6 10 HISTORY OF SCIENCE. 



of barytes ; magnesium, the metal of magnesia, and others. It was in 

 order to enable Davy to pursue experiments which were yielding 

 results of so novel and surprising a kind, that the members of the 

 Royal Institution had a galvanic battery constructed on a scale of 

 then unprecedented magnitude, and with this some of his later results 

 were obtained. 



Another investigation of Davy's of no little importance to chemistry 

 related to the substance we now call chlonne the name proposed for 

 it by Davy. Chlorine was discovered by Scheele (page 352), and 

 was some years afterwards carefully examined by Berthollet, who con- 

 cluded it to be a compound of muriatic acid (page 379) and oxygen. 

 This view was accepted, and for twenty years chlorine was known as 

 " oxy-muriatic acid gas" Davy in his turn submitted " oxy-mnriatic 

 add" to a very searching examination, in which he was led to the 

 discovery of new compounds. He declared that the gas was an ele- 

 mentary (or undecompounded) substance, combining with substances 

 to form bodies analogous to the oxides. These substances are now 

 called chlorides, as, for instance, in the case of common table salt, 

 which chemists designate chloride of sodium, or sodium chloride. 



Soon after Davy's researches on chlorine he attempted without 

 success to isolate fluorine. But at this time an accidental discovery 

 made by M. Courtois, a manufacturer of saltpetre at Paris, added to 

 the chemist's inventory a new substance belonging to the same small 

 peculiar group of elements as chlorine and fluorine. Courtois found 

 that the metallic vessels he used in preparing soda from seaweeds 

 became much corroded. He investigated the cause, and found it in 

 a substance separable without much difficulty from the ashes, a sub- 

 stance which in the solid form resembled plumbago in appearance, 

 and which, when heated, gave rise to a vapour of a magnificent violet 

 colour. This was iodine, for such was the name Davy gave it when 

 he, like other eminent chemists, had examined it. His conclusion 

 was decided that iodine was an undecomposable substance similar in 

 its general chemical behaviour to chlorine and fluorine. 



Many pages might yet be filled with the mere statements of Davy's 

 chemical investigations and discoveries. Perhaps the selection we 

 have made will suffice to convey to the general reader some notion 

 of the extension chemistry owes to Davy. There is, however, one 

 discovery or, rather, invention of Davy's which is so familiarly 

 identified with his name as to deserve mention here. If it has not 

 contributed to the structure of chemical science, it has contributed 

 directly to human welfare. It need hardly be said we here refer to 

 the Safety Lamp. The " Researches on Flame " which resulted in the 

 invention of the "Davy lamp," constitute an elegant model of scientific 

 investigation. We may see in these papers how Davy was led to adopt 

 the now well-known cylinder of wire gauze, in which he enclosed an 

 ordinary oil-lamp, as represented at Fig. 311. Apiece of common 



