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 siderable proportion of the solvent being retained. The stabilizer, being a substance with an affinity for nitrogen dioxide (NO2 ), is intended to prevent the free presence of nitrous acid should any decomposition occur. It is claimed for the stabilizer that it at the same time acts as a detector and shows when decomposition is occurring, by means of the resulting discoloration; but this claim does not appear to have been clearly established. The shape of the powder is different from that of cordite. The mixture is extruded through dies, for charges for smaller guns in a tubular form, and for larger guns as a stick with several longitudinal holes; as it is extruded, it is cut into short lengths, the lengths having a proportion- ate relation to the diameter of the hole in the stick. This shape enables greater ease in making up cartridges than with cordite, requiring merely the weighing of the charge on scales as against cutting lengths of cordite according to size and weight. Nitrocellulose tubular is not so powerful as cordite, and therefore larger charges are required; it is hygroscopic, and consequently, if cartridges become damp, considerable variations in ballistics may result ; it is not so stable in storage as cordite. On the other hand, it is more uniform in burning (at a slower rate and with a lower temperature than cordite), and so causes much less erosion in a gun; and, further, the loading temperature has less effect on ballistics than with cordite, and the regularity in worn guns is better. The colour of the grains varies very much and may be buff, brown, dark blue or even nearly black, perhaps owing to slight changes in the stabilizer present ; but practically no difference in stability has been detected, except when the colour becomes brick-red, or rusty, when it may be concluded that corrosion has set in.

A nameless powder has been made in America for which it is claimed that, with field guns firing this kind of powder, it is possible for the eyes of the gunner to see the muzzle of the gun at the moment of firing, and that the flash is imperceptible at a distance of a mile. The composition of the American powder is approximately 60 % of nitro-cotton, stabilized only with potassium carbonate, 25–28 % of nitroglycerine treated in the same way, 5–7 % of diphenylphthalic diethylester of the phthalic acid obtained by estenfying phthalic anhydride with ethyl-alcohol in the presence of sodium bisulphate and 35% of neutral potassium tartrate; vaseline or mineral jelly up to 5 % is used to balance the composition. The dimensions of the powder-sticks and the exact composition depend upon the form to be used and have to be calculated.

The German propellant used with the 77-mm. gun was in the form of tubular sticks, and was a ballistite containing a stabilizer of “centralite” type, the stabilizer being the thio-urea derivative corresponding to diphenyldimethyl-urea. This last substance has been found frequently in German powders; it is very resistant to the action of acids and alkalis and is oxidized by fuming nitric acid only after prolonged heating at a high temperature.

A ballistite containing 60% of nitrocellulose and 38% of nitro- glycerine has been used by the Germans; and also a mixture of nitrocellulose (66-16%), nitroglycerine (25.97%), sym. diethyl- diphenyl-urea (5-64%), volatile matter (0.91%), mineral matter (1.32 %). With certain guns the Germans tried a mixture of ammo- nium nitrate (84.5%), carbon (15.0%), ammonium chloride (0.5%), in a compressed block, in the shape of an annulus, which was inserted in the cartridge-case above a charge of ordinary nitrocellulose powder. What was the exact result of this combination is not clear.

Among sporting powders which were tried during the war as propellants might be quoted as an example E.C.3—a powder made by a private company—which was used rather largely in trench warfare. This also is a nitrocellulose powder, which after forming to required shape is treated with acetone so that the outer surface is hardened. It is claimed for this process that the pressure during burning is more evenly distributed and more regularly maintained, since the hardened skin of the powder allows of slow burning to commence with and the porous interior allows more rapid action later on. (F. M. R.)

PROTHERO, SIR GEORGE WALTER (1848–), English man of letters, was born in Wilts. Oct. 14 1848. Educated at Eton, King's College, Cambridge, and the university of Bonn, he became fellow of King's College, Cambridge, and was history lecturer there from 1876 to 1894, when he became professor of history at the university of Edinburgh 1894–9. He was a member of the Royal Commission for Ecclesiastical Discipline 1904–6. In 1899 he succeeded his brother Rowland (afterwards Lord Ernie) as editor of the Quarterly Review. He was also editor of the Cambridge Historical Series and co-editor of the Cambridge Modern History. During the World War he was head of the historical section of the British Foreign Office, and in that capacity attended the Peace Conference in Paris (1919). He was created K.B.E. in 1920. Amongst his publications are Life and Times of Simon de Montfort (1877), Memoir of Henry Bradshaw (1889), and various volumes of historical papers, as well as the British History Reader (1898).

PROTOPOPOV, ALEXANDER DMITRIEVICH (1864–1918), Russian statesman, was born in 1864 and educated in a military school. He served for some time in the army, but he soon left the service and went into business. As a big landowner of the Simbirsk province he took an active part in the Zemstvo life and was elected member of the executive board of the Simbirsk Zemstvo and marshal of the nobility of the Simbirsk province. In 1907 he was elected member of the third and subsequently of the fourth State Duma, where he joined the left wing of the Octobrist (Moderate Liberal) party. Later be became vice-president of the State Duma. The first unfavourable rumours with reference to him arose in connexion with an interview with Herr Max Warburg, the German financier at Stockholm. In March 1916 he visited the capitals of western Europe as one of the leaders of the Russian parliamentary delegation. On his return journey he privately met at Stockholm Herr Warburg, the head of the Scandinavian section of the German Committee on Food Supplies. The importance of the conversation was, however, greatly exaggerated by the press, and also by Protopopov himself. At the beginning of Oct. 1916 Protopopov was appointed, through the influence of the Emperor, Minister of the Interior, in succession to Khvostov, and thus entered the Sturmer Cabinet. A former leader of Liberals, he proved to be now the strongest upholder of reaction. He enforced the censorship with unexampled rigour, and his interference with the food-supply work of the Zemstvos and Towns Union created a serious danger to the activities of these organizations. At a stormy meeting held at the Duma he was asked by his political friends to resign his post, and when he refused to do so they struck his name off the list of members of the party. Hated by the Liberal circles and the Duma, Protopopov not only supported the reactionary policy of Sturmer and Prince Galitzin with the utmost energy, but he is said also to have been one of the secret organizers of the disturbances of Feb. 1917, which he proposed to suppress by military force, and which, unexpectedly for him, resulted in the overthrow of the Empire and of himself. He was arrested by the Provisional Government and committed for trial. He remained for many months in the Peter and Paul fortress and was executed by order of the Extraordinary Commission in Sept. 1918.

PROTOZOOLOGY (see ) is that branch of zoology which is concerned with the group of animals known as the Protozoa. It is not, as its name might seem to imply, a primitive form of zoology. As a science it is comparatively young, but, owing chiefly to the practical importance of some of the animals with which it deals, it had in 1921 already become one of the largest and most cultivated fields in biology. The Protozoa are very interesting animals, from both the practical and the theoretical standpoint. Nevertheless, they are all small, and most of them of microscopic dimensions. To the general public they are therefore invisible, and consequently unknown, except by the conspicuous results such as diseases which they occasionally produce. In common speech they are still nameless, though they are popularly included among “animalcules” and “microbes”. But these are unscientific and unnatural groups, which comprise all microscopic creatures, both animals and plants; and consequently the Protozoa are still confused, in the popular mind, with other “microbes,” such as the Bacteria, with which they have no connexion.

It will be evident that protozoology, as an independent science, must necessarily have arisen as a comparatively late offshoot of zoology. Its history is bound up with that of the microscope, an instrument which bears much the same relation to protozoology that the telescope does to astronomy. Before microscopes were invented no Protozoa could have been clearly visible. With the first lenses, the largest and most conspicuous of them were discovered; and as microscopes were improved, more and more minute creatures gradually became known. Out of the confusion of forms which the microscope has continued to reveal, the Protozoa have ultimately emerged as a well-defined group of animals, and, as a result, those who study these animals have slowly built up a new section of zoological science.