=Ammonium Salts.=--If the body is fresh, and yet the salts of ammonium present in large amount, it is safe to conclude that they have an external origin; but there might be some considerable difficulty in criminal poisoning by a neutral salt of ammonium, and search for it in a highly putrid corpse. Probably, in such an exceptional case, there would be other evidence. With regard to the quantitative separation and estimation of the fixed alkalies in the ash of organic substances, the reader is referred to the processes given in “Foods,” p. 99, _et seq._, and in the present work, p. 121. PART V.--MORE OR LESS VOLATILE POISONOUS SUBSTANCES CAPABLE OF BEING SEPARATED BY DISTILLATION FROM NEUTRAL OR ACID LIQUIDS. HYDROCARBONS--CAMPHOR--ALCOHOL--AMYL NITRITE--ETHER--CHLOROFORM AND OTHER ANÆSTHETICS--CHLORAL--CARBON DISULPHIDE--CARBOLIC ACID--NITRO-BENZENE--PRUSSIC ACID--PHOSPHORUS. I.--Hydrocarbons. 1. PETROLEUM. § 137. Petroleum is a general term for a mixture of hydrocarbons of the paraffin series, which are found naturally in certain parts of the world, and are in commerce under liquid and solid forms of various density. Crude petroleum is not imported into England, the original substance having previously undergone more or less rectification. The lighter and more volatile portions are known under the name of cymogene, rhigolene, gasolene, and naphtha. § 138. =Cymogene= has a specific gravity of ·590, and boils at 0°. It has been employed in refrigerating machines. It appears to consist chiefly of butane (C₄H₁₀). § 139. =Rhigolene= is now used in medicine in the form of spray to produce local anæsthesia. It boils at 18°, and has a density of ·650. § 140. =Gasolene= has a density of ·680-·688; it has received technical applications in the “naphthalising” of air and gas. § 141. =Benzoline= (=mineral naphtha=, =petroleum naphtha=, =petroleum spirit=, =petroleum ether=) is a mixture of the lighter series of hydro-carbons; the greater part consists of heptane, and there is also a considerable quantity of pentane (C₇H₁₆) present. The specific gravity varies from ·69 to ·74. It is very inflammable, and is used in sponge lamps, and also as a solvent for gutta-percha, naphthalene, paraffin, wax, and many other bodies. By the practical chemist it is much employed. The similarity of the terms _benzoline_ and _benzene_ has caused benzoline to be often confused with _benzol_ or _benzene_, the leading constituent of coal-tar naphtha (C₆H₆). Mr Allen[132] gives in the following table a summary of the chief points of distinction, both between petroleum naphtha, shale naphtha, and coal-tar naphtha. The table is founded upon the examination of particular samples, and commercial samples may present a few minor deviations. [132] _Commercial Organic Analysis_, vol. ii. p. 31.