Vitriol
2 of the 7 encyclopedias on this shelf carry an entry for Vitriol. Both are reproduced below, so you can see where they agree and where they differ.
Aiton's Encyclopedia (1910)
Important: Safety Warning This page directs the use of hazardous substances — such as benzine, gasoline, lye, or strong acids — for household tasks. These materials are highly flammable, corrosive, or poisonous, and the practices described have caused fires, burns, and poisonings. In addition, this page states that a trace of copperas added to a reservoir kills the typhoid germ. It does not, and treating drinking water this way would leave it contaminated while appearing to have been made safe. This information has been left in for historical purposes but should not be acted upon. vit'ri-ul, or Oil of Vitriol, sulphuric acid. It is composed of hydrogen, sulphur, and oxygen. It is an oily acid with a strong affinity for water. A drop poured on wood absorbs the moisture and leaves a charred spot. Poured on the skin of one's hand it draws out the water and leaves a painful sore difficult to heal. In the laboratory sulphuric acid must be handled with care. A slight amount spilled on woolen clothing will eat holes in it, or, if the amount be sufficient, it will burn the body, causing death. Vitriol or sulphuric acid is used in many industries where it is desired to free substances from animal or vegetable matter. Blue vitriol, consisting chiefly of sulphur and copper, is the basis of Scheele's green and paris green . It is used extensively in coloring papers, calico printing, gravity batteries, electro-plating, and especially as a spray in vineyards to kill insects, etc. Green vitriol, or copperas, literally, copper rose, is a sulphate of iron. It is usually of a green color, and is derived commercially from iron pyrites. It is employed in dyeing black, and as a developing agent in photography. Copperas forms an excellent black ink. It is also added to water, say a pound to the gallon, for a disinfectant for sinks and sewers. The slightest trace, a drop in a million, added to water in a reservoir, kills the typhoid germ. White vitriol is a sulphate of zinc. Red vitriol a sulphate of cobalt. See Spraying ; Paris Green ; Dyeing
Collier's New Encyclopedia (1921)
a commercial name for sulphuric acid, and for several salts which that acid forms. The term vitriol was in common use among the alchemists, and was derived from vitrium, "glass," on account of the glassy appearance and transparency of copperas or green vitriol (the sulphate of iron). From copperas the alchemists prepared a strong sulphuric acid by distillation, whence also the acid was termed the oil of vitriol. In addition to these there is yet commercially known blue vitriol, which is a sulphate of copper, and white vitriol, the sulphate of zinc (zinci sulphas of the "Pharmacopeia") much used in medicine.