288 CHEMISTRY. 



air in sufficient quantity to be seen very readily in the 

 spectroscope by agitating the air ; clapping the hands to- 

 gether or dusting a book will make the yellow line flash 

 out brilliantly, the particles of dust always containing 

 salt. 



Of strontium, the -^-^ of a milligramme is capable of 

 detection ; of calcium the same ; of lithium, the ^-^ of a 

 milligramme. 



403. Discovery of New Elements. So great is the deli- 

 cacy of spectrum analysis that many known elements have 

 been found more widely distributed than was previously 

 supposed, and four new elementary substances have been 

 discovered which existed in such minute quantities as to 

 be overlooked by the ordinary methods of examination. 

 Bunsen and Kirchhoff, the discoverers of spectrum analy- 

 sis, almost immediately after their invention discovered 

 two elements, ca3sium and rubidium, which belong to the 

 class of alkaline metals. Caesium was recognized by two 

 blue lines in its spectrum, which did not correspond in 

 position to the blue line of strontium, and rubidium by 

 two red lines. The material examined was the residue 

 from the evaporation of certain mineral waters. 



Since then thallium has been discovered by Prof. Crookes, 

 of London, and indium by Profs. Reich and Richter, of Sax- 

 ony. These bodies are mere chemical curiosities, occur- 

 ring in far too small quantities to become of commercial 

 value unless some unexpected and rich source is awaiting 

 discovery. In the chapters on metals we have barely al- 

 luded to them by name. 



404. Spectra of the Heavy Metals. The heavy metals 

 and their salts as a rule do not give spectra when heated 

 in the non-illuminating flame of a Bunsen burner, but this 

 is only because the temperature is not high enough to vol- 

 atilize them. To obtain spectra of these substances, there- 



