Page:Popular Science Monthly Volume 39.djvu/640

622 would probably have tapped their foreheads significantly and perhaps even winked at one another. But to-day stellar chemistry is a recognized branch of cosmical research. If a ray of sunlight be passed through a glass prism, it gives a bright, continuous 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 former 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 admirably 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 glassblower'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 volume 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