The teachings of Dr. Mueller and Mouras went unheeded for a long time, on account of the chemical processes then in vogue. It was maintained by those who were supposed to know, that lime and other antiseptic substances were particularly valuable in sewage purification, because they destroyed living organisms, such as bacteria, which give rise to putrefaction and fermentation. They contended that if all the organisms could be destroyed, that sewage would be rendered unobjectionable. So conditions stood when in January, 1887, Mr. Dibden read a paper before the Institute of Civil Engineers, in which he pointed out that the very essence of sewage purification was not the destruction of bacterial life, but the resolution of organic matter into other combinations by the agency of the micro-organisms. He pointed out, further, that a septic and not an antiseptic action was what was wanted, consequently any process which arrested the activity of the bacteria was the reverse of what was desired. Dibden's paper had the effect of turning investigation in the right direction, but a world of experimenting on a practical scale would be necessary before the practice of sewage purification could be established on a safe, sound and scientific footing. It remained for the Massachusetts State Board of Health to conduct those investigations, and so thoroughly was it accomplished that the records of their experiments furnish the basis for sewage purification practice in the United States. The experiments have been carried on since 1887, and the thoroughness and value of these investigations can be judged by the fact that during one period of twenty-two months four thousand chemical examinations were made in addition to the microscopic examinations. Following the historic investigations of the Massachusetts State Board of Health, numerous engineers and investigators commenced applying to practice the principles there laid down, and with such good results that there are upwards of 200 purification plants in the United States to-day, and in Pennsylvania alone plans are under way at the present time for over one hundred sewage disposal works. Such a showing is encouraging, and leads to the hope that within a decade no city of any importance within the States will be pouring impurified sewage into public streams or lakes.