Soda
2 of the 7 encyclopedias on this shelf carry an entry for Soda. Both are reproduced below, so you can see where they agree and where they differ.
Collier's New Encyclopedia (1921)
the protoxide of the alkaline metal sodium. It may be procured in an anhydrous state by burning the metal in dry air or oxygen. It is of a white color, greedily abstracting water from the air, which cannot be expelled by heat. In this state it forms hydrate of soda, or caustic soda. It is so similar in its properties to hydrate of potash that it need not be fully described here. Carbonates of Soda. - There are three of these, the ordinary monocarbonate, or common washing soda, Na2CO3, which in its crystalline form contains 10 equivalents of water; the sesquicarbonate, Na2 CO3+2NaHCO3+3Aq, which occurs in the mineral kingdom as trona and urao; and the bicarbonate, NaHCO3, which is prepared by passing carbonic acid through a concentrated solution of the carbonate till saturation takes place. It is also prepared by exposing the crystallized monocarbonate to the action of a current of carbonic acid; but in this method of making it only the outside portions of the converted crystals should be used, the inner parts being only partially changed. It is ground and dried at a very gentle heat, care being taken to avoid a high temperature, which would cause the formation of the sesquicarbonate. Bicarbonate of soda crystallizes in prisms. It occurs in commerce as a white crystalline powder, which is gradually converted into the sesquicarbonate by exposure to the air. It is much used in medicine. The properties of the monocarbonate are described under manufacture of soda.
The Encyclopedia of Founding (1892)
A compound of oxygen and a metallic basis called sodium. It was formerly called mineral alkali, as it is found in mineral seams and crusts, and in great abundance in certain lakes in Egypt being brought thither by the water which enters from the neighboring country during the overflow of the Nile, and precipitated by evaporation during the dry season. Barilla is impure soda obtained by burning plants near the sea. Kelp is obtained by burning seaweed. For the purposes of commerce, soda is obtained from common salt. See SALT; SODIUM.