334 GREAT MEN OF SCIENCE 



with the strong currents from his new elements, provided 

 him with sources of Hght of hitherto unattained intensity; 

 and for the purpose of measuring them he invented the 

 grease-spot photometer, which for fifty years was the only 

 photometer used. A later refinement was to replace the 

 grease-spot by a special cube of glass. 



His preparation of new elements in a pure state led him to 

 invent the ice calarimeter, in order to be able to measure, for 

 the purpose of atomic weight determination, the specific 

 heats of the elements with great accuracy, in spite of the 

 small quantities available. This calorimeter again provided 

 the means for further investigations, and remained for a very 

 long time unequalled by any other method. 



Bunsen had also not a little to do with investigations con- 

 cerning the interior of the earth, volcanic phenomena, and 

 the formation of rocks. On a scientific journey to Iceland 

 (1846) he discovered the nature of geysers, and later investi- 

 gated the dependence of the melting-point of materials upon 

 pressure, most substances being specifically heavier in the 

 solid than in the liquid state. He found that the melting- 

 point rose rapidly as the pressure increased, and drew atten- 

 tion to the high pressures which undoubtedly exist in the 

 interior of the earth, where we must therefore assume a 

 considerably increased melting-point for rocks and substances 

 composing them. This agrees with the fact that the interior 

 of the earth must be for the most part solid, as already 

 follows from Newton's investigation of the phenomena of 

 tides, inasmuch as the body of the earth only takes part in 

 tidal motion to a very small extent, a fact which cannot be 

 ascribed solely to the solid state of the surface. Further- 

 more, Bunsen's results concerning melting-points formed a 

 valuable confirmation of a calculation which at that time 

 had just been based upon the thermo-dynamics of Clausius 

 and W. Thomson. This same calculation led the opposite 

 kind of behaviour to be expected in the case of ice, which is 



