ADDEESS. 9 



forty atmosplieres without prodacing 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 expansion up to twelve 

 atmospheres ; though more intense than at ordinary pressure, they 

 remained narrow and clearly defined. 



It follows, therefore, that a continuous spectrum cannot 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 vapour, 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, that is, of molecules containing different kinds 

 of atoms, but simply of a more complex molecule, which may be mad& 

 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 absorption 

 spectrum of oxygen, the bands may have an intensity proportional to the 

 square of the density of the gas, and may be due either to the formation, 

 of more complex molecules of the gas with inci'ease of pressure, or it may 

 be to the constraint to which the molecules are subject daring their 

 encounters with one another. 



It may be thought that at least in the coincidences of bright lines we 

 are on the solid ground of certainty, since the length of the waves set up. 

 in the ether by a molecule, say of hydrogen, is the most fixed and abso- 

 lutely permanent quantity in nature, and is so of physical necessity, for 

 with any alteration the molecule would cease to be hydrogen. 



Such would be the case if the coincidence were certain ; but an 

 absolute coincidence can be only a matter of greater or less probability,, 

 depending on the resolving power employed, on the number of the lines 

 which correspond and on their characters. When the coincidences are 

 very numerous, as in the case of iron and the solar spectrum, or the lines- 

 are characteristically grouped, as in the case of hydrogen and the solar 

 spectrum, we may regard the coincidence as certain ; but the progress of 

 science has been greatly retarded by resting important conclusions upon 

 the apparent coincidence of single lines, in spectroscopes of very small 

 resolving power. In such cases, unless other reasons supporting the 

 coincidence are present, the probability of a real coincidence is almost 

 too small to be of any importance, especially in the case of a heavenly- 



