622 THE POPULAR SCIENCE MONTHLY. 



would probably have tapped their foreheads significantly and 

 perhaps even winked at one another. But to-day stellar chemis- 

 try is a recognized branch of cosmical research. If a ray of sun- 

 light be passed through a glass prism, it gives a bright, continu- 

 ous spectrum, varying in color from red to violet. If the source 

 of light be a vapor of a metal, or metallic salt, the continuous 

 spectrum is replaced by one or several bright lines whose position 

 and color are invariable for the same metal. If a more intense 

 white light be allowed to pass through the metallic vapor, the for- 

 mer bright lines appear black, but are easily recognized by their 

 number and position. In the solar spectrum a whole series of 

 such black lines are distinguishable, and by correspondence they 

 are believed to indicate the presence of at least seventeen of our 

 earth elements, while there appears to be at least one element in 

 the solar atmosphere for which we have no counterpart on earth. 

 A similar study of the light of the stars has disclosed in their 

 atmosphere a number of earth elements and has indicated the 

 presence of others unknown on earth. This little piece of flint 

 glass, ground into the shape of a triangular prism, has proved the 

 " open sesame " to secrets so profound that in its absence they 

 must have been regarded as belonging to the great domain of the 

 unknowable. It is something of a triumph for the near-sighted 

 philosopher on our planet to announce that he has discovered 

 magnesium on the star Aldebaran and sodium on Sirius. 



While the refractive power of glass opens so many wonderful 

 possibilities, its simple transparency is a quality which adapts it 

 for many less ambitious uses. Much of the work of science is that 

 of measurement. Sir William Thomson has indeed said that in 

 any branch of research we have only so much science as we have 

 mathematics. For this service of measurement, glass is admira- 

 bly adapted. The measurement of heat by the thermometer is 

 an example of a frequent and important operation, while the 

 manufacture of the instrument itself is a type of many similar 

 processes. 



The ordinary thermometer measures heat by the expansion of 

 some such liquid as mercury. The increase in bulk for any slight 

 increment of heat would be too small, however, to be perceptible 

 in a mass of the fluid metal. Hence the necessity for the glass- 

 blower's skill by which the increased volume is made sensible to 

 the eye. By having a comparatively large bulb in connection 

 with a tube of very fine bore, the slightest expansion in the vol- 

 ume of the mercury becomes at once apparent by a relatively 

 large change of level of the fluid in the tube. The greater the 

 discrepancy between the bulb and the tube, the more sensitive the 

 instrument. 



The operation of making a thermometer begins in the crucible- 



