of the new body), Mr. Crookes succeeded in discovering a comparatively large quantity of it in a sulphur mineral found in Belgium. The new element was found to be a heavy metal, closely resembling lead in its properties. It was named by the discoverer, thallium, from the Greek word thallos, a green twig, from the brilliancy of the single green line that indicates its presence. In like manner, Messrs. Reich and Richter have discovered a fourth new metal, which has been named indium, from its principal lines being found in the centre of the indigo of the spectrum.
The delicacy of spectrum analysis may be imagined from the fact that a quantity of sodium amounting to less than the two-millionth of a grain can be detected by its means. Indeed, it has taught us that sodium in one form or other exists almost everywhere. This mode of analysis is only serviceable to indicate the composition of any salt or other substance, the quantities of the different elements found by its use having no influence on the appearances brought out by the prism. Thus, a substance which has only been contaminated with sodium from being handled by warm fingers, will show the yellow bands as strongly as if it contained a large proportion of that metal.
For ordinary experiments in spectrum analysis the apparatus used is very simple. It consists of a tube with a fine slit at one end, and a convex lens at the other, for concentrating the light from the coloured flame upon the centre of the prism. After the light passes through the prism, it is examined by a small telescope of low magnifying power. The lamp used may be either a spirit-lamp or a colourless gas flame into which the substance to be examined is introduced upon a platinum wire.
We now come to another very important discovery, made by means of our prism and narrow slit—the
