Sir C. Lyell on the Mineral Waters of Bath. 19 
have wholly escaped detection by ordinary tests, are made known 
to the eye by the agency of light. Thus, for example, a solid 
substance such as the residue obtained by evaporation from a 
mineral water is introduced on a platinum wire into a colorless 
gas-flame. The substance thus volatilized imparts its color to 
the flame, and the light, being then made to pass through a 
rism, is viewed through a small telescope or spectroscope, as it 
is called, by the aid of which one or more bright lines or bands 
are seen in the spectrum, which, according to their position and 
color, indicate the presence of different elementary bodies. 
Professor Bunsen, of Heidelberg, led the way, in 1860, in the 
application of this new test to the hot waters of Baden-Baden 
and of Diirkheim in the Palatinate. He observed in the spec- 
trum some colored lines of which he could not interpret the 
meaning, and was determined not to rest till he found out what 
evaporate fifty tons of water to obtain 200 grains of what proved 
to be two new metals. Taken together, their proportion to the 
ger quantities, may furnish medical science with means of com- 
bating diseases which have hitherto baffled all human skill. 
While I was pursuing my inquiries respecting the Bath wa- 
ving about as high a temperature as that of the Bath waters, 
and of which, strange to say, no account has yet been published. 
