DISCOVERY BY COMPARISON OF FACTS. 579 



absorbed. He also concluded that heated vapour of sodium 

 absorbs rays of the same colour as those it emits whilst 

 burning. By further experiments of comparison he dis- 

 covered that the vapour of potassium and other individual 

 metals all possess the property of absorbing rays of the same 

 colour as they emit in burning ; and he thus explained 

 the persistent mystery of ' Fraunhofer's (or rather Wollas- 

 ton's) lines,' which had for a period of forty or fifty years 

 baffled the insight of many investigators. He showed 

 that the sun is surrounded by an atmosphere containing a 

 number of metallic vapours, which absorb particular por- 

 tions of the white light emitted by the sun, and cause the 

 numerous dark lines in the solar spectrum. This discovery 

 also furnished a method by means of which the com- 

 position of other luminous heavenly bodies might be 

 ascertained. 



Tabern Bergman, Professor of Chemistry at Upsala, 

 appears to have been one of the first to observe, in the 

 year 1775, by comparison of chemical facts, that chemical 

 union is determined not by simple elective affinity only, 

 but by the sum of the affinities of the bodies mixed 

 together ; and thus discovered the idea of double decom- 

 position by balance of affinity. By similar means, Ber- 

 thollet, in 1803, further noticed that both simple elective 

 affinity and also double decomposition were each affected 

 by the relative quantities of each of the substances present. 



b. By comparison of facts with hypotheses. Many 

 of the numerous discoveries in the comparative sciences 

 of anatomy, histology, embryology, and physiology have 

 been made by this method. It was largely by means 

 of comparison that Groethe, about the year 1790 (and 

 before him Wolff and Linnaeus also, but vaguely), suggested 

 the discovery that all the different parts of a plant are 

 only modified stems and leaves, so altered by surrounding 



p p 2 



