was the only method recognized by the earlier investigators. This method of statement has some disadvantages: it fails to take into account the different sizes of particles which are carried in suspension by different waters, and at different times. Thus the Merrimac River in a great flood may carry 100 parts in 100,000 of fine sand in suspension, and still it could hardly be called muddy; while another stream carrying only a fraction of this amount of fine clay would be extremely muddy. Further, an accurate determination of suspended matters is a very troublesome and tedious operation, and cannot be undertaken as frequently as is necessary for an adequate study of the mud question. Turbidity is principally important as it affects the appearance of water, and it would seem that optical rather than gravimetric methods should be used for its determination. Various optical methods of measuring turbidity have been proposed. The general method employed is to measure the thickness of the layer of water through which some object can be seen under definite conditions of lighting. The most accurate results can probably be obtained in closed receptacles and with artificial light. Such a method has been used by Mr. G. W. Fuller at Louisville and Cincinnati in connection with his experiments, and is described by Parmelee and Ellms in the Technology Quarterly for June, 1899. This apparatus is called by Mr. Fuller a diaphanometer.