340 
r OCULAR SCIENCE REVIEW. 
chemical composition of the bodies by which they are pro- 
duced. 
But the case is very different when the spectrum of a gaseous 
body is examined. Then we have an interrupted spectrum, 
composed of bright lines of light of certain colours only, with 
intervals between them more or less completely dark. When- 
ever an interrupted spectrum composed of bright lines is seen, 
we infer that we are dealing with the spectrum of a transparent 
gaseous body in a state of intense glowing heat. Each gas or 
vapour emits light of a particular kind, which is collected into a 
line or group of lines peculiar to itself. If the position of these 
lines be accurately measured, it is found that the same sub- 
stance always gives rise to lines which occur invariably, exactly 
in the same part of the spectrum. Hence these lines may be 
made use of as tests of the particular substance by which they 
are produced. I showed you, just now, chemical tests of silver, 
copper, and iron. Now let us look at the optical tests, which 
are not less certain. Silver, for example, when heated suffi- 
ciently to distil it in vapour, emits a brilliant green light, which 
is mainly concentrated into two intense green bands, fig. 3, the 
other part of the spectrum being produced by the charcoal on 
which the silver rests. Copper also emits a green light, but this is 
seen to consist of a more complex system of bright bands, fig. 4. 
Iron, when volatilised at a still higher heat, in like manner gives 
a light with a system of bands still more complicated and nume- 
rous. Magnesium likewise furnishes an intense green band, 
which is really composed of three, so closely approaching each 
other as to appear on the screen but one. Each metal and each 
chemical element has in fact its own special set of bands. Each, 
when converted into vapour, vibrates in a definite way, pro- 
ducing a special set of luminous vibrations of fixed frequency, 
just as when a particular tuning-fork is struck, it occasions a 
series of waves of sound which occur with the particular fre- 
quency characteristic of its peculiar musical note. If, there- 
fore, we can determine with accuracy the position and number 
of lines in the spectrum of each chemical element, we can at 
once recognise its presence whenever we see its light, by simply 
measuring the position of these lines. 
Why, then, does a substance, when in the solid or the liquid 
form, not produce a spectrum like that which it furnishes in the 
gaseous state ? liodies, when in a solid or liquid form, are tied 
together l)y the attraction of their particles, and consequently 
their vibrations appear to be those of the mass, not those ot 
their constituent atoms ; whereas, in the state of gas or vapour, 
their constituent particles are widely separated from each other, 
and each is free to move independently of the rest. 
You will now easily perceive that this optical method of ana- 
