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SOLOTHURN. 333 SOLUTION. tal, Solotluirn. Population, in 1888, 85,621; in 1900, 100,7(i2, of whom the Catholics form over three-fourths. German is the predominating lanjruage. The liistory of the canton centres chielly around its capital, Solothurn, which dates from pre-Roman times, and which in 1218 became a free Imperial city. The burgliers were associated with Bern in the struggles against the petty princes of the region. Solothurn was formally admitted into the Swiss Confederation in 14S1, by which time it had extended its rule over the region now constituting the canton. The aristo- cratic regime which had long existed in the can- ton came to an end in 1S30. SOLOTHURN. The capital of the Canton of Solothurn, Switzerland, on the Aar, about 20 miles north-northeast of Bern (Map: Switzer- land, B 1 ). It is a walled city with broad streets and numerous churches. The Cathedral of Saint Ours (the cathedral church of the Bishopric of Basel) is a cruciform structure of the eighteenth century, built on the site of an old church dating from 1050. Other interesting architectural struc- tures are the ancient clock-tower and the cloth hall with its collection of weapons. The town library contains about 40,000 volumes. There is a natural-history museum with valuable zoologi- cal and paleontological collections. The environs of the town are exceedingly picturesque and abound in villas and resorts. The chief industries are Avatchmaking and stone-quarrying. Popula- tion, in moo. 10,100. SOLOVIEFF, so'hVvyef, Sergei Mikh.^il- oviTCH (1820-79). An eminent Russian historian, born and educated at Moscow. From 1842 to 1844 he was abroad as tutor in Count Stroganoff's family, attending the lectures of Ranke. Bijckh, and Jlichelet. His thesis. The Relations Beticeen Xorr/orod and the (IranrJ Princes (1845). and his dissertation, Eistori/ of the ReUitions Amonti the Princes of the House of Rtirik ( 1847 ) , established his reputation, and he was appointed professor of Russian history at Moscow. Subsequently he was dean of the Historieo-Philologioal Faculty and rector of the university for a number of years. His Histori/ of Russia in 29 volumes (7th ed., 1870) was the first thoi'ough treatment of the subject from the earliest period to 1774. He wrote also a number of historical text-book?., in- cluding Historical Letters (1858), Historij of the Fall of Poland (1863), and Political and Diplo- matic History of Alexander I. (1877). SOLSTICE (Lat. solstithim, from sol, sun + sistcre, to stand, reduplication of stare, to stand). That point in the ecliptic (q.v.) at which the sun is farthest removed from the celes- tial equator, and where it is eonseqiiently at the turning point of its apparent course. There are two such points in the ecliptic, one where it touches the Tropic of Cancer, the other where it touches that of Capricorn. (See Tropic.) The former is the summer and the latter is the u-^inter solstice to those who inhabit northern latitudes, and vice versa. The term is also, em- ployed to signify the date at which the sun attains these two points in its orbit, viz. June 21st and December 21st. SOLUBLE GLASS. See Watek-Glass. SOLUTION (Lat. solutio, from solvere, to loose, dissolve, from so-, se-, apart, away -j- luere, Gk. Xiety, lycin, to loose). In chemistry, a term applicable to any mixture that can be formed by the interdiffusion of two or more substances, gaseous, liquid, or solid. A mi.xture so formed is invariably homogeneous, i.e. its ingredients do not exist alongside of one another in separate masses, and therefore cannot be distinguished separately even by means of a powerful micro- scope. For the distinction between a homo- geneous mixture and a chemical compound, see the article Chemistry. Gaseous Mixtures. The formation of these is not limited to any particular set of sub- ■ stances, as is the case with liquids and solids; all gases are capable of mutual interpenetration by diffusion and hence of forming homogeneous mixtures. In a gaseous mi.xture the properties of each ingredient are practically unaffected by the presence of the other ingredients. Therefore, provided no chemical reaction takes place, a gas- eous mixture obeys the laws of gases (viz. those relating to the mutual dependence of volume, pressure, and temperature) as if it were an isolated gaseous substance. Liquid Solutions. These may be formed by liquids with gases, by liquids with other liquids, and by liquids with solids. The mass of any gas absorbed by any liquid is proportional to the pressure of the gas (Henry's law) and diminishes w'ith increasing temperature. Of course, even under the same conditions of pressure and temperature the solu- bility of different gases in some liquid is not the same: thus carbonic acid gas is much more soluble in water than oxygen. The solubility in the case of each system consisting of a gas and a liquid is termed by Bunscn the 'coefficient of absorption.' To understand clearly the meaning of this terra imagine some gas in contact with a given liquid and maintained at some tempera- ture t, under a pressure equal to the normal pressure of the atmosphere; imagine that when no more of the gas is being absorbed, all the gas contained in one cubic centimeter of the solution is driven out of it, confined separately, and cooled off to 0° Cent.: the volume that the gas will then occupy is its coeflieient of absorption with re- spect to the given liquid at the temperature t. In the case of gases (such as ammonia, with respect to water) that are copiously soluble, i.e. whose coefficient of ab.sorption is very large, that coefficient itself is variable, not only with the temperature, but also with the pressure of the gas ; in other words, such gases fail to obey Henry's law — probably because they enter, to a greater or less extent, into chemical comlun.a- tion with the solvent liquid. Why the coefficient of absorption should be exactly w-hat it is, whether Henry's law is obeyed or not. is not yet understood. Nor do we understand clearly the state of a gas when absorbed by a liquid. Are its molecules combined with those of the solvent in the form of hydrates, or do they exist in the solvent independently? On the other hand, it has been demonstrated that if a gas obeys Henry's law its molecules in solution are neither dissociated into simpled molecules nor associated with one another. It has also been shoTATi that dilute solutions of gases in liquids obey the laws of osmotic pressure as