74. CELESTIAL SPECTROSCOPY. 
lines. An individual molecule of sodium will be sensibly in the same 
condition, considering the relatively enormous number of the molecules 
of the other gases, whether the flame is scantily or copiously fed with 
the sodium salt. With a small quantity of sodium vapor the intensity 
will be feeble except near the maximum of the lines; when, however, 
the quantity is increased, the comparative transparency on the sides of 
the maximum will allow the light from the additional molecules met 
with in the path of the visual ray to strengthen the radiation of the 
molecules farther back, and so increase the breadth of the lines. 
In a gaseous mixture it is found, as a rule, that at the same presstire 
or temperature, as the encounters with similar molecules become fewer, 
the spectral lines will be affected as if the body were observed under 
conditions of reduced quantity or temperature. 
In their recent investigation of the spectroscopic behavior of flames 
under various pressures up to forty atmospheres, Profs. Living and 
Dewar have come to the conclusion that though the prominent feature 
of the light emitted by flames at high pressure appears to be a strong 
continuous spectrum, there is not the slightest indication that this con- 
tinuous spectrum is produced by the broadening of the lines of the same 
gases at low pressure. On the contrary, photometric observations of 
the brightness of the continuous spectrum, as the pressure is varied, 
show that it is mainly produced by the mutual action of the molecules 
of a gas. Experiments on the sodium spectrum were carried up to a 
pressure of forty atmospheres without producing any definite effect on 
the width of the lines which could be ascribed to the pressure. In a 
similar way the lines of the spectrum of water showed no signs of ex- 
pansion up to twelve atmospheres; though more intense than at ordi- 
nary pressure, they remained narrow and clearly defined. 
It follows therefore that a continuous spectrum can not be considered, 
when taken alone, as a sure indication of matter in the liquid or the 
solid state. Not only, as in the experiments already mentioned, such 
a spectrum may be due to gas when under pressure, but, as Maxwell 
pointed out, if the thickness of a medium, such as sodium yapor, which 
radiates and absorbs different kinds of light, be very great, and the 
temperature high, the light emitted will be of exactly the same composi- 
tion as that emitted by lamp-black at the same temperature, for the 
radiations which are feebly emitted will be also feebly absorbed and 
can reach the surface from immense depths. Schuster has shown that 
oxygen, even in a partially exhausted tube, can give a continuous spec- 
trum when excited by a feeble electric discharge. 
Compound bodies are usually distinguished by a banded spectrum; 
but, on the other hand, such a spectrum does not necessarily show the’ 
presence of compounds—thatis, of molecules containing different kinds 
of atoms—but simply of a more complex molecule, which may be made | 
up of similar atoms, and be, therefore, an allotropic condition of the 
same body. In some cases—for example, in the diffuse bands of the, 
