1 82 PROGRESS OF SCIENCE. 



nently. This was accomplished later by Niepce and Daguerre. 

 Davy's practical experiment was the earliest step towards 

 photography. He further discovered ELECTRICITY to be AN 

 ILLUMINANT ; for severing the connecting wire of the electric 

 battery, and tipping its points with carbon a sluggish 

 conductor he obtained, not a spark, but actually a dazzling 

 arch of white flame, called voltaic arc.* He used carbon, 

 a poor conductor, because the intense heat would melt the 

 wires, whereas carbon is infusible ; the carbon points being 

 made to touch, and being then slightly separated, Davy 

 obtained the wonderful result just mentioned the new 

 illuminating power, electric light. The light, it may be 

 mentioned, is not produced by the combustion of the carbon, 

 but from bringing the solid particles (of it) into intense white 

 heat a heat in which platinum itself is melted as sealing- 

 wax in a candle-flame. With Davy the French school of 

 chemistry, which had headed science for a long time, receded 

 to the second rank, England taking the lead, and keeping 

 under the banner of Davy, Dalton, and Faraday. 



1778 1850. Gay-Lussac investigated IODINE, and 

 showed it to be an element analogous in its chief reactions 

 to chlorine, thereby putting an end to the oxymuriatic 

 theory; investigated also cyanogen, and fulminating silver; 

 he discovered THE LAW "Equal volumes of elementary gases 

 contain equal numbers of atoms " in other words, correspond 

 to equivalent weights of the substances (1809) a confirma- 

 tion of Dalton's atomic theory. Yet, exceptionally, phos- 

 phorus, arsenic, mercury, and cadmium, do not conform to 

 this law the two first giving vapours twice as dense, the 

 two last half as dense, as it is expected they should. 



1779 1848. Berzelius, one of the founders of modern 

 chemistry: was familiar with all its branches; discovered 

 several elements, and reduced from their oxides thorium, 

 calcium, barium ; developed the use of the BLOWPIPE into 

 a system of qualitative analysis ; made the greatest number 

 of accurate analyses of any chemist in his time ; identified 

 electrical polarity with chemical affinity every atom being 

 regarded by him as both positively and negatively electrified, 



* See Volta. 



