The Spectroscope as a Detective.

Manufacturer and builder 12, 1876

The most usual application of the spectroscope consists in observing the luminous bands or lines in the spectra of the flames of different combustible bodies, or of bodies introduced into a flame; and as those lines differ for each element or compound, it gives a ready means to the chemist to determine what substances he has to deal with, providing they are only volatile enough to affect the flame and he can produce a flame or heat of sufficiently high temperature.

This subject has already been explained on page 68 of our March number for 1870, to which we refer the reader for details, and we will only add that since that time this method has been applied to the heavenly bodies, and it has been shown not only that sun and stars owe their bright luminosity to an exceedingly high temperature, but it has even been proved what the chemical nature of the substances is, which are in such a bright incandescent state as to illuminate the universe.

Another application of the spectroscope is to intercept the ordinary day, sun, or lamp light before passing it through the instrument, by placing in front of the same a glass vessel containing a colored liquid the nature of which it is desired to investigate. One of the simplest means is to place the liquid in a test-tube, and suspend this in front of the slit of the instrument, as represented in the adjoined engraving; the light passing through the liquid, undergoes a kind of filtration; some rays are absorbed, and being unable to reach the instrument, manifest their absence by broad dark bands in the spectrum, which otherwise shows all the colors in their regular order, which are red, orange, yellow, green, blue, indigo, violet, and lavender.

If, for instance, in this way various red solutions are examined — say a solution of carmin, of anilin red, of sulpho-cyanid of iron, a decoction of Brazil wood, etc., they will, notwithstanding the colors may be so much alike as to make it difficult to distinguish one from the other, show in the spectrums mentioned various absorption lines; they all will, of course, absorb more of the yellow, green, and blue than of the red, orange, and violet; but the parts absorbed, and their extent or the width of the dark bands, will differ in each in such a characteristic way as to leave no doubt about their nature; also the brightness and extent of the colors not absorbed will differ, some will show a larger and some a lesser part of the violet or lavender, and in others this color will also he absorbed and only the red shown, with or without the orange. If blood is largely diluted with water, the liquid shows a yellow tinge, while it is easy to make a similar shade with gumboge or saffron; however, when compared in the way described and illustrated in our figure, the blood will show two characteristic dark bands in the yellow and green, which no other substance will show. It is evident that this may be quite important in legal investigations.

Another method is to look through the spectroscope at a strongly illuminated color. The best for this purpose is where a small spectroscope is attached to the microscope in the form of an eye-piece. Also by this method the material of colors may be recognized from one another, which it otherwise would be impossible to distinguish without destroying the material in order to make a chemical analysis.

We have in a former number referred to a case where we proved by these means, before a court of justice, that two signatures were both made by an ink of Prussian blue, thus upsetting the claim that the inks were different.

Without the spectroscope the only means by which to settle this matter would be to apply chemical reagents to the signature in question, and thus destroy perhaps part of them. Thus, for instance, to distinguish a signature iu indigo blue ink from one written with Prussian blue, a drop of sulphuric acid would show the difference, as it obliterates the Prussian blue and not the indigo. The spectroscope however demonstrated the identity of the two inks without any injury whatsoever to the papers in dispute.

Other colored inks may be also thus distinguished, but not a deep black, as this is properly not a color, and absorbs all the rays equally, whether it be India ink, iron ink, chromate of potash ink, lampblack, etc.