As is well known, bone-meal is a popular fertilizer. In this instance, although the fatty content of the crude bones may be high, the processes of degreasing have been advanced to such a stage of perfection as to bring about virtually the total elimination of this objectionable constituent. The fertilizer, if properly prepared, will not carry more than 1 per cent. of grease. The bones undergo a very thorough treatment, because this waste is able to feed several industries. Sewage is also coming more widely into favour as a fertilizer, as I explain in another chapter, while residues incurred in other ramifications of industry are now being carefully collected instead of being permitted to dissipate into the air or to pass to the furnaces for combustion. The dust arising from the reduction of woollen rags into shoddy forms an excellent hop manure. Dried blood is another first-class fertilizer--in fact it would be difficult to enumerate all the wastes which can now be profitably exploited for their soil-nourishing values. Speaking broadly, it may be stated that any refuse which, upon investigation, is able to yield 3 or more per cent. of nitrogen demands further examination for the discovery of the cheapest ways and means to reduce it to a fertilizer for sale at an attractive figure. If price be right no apprehensions need be entertained concerning disposal; the farmer will absorb the plant food, to nourish his crops, with eagerness. CHAPTER XVIII SAVING THE SEWAGE SLUDGE In matters pertaining to sanitation and the movement of sewage Great Britain undoubtedly leads the world. There our conquest ends. From that point onwards we can only point to lamentable inefficiency. For instance, the lay-out of the main drainage system of London, undoubtedly the finest illustration of such engineering in the world, has involved a capital expenditure of £12,514,606--$62,573,030. By the provision of enormous conduits and feeders the excrementitious matter from residences, offices, workshops, and factories of the metropolis is borne for miles to central stations. In this manner those natural and trade wastes, construed as being inimical to health, are removed swiftly and hygienically, and we compliment ourselves upon our prowess, which certainly is justifiable so far as it goes.