io8 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



is held in solution by fixed air in much the same way as 

 the chalk and magnesia in the water investigated by 

 Cavendish. This property of fixed air was discovered by 

 T. Lane, an English apothecary, who described his experi- 

 ments in the Philosophical Transactions for 1769, as follows : 



"A wide-mouthed bottle, containing half a pint of 

 distilled water and sixty grains of steel filings, was suspended 

 forty-eight hours over some distiller's molasses, in brisk 

 fermentation ; so as to receive the fixed air escaping from 

 the fermenting liquor ; the surface of which was ten inches 

 below the mouth of the bottle. Immediately after its 

 removal, the clear water was decanted from the filings and 

 ochrous sediment." 



" This liquor had a brisk and ferruginous taste, with a 

 flavour of the molasses. An infusion of galls, or green tea, 

 soon changed part of it to a colour like ink. The remainder, 

 being exposed to the open air, presently became turbid, 

 threw up a party coloured pellicle, and deposited a yellowish 

 sediment." 



" The water now retained but little power of tinging with 

 galls; and in a few days lost this property entirely" ("On 

 the Solubility of Iron in Simple water, by the intervention 

 of Fixed Air," Phil. Trans., 1769, 59, 218). 



These simple experiments, carried out 140 years ago, disclose 

 the two essential features of the modern theory of the rust- 

 ing of iron, namely, (i) that iron is dissolved by carbonic 

 acid to a colourless solution, and (2) that this solution 

 deposits a yellow rust on exposure to the air. 



Black (1755) discovers a second series of salts derived 

 from fixed air. Whilst there is some uncertainty as to 

 whether Black recognised fixed air as an acid, there is no 

 doubt as to his discovery of the existence of a salt containing a 

 larger proportion of fixed air than ordinary potash. 



" That the fixed alkali, in its ordinary state, is seldom en- 

 tirely saturated with air, seems to be confirmed by the 

 following experiment. I exposed a small quantity of a pure 



