Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/307

 L A N L A N 291 but which subsequently passed through many editions with varying titles and in several languages. Madame Merian stated that the common South American species, now known as Fulgora laternaria, L., was highly luminous at night, so much so that she was enabled to read by the light of one only, and that when several were confined together the interior of the box appeared all ablaze. No one doubted these statements, and the illustrious Linnaeus used the words &quot; Prominente fronte noctu lucem vivacissimam spargit &quot; in diagnosing the insect in his Systema Naturae. Moreover, it was believed that, because one species had been asserted to be luminous, others allied thereto must possess the same power ; the specific names used by Linnaeus, such as candelaria, phosphorea, noctivida, lucernaria, and flammea, may be adduced as instances. Of these one only, the F. candelaria of China, has become (with the original laternaria} a subject of controversy, for it also was asserted to be luminous. As time wore on many intelligent naturalists and other travellers visited both South America and China, and they concluded that the light must be produced only under very exceptional conditions, or that the original statement was an error, for they could not detect any luminosity, nor, as a rule, was such a property believed in by the natives of the regions. Quite recently many naturalists of undoubted authority have resided for years in the districts where these insects occur without having .personally detected luminosity (though directly in search of it), and without obtaining any indications of the existence of such a belief in the minds of the natives. On the other hand, there have been a few travellers who have professed to be able to confirm Madame Merian s statements, both from personal observation and from information derived from native sources. Possibly the last of these was within the last twenty years, and his assertion concerned F. candelaria, and upon his statement an entomologist of repute, lately deceased, maintained to the last his belief in the luminous powers, ^^ 7 ith him all faith in this direction has probably passed away. It is not for us to attempt to define the reasons for Madame Merian s positive and circumstantial statements. The preponderance of negative testimony is so crushingly great that Fulgora may be regarded as eliminated from the category of luminous insects. LANTHANUM. It will be convenient to notice under this heading the group of closely allied metals LAN THANUM, CERIUM, and DIDYMIUM. In an abandoned copper mine at Ridclarhyttan, West- manland, in Sweden, there occurs a heavy compact mineral, which, though pretty abundant there, is hardly met with anywhere else. This mineral was long mistaken for tungsten (syn. scheelite), until Klaproth of Berlin in 1803 found in it a peculiar earth, which he called ochroito earth, as it becomes yellow when heated in air. About the same time Berzelius and Hisinger made the same discovery and, (rightly) presuming the new earth to be an oxide of a new metal, they called the latter cerium (after the planet Ceres, the then latest discovery in astronomy) and the mineral cerite, which names have been retained to this day. Only the name &quot;cerium&quot; now has a more specific meaning, it having been shown by Mosander (in 1839-41) that Berzelius s cerium is a mixture of three metallic radicles, namely, cerium proper, lanthanum (from Xa.vOa.vciv, &quot;to be concealed&quot;), and didymium (from SiSv/aos, &quot;twin&quot;). These metals are very closely related to one another in their chemical character, and may be conveniently treated together. The extraction from cerite, of the oxide group, offers no difficulty. According to Marignac (Ann. Chim. Phys. [3], vol. xxvii.), the powdered mineral is made into a thick paste with oil of vitriol, and the reaction which sets in allowed to accomplish itself. The remaining dry white powder is placed in a crucible, and kept there for a long time at a temperature below redness, but sufficient to chase away the bulk of the free sulphuric acid. The residue is added in small instalments to a quantity of cold water, and the gangue (ferruginous silica) filtered off. The solution is boiled, when the greater part of the cerite-oxides comes down in the form of sulphate almost free from foreign oxides. The sulphates can be purified by redissolving them in the least quantity of water at 5 to 6 C. /filtering, and reprecipitating by boiling. What remains in the mother-liquors is recovered by precipitation with sulphate of potash (which must be added as a solid and in sufficient quantity to saturate the solution) as an alum-like double sulphate. The purified sulphates are dissolved in cold water, precipitated as oxalates by means of oxalate of ammonia, and the washed oxalates ignited, when the pure cerite-oxide mixture remains. The separation of the three oxides from one another offers very great difficulties. Comparatively easy is the extraction of approximately pure oxide of cerium by Berzelius s method. Dissolve the mixed oxide (which must be free of sulphate if the method is to succeed) in nitric acid, evaporate to dryness, ignite the residue, and treat it with nitric acid diluted with one hundred times its weight of water. Only lanthanum and didymium dissolve, impure binoxide of cerium (CeO^) remaining, which can be further purified by treatment with more concentrated nitric acid, which, however, besides the lanthanum and didymium, dissolves a good deal of the cerium itself. This method (like any of the rest) is founded upon the fact that salts of sesquioxide of cerium (Ce 2 O 3 ) are readily oxidized into salts of the feebly basic binoxide Ce0 2 under circumstances which effect no higher oxidation in La 2 3 or Di 2 O 3. For the preparation of the oxides of lanthanum and didymium we may utilize the nitric mother-liquors obtained in the extraction of cerium-oxide. These are evaporated to dryness, the residue is ignited, and treated with very dilute nitric acid, which dissolves the lanthanum and didymium with only little cerium (Mosander, Marignac). A more complete elimination of the cerium is effected (Bunsen) by converting the nitrates into sulphates (by evaporation with sulphuric acid to dryness, and igniting the residue), dissolving these in sulphuric acid water, and boiling with powdered magnesite (MgC0 3 ). From the filtrate the lanthanum and didymium are precipitated (after acidulation by muriatic) with oxalic acid, and the oxalates filtered off, washed, and ignited. By repeating the magnesia and oxalic acid process two or three times, the oxides are obtained cerium-free. They are then made into anhydrous, neutral sulphates ; these are dissolved in a minimum of water at to 5 Q, and the solution is heated to 30 to 35 C., when lanthanum sulphate chiefly separates out in small crystals, which are filtered off with the help of a filter-pump. A relatively lanthanum-free didymium sul phate remains dissolved (Mosander). The metals were known only in a powdery form up to 1876, when Hillebrand and Norton succeeded in preparing them in a compact form by the electrolysis of the fused chlorides. The three metals are very similar to one another ; they are steel-grey ductile true metals, melting at a somewhat lower temperature than silver. Specific gravities range from G 1 ! to G G. They are more readily inflammable than magnesium. The atomic weights of the three elements are now (1882) quoted as Ce = Hl, La =139, Di=147. Oxides and Salts. Cerium has long been knovn to form two oxides, namely, a lower (&quot;cerous&quot;) oxide, which is a pretty strong, and a higher (&quot;eerie &quot;) oxide, which is a feeble base. 16 ( = 0) parts of oxygen are combined, in the former with 92 ( = &quot;F&quot;) parts, in the latter with |F parts of metal. Formerly F was looked upon as the atomic weight, and the oxides formulated as FO and F 5 4 respee-