476 



THE POPULAR SCIENCE MONTHLY. 



period of vibration is increased, and consequently that the length of 

 the wave of sound sent out is lengthened. Now, suppose a molecule 

 or atom, the spectrum of which I am acquainted with, enters into com- 

 bination with another ; and suppose that the vibrations of the second 



molecule are weak, or lie outside 

 the visible range of the spectrum : 

 then the most simple assumption 

 which I could make would be that 

 the addition of the new molecule 

 is equivalent to an increase of the 

 mass of the other. An increase 

 of mass without alteration of the 

 force of the molecule will, as in the 

 case of the tuning-fork, lengthen 

 the period of vibration, and in- 

 crease the wave-length. If a case 

 of that kind were actually to hap- 



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pen, I should observe the whole 

 spectrum shifting toward the red ; 

 and this is what is observed in the 

 few simple cases to which I have 

 referred. The first observation to 

 that effect is due to Professor Bun- 

 sen, of Heidelberg. Examining 

 the absorption spectra of different 

 didymium salts, he found such a 

 close resemblance between them, 

 that no difference could be detect- 

 ed with instruments of small pow- 

 ers ; but with larger instruments it 

 was found that the bands varied 

 slightly in position, that in the 

 chloride they were placed most 

 toward the blue end of the spec- 

 trum, that when the sulphate was 

 substituted for the chloride a slierht 

 shift toward the less refrangible 

 end took place, -and that a greater 

 shift in the same direction occurred 

 on examining the acetate. Prof. 

 Bunsen remarks that ttie molecular weight of the acetate is larger 

 than that of the sulphate, and that the molecule of the sulphate, 

 again, is heavier than that of the chloride. He adds : " These differ- 

 ences in the absorption spectra of different didymium compounds can 

 not, in our present complete state of ignorance of any general theory 

 for the absorption of light in absorptive media, be connected with 



