PROGRESS IN CHEMISTRY. 237 



theory that a chloride was a compound of the unknown basis of oxy- 

 muriatic acid with oxygen and the oxide of a metal, he introduced the 

 simpler and correct view that a chloride is merely a compound of the 

 element chlorine with a metal. In 1813 he established the similar 

 nature of fluorine, pointing- out that on the analogy of the chlorides it 

 was a fair deduction that the fluorides are compounds of an undiscov- 

 ered element, fluorine, with metals; and that hydrofluoric acid is the 

 true analogue of hj^drochloric acid. The truth of this forecast has 

 been established of recent years by Henri Moissan, who isolated gas- 

 eous fluorine l)y subjecting a mixtui-e of hydrofluoric acid and hydro- 

 gen potassium fluoride contained in a platinum U tube to the action of 

 a powerful electric current. He has recently found that the tube may 

 be equally well constructed of copper; and this may soon lead to the 

 industrial application of the process. The difliculty of isolating fluor- 

 ine is due to its extraordinary chemical energy, for there are few sub- 

 stances, elementary or compound, which resist the action of this pale, 

 yellow, sutt'ocating gas. In 1811 iodine, separated by Courtois from 

 the ashes of sea plants, was shown ])y Dav}^ to be an element analo- 

 gous to chlorine. CTa3'-Lussac subsequentl}' investigated it and pre- 

 pared many of its compounds; and in 1826 the last of these elements, 

 bromine, was discovered in the mother liquor of sea salt by Balard. 

 The elements of this group have been termed ''halogens," or "salt 

 producers." 



JOHN DALTON's theory. 



While Davy was pouring his researches into the astonished ears of 

 the scientific and dilettante world John Dalton, a Manchester school- 

 master, conceived a theory which has proved of the utmost service to 

 the science of chemistry, and which bids fair to outlast our day. It 

 had been noticed by Wenzel, by Kichter. by WoUaston, and by Cavendish 

 toward the end of the last century that the same compounds contain 

 the same constituents in the same proportions, or, as the phrase runs, 

 "possess constant composition." WoUaston, indeed, had gone one 

 step further, and had shown that when the vegetable acid, oxalic acid, 

 is combined with potash it forms two compounds, in one of which the 

 acid is contained in twice as great an amount relatively to the potash 

 as in the other. The names monoxalate and binoxalate of potash wert> 

 applied to these compounds to indicate the respective proportions of 

 the ingredients. Dalton conceived the happy idea that by applying 

 the ancient Greek conception of atoms to such facts the relative weights 

 of the atoms could be determined. Illustrating his views with the two 

 compounds of carbon with hydrogen, marsh gas and olefiant gas, and 

 with the two acids of carbon, carbonic oxide and carbonic "acid," he 

 regarded the former as a compound of one atom of carbon and one of 

 hydrogen and the ^second as a compound of one atom of caihon and 



