Dll. MILLED S EXETED LECTUDE. 339 
with all degrees of intermediate frequency between. We may 
say that red is the bass, and violet the treble of colours. 
Few things in the progress of science are more remarkable 
than the manner in which discoveries in one branch of enquiry 
often prove of the greatest importance to the advancement of 
other branches of knowledge, with which they appear, at first, 
to have no connection. A striking instance of this kind occurs 
in the manner in which optical science has aided the studies of 
the chemist. By means of chemical analysis, it has been dis- 
covered that the various substances which are found upon the 
earth may be separated into a comparatively small number of 
bodies, out of which no other kind of matter may be separated. 
Out of sulphur, for example, nothing but sulphur can be 
obtained. These the chemist terms elements, and out of these 
all the different substances with which we are familiar are 
formed. For instance, the air we breathe is composed mainly 
of a mixture of two such elementary bodies — viz. the gases 
oxygen and nitrogen; water consists of oxygen chemically 
united with the gaseous element hydrogen ; and among the 
elements are fhe various metals — gold, silver, iron, copper, 
magnesium, sodium, and so on. These different substances the 
chemist distinguishes from one another by means of certain 
chemical tests. For instance, I may, by the addition of ammonia 
to a certain solution, find copper by the beautiful blue tinge pro- 
duced. In like manner, I may, by the white cloud occasioned 
on adding common salt to a second vessel, ascertain the presence 
of silver ; while in a third, the presence of iron is not less cer- 
tainly revealed by the red colour produced on adding potassic 
sulphocyanide. Within the last few years optics has come to 
the aid of chemistry in a manner which I must now endeavour 
to explain. 
We have seen that this spectrum of glowing charcoal is con- 
tinuous from end to end. Provided that the ignited material 
be a solid, its chemical nature has no influence upon the colour 
of the light which it emits. Whether, for example, the heated 
body consist of lime, magnesia, flint, cla}^, charcoal, iron, or 
platinum, so long as the substance is in the solid form a con- 
tinuous spectrum is obtained, containing rays of every degree of 
refrangibility, and of every colour, from the deepest red to the 
extreme violet. The spectrum of an ignited cloud of solid par- 
ticles, such as that produced by soot or any solid suspended 
matter, such as phosphoric anhydride when phosphorus is 
burned in oxygen gas, is also continuous. The same continuous 
spectrum is also produced by a white-hot liquid, such as melted 
copper or cast iron, and no difference dependent upon the 
chemical nature of the substance can be perceived in any of 
these cases. Such spectra, therefore, teach us nothing of the 
