In the course of these experiments Professor Stokes observed that even quartz of a certain thickness is not transparent to invisible lines of the highest refrangibility, for the highest aluminium line, which is double, could only be seen by rays passing through the edge of the prism. This leads to another branch of the subject, namely, the absorption of the invisible rays by solids, liquids and gases. Mr. Wm. Allen Miller has shown from his own experiments that bodies pervious to the chemical rays in the solid form, are so also in the liquid and gaseous form; that colourless transparent solids which absorb the photographic rays, absorb them more or less also in their liquid and gaseous states. He has moreover found that the following substances have the same maximum transparency:—rock crystal, ice, and fluor spar among solids, water among liquids, the three elementary gases and carbonic acid among gaseous substances. The most opaque to the invisible rays are, nitrate of potash, bisulphide of carbon, and sulphuretted hydrogen. It appears that a thin plate of mica is intensely opaque to all the invisible rays except a small portion of them of the lowest refrangibility. The absorptive property however is partial: an absorptive substance either cuts off a portion of the light of a fluorescent spectrum or stripes it with dark lines: each substance absorbs rays peculiar to itself. Those employed by Professor Stokes were the alkaloids and glucosides, and he assumed the spectrum of tin for their examination because it has a long interval of continuity. The fluorescent property of yellow uranite was discovered by Professor Stokes some years ago, and now he has added another fluorescent mineral in adularia or moonstone; from its natural faces and planes of cleavage alike a beautiful blue fluorescence emanated under the induction spark. As the same was observed in colourless felspars generally, Professor Stokes concluded that fluorescence is an inherent property in the silicate of alumina and potash constituting the crystal of moonstone. The blue fluorescence extended to a very sensible though small depth within the substance. A particular variety of fluor spar found at Alston Moor in Cumberland, which is very pale by transmitted light, shows a strong blue fluorescence, and is eminently phosphorescent on exposure to the electric spark. It is the same kind of crystal in which Sir David Brewster originally discovered the property of fluorescence. On presenting such a crystal to the spark passing between aluminium terminals, besides the usual blue fluorescence, there was another of a reddish colour extending not near so far into the crystal, produced by the rays belonging to the strong lines of aluminium of extreme refrangibility.