416 Scientific Intelligence. 
wire. A reading of the scale is to be made for each position of the 
spectrum ; this reading however is not necessary for those who know the : 
particular spectra by repeated observation. 
odium.—The spectral reactions of sodium are the most delicate of all. 
_ The yellow line Nae the only one found in the sodium spectrum eorres- _ 
visible, in m 
phere even for a short time, rarely fail to produce the characteristic spec- 
a Striking a dusty book for example produces at a distance of sev- 
eral steps the strongest sodium reaction. j 
