Gun-cotton differs but slightly in appearance from ordinary cotton. It has a harsh feel and is less flexible than common cotton. It becomes highly electrified when rubbed between the fingers and appears luminous when rubbed in the dark. It is entirely insoluble in hot or cold water, but dissolves in a mixture of ether and ammonia. It will rarely take up more than two per cent. of moisture from the atmosphere. It is insensible to pressure, percussion or friction, unless closely confined or firmly compressed. It burns with a flash, but without explosion if brought into contact with burning or incandescent bodies. Wet gun-cotton will not burn or explode. Its ignition temperature is 360 degrees F. Pure gun-cotton will undergo no spontaneous decomposition and is the safest explosive known. Although it will not explode when wet, it may be detonated in this condition by a Mercury Fulminate Detonator with a small initial charge of dry gun-cotton in contact with it. =Nitro-Glycerine.= Nitro-glycerine is a nitric ether, or specifically a glyceryl tri-nitrate. Different degrees of nitration yield the mono-di- and tri-nitro glycerine, respectively; the latter being the nitro-glycerine of commerce. It is made by treating an exceedingly pure quality of glycerine with a mixture of nitric and sulphuric acids, the proportions commonly adopted being 3 parts of nitric acid, 5 parts of sulphuric acid, and from 1 to 1.15 parts of glycerine. The glycerine is added very slowly and with constant stirring. The agitation of the mixture is now usually accomplished by compressed air. When made from the purest ingredients, nitro-glycerine is an oily looking fluid, as clear and transparent as water. When freshly made it is whitish and opaque, but on standing it clears. The specific gravity at normal temperature is about 1.6 deg. F., when frozen 1.735 deg. F. Nitro-glycerine dissolves in alcohol, ether, methyl-alcohol, benzine, etc. Freshly made, opaque nitro-glycerine freezes at from 2.2 deg. F. to 7.6 deg. F., while the transparent, or clear, product freezes at from 39.2 deg. F. to 37.4 deg. F. In its frozen state it is less sensitive to shock or concussion than when it is in liquid. It may be completely evaporated when at a temperature of 158 deg. F. Its ignition temperature or firing point is 356 deg. F. Exposed to a temperature of 365 deg. F. it boils with evolution of vapors; at 381.2 deg. F. it volatilizes slowly; at 392 deg. F. it evaporates rapidly; at 422.6 deg. F. it detonates violently. From this point its behavior changes, passing through temperatures at which it explodes with constantly lessening violence until, at a dark, cherry-red heat, it assumes a spheroidal state and fails to explode. This applies to small quantities only. When gradually heated it is certain to explode at 356 deg. F.