P L P L 315 ized with soda and half with ammonia, we obtain an optically inactive solution containing a mixture of the two double salts. If this solution is allowed to crystallize each salt crystallizes independently, and the crystals can be separated by picking them out. Further, a super saturated solution of the one double salt is not made to crystallize by contact with a crystal of the other, so that FIG. 12. Double Tartrate of Soda and Ammonia, a, right-handed ; b, left-handed. if we make a supersaturated solution of the inactive mix ture and drop into the vessel, at different places, two crystals one of the right the other of the left salt, crystal lization occurs at each place, at the one of the one kind and at the other of the other. (2) By the action of another optically active substance. While the salts of the two opposite tartaric acids with an inactive base are precisely alike in solubility, density, and other physical characters, and, if they crystallize, crystal lize in the same form (or in enantiomorph forms), it is not at all so when the base is optically active ; thus right tartaric acid forms a crystalline salt with left asparagine, while with the same base left tartaric acid gives an uncrystallizable compound. (3) By the action of living ferments. The minute fungi which act as ferments do not show any right- or left-hand- odness as far as their obvious anatomical structure is con cerned, but Pasteur has shown that some of them are, if we may use the expression, physiologically asymmetrical. As an example we may give the very interesting case of mandelic acid. This acid, which stands to benzoic alde hyde (bitter almond oil) in the same relation as lactic acid does to common aldehyde, contains one asymmetric carbon atom in its molecule. It is optically inactive, and there fore, if Le Bel and Van t Hoff s theory is true, it must be a mixture of two oppositely active acids. Now Lewko- witsch found that when Penicilium glaucum is cultivated in a solution of mandelic acid fermentation takes place. This goes on until exactly half of the acid is decomposed, and what remains has all the properties of mandelic acid, but is optically active ; it is the right-handed component of the mixture, the growing fungus having consumed the other. There is an interesting peculiarity of tartaric acid dis covered by Pasteur (to whom we owe nearly all our know ledge of the relations between optical activity and crystalline form in tartaric acid) which is of importance in connexion with the theory we have just been explaining. We have not only right and left tartaric acid and racemic acid, the inactive compound of the two, but also a kind of tartaric acid which is inactive but incapable of being separated into the two oppositely active acids. Now the chemical formula of tartaric acid is H H I! I I I! C C* 0* I I I I 0000 I I I I H H H H It will be observed that the carbon atoms marked * are asymmetric, and that they occupy precisely similar posi tions in the molecule. Each of them is combined with H, OH, COOH and CH(OH)COOH. If in both of them these four things are arranged in the same order there is helicoidal asymmetry the one order giving the one, the other the other enantiomorph form. But if the one has one order and the other the opposite, then there is in the whole molecule no helicoidal asymmetry, as the two halves exactly balance one another. There is not, as in racemic acid, a compound of one molecule of each of the two opposite active acids, but rather a compound of half a molecule of each, and we should not expect such a com pound to be easily separable. Jungfleisch has shown that if any one of the four tartaric acids (right, left, racemic, and inactive) is mixed with a little water and kept for some time at a temperature of about 200 C., it is converted into a mixture of racemic and inactive tartaric acids, so that, as racemic acid can be divided into right and left tartaric acids, it is possible to prepare any one of the four from any other. (A. c. B.) POLARIZATION OF LIGHT. See LIGHT and WAVE THEORY. POLAE EEGIONS la II. nnHE polar regions extend respectively from the Arctic J_ and Antarctic Circles, in 66 32 N. and S., to the north and south poles, the circles being 1408 geographical miles from the poles. The intense cold and the difficulties of ice navigation have made the discovery and examination of these regions a slow and hazardous task. Millions of square miles are still entirely unknown. In the present article the history of the progress of discovery within the north polar region will be traced, and some account of its physical geography will follow. A similar review of work in the south polar region will conclude the article. NORTH POLAR REGION. } xht The Arctic Circle is a ring running a little south of the northern shores of America, Asia, and Europe, so that those shores form a fringe within the polar region, and are its boundary to the south, except at three openings, those of the North Atlantic, of Davis Strait, and of Behring s (more properly Bering s) Strait. Ap The width of the approach to this region by the Atlantic robes. Ocean, in its narrowest part, is G60 miles, from the Norwegian Islands of Lofoten to Cape Hodgson on the east coast of Greenland. The width of the approach by Davis Strait in the narrowest part, which is nearly on the Arctic Circle, is 165 miles; and the width of Behring Strait is 45 miles. Thus out of the whole ring of 8640 miles along which the Arctic Circle passes about 900 miles is over water. This great environment of land is an important feature in the physical condition of the north polar region. It influences the currents and the movements of ice, which are still further affected by the archipelago? lying to the northward of the fringing coast-lines. The larger opening into the north polar region by way of the Atlantic is divided from Davis Strait by the vast mass of Greenland, which, extending for an unknown distance to the north, crosses the Arctic Circle and ends in a point at Cape Farewell in 59 48 N. lat. It was inevitable that the routes across the Arctic Circle by the Atlantic and Davis Strait should first become known, because these openings to the polar regions are nearest to the temperate regions inhabited by the exploring nations of Europe. A rumour respecting Thule, an island on the Arctic Tlmle. Circle, first brought by PYTHEAS (q.v.}, and afterwards
! doubted, was the extent of the knowledge of the north