The value of dung manures depends not merely upon the protein content, but also upon its content of phosphate and potash, as well as other organic matter. The protein breaks down into simpler nitrogenous compounds, and eventually, through ammonium carbonate, it becomes nitrate. Nitrogenous manures darken leaves and increase growth considerably. Dung manures are deficient in phosphates and potash and are of value partly as nitrogenous manures producing growth, and partly as dressings of organic matter for soil. From both points of view it is desirable that the manure should be well decayed. Fresh dung manures are both wasteful and injurious to soil, except perhaps to very stiff clays. They are wasteful inasmuch as much ammonia escapes, and injurious inasmuch as they cause the "denitrification" of the valuable nitrates already in the soil. When possible dung manures should be kept under cover. Free exposure to air and rain will sometimes reduce its value by one half. It should be stored until "sweet," and until the straw has rotted and become "short." This takes usually several months. A ton of well-rotted farmyard manure contains very approximately 10-12 lbs. nitrogen, about the same amount of potash, and about half that quantity of phosphates. It is, however, very variable. Horse manure is rather richer than cow manure, but more liable to loss on storage. Pig manure is intermediate between them. Sheep manure is distinctly richer in protein, and has therefore greater value as nitrogenous fertilizers. Poultry droppings are richer still, perhaps partly because they include the urinary products. When fresh they contain 18-25 lbs. nitrogen, 12-24 lbs. phosphate, and 6-12 lbs. potash per ton. When dried they have about double the value. Pigeon manure is even richer, and the pigeon loft scrapings have a manurial value about double that of dry hen manure, and eight times that of farmyard manure. Guano is much decayed droppings of sea birds on the tropical coasts of Africa and America. The supplies are now quite exhausted, and the market guanos are chiefly artificial fertilizers. * * * * * There is one other animal protein which must be referred to before this volume is concluded, viz. silk. This is obtained from the cocoon of the "silkworm," which is the general name given to the larvæ of certain bombycid moths. These larvæ feed on the leaves of the mulberry, and when ready to pupate produce a considerable supply of a soft and delicate thread which is wound round about the larva itself. This is the raw silk, and it is unwound from the cocoon in a machine called the "silk-reel," and may then be wound into a thread. Two or more threads twisted together form "thrown-silk." Silk threads are also woven into cloth of characteristic texture and appearance. This protein thus forms the raw material of one of the most important textile industries. * * * * *