IMMUNITY. 500 IMPACT. wheel. (4) Removal of the spleen is followed by su.sceptibilitr to disease (Bardacli). (3) I'ombiiiiiKj various iincrourtioiiiniiis destroys im- munity in some experimental cases. TiiEOKiE.s OF Im.mvmty. In 1S80 Pasteur tauj,'ht that the microorgiinisni, by its growth in the body, uses up sonic substances necessary for its existence, and then perishes. If the removal of this substance be complete, perfect immunity results. This is the cxhtiimlioii theory. .Stern- berg combats it, sayinfr that if it were true, we mu>t have in each of our bodies certain siiiallpo.K material, measles iimtcrial. and scarlet-fever material, etc., each of which must be e.xhausted by its appropriate microorganism, thus neces- sitating an almost inconceivably complex body chemistry. The retention theory was advanced in 18.S0 by C'hauveaii. who suggested that the growth of the bacteria in the body probably originated some substance prejudicial to their further dcvelo|)ment. The phufiocytosis theory was suggested by Carl Roser in ISl^l, received attention from Sternberg, and al-^o from Kix-h, but was not advanced with any insistence fiU in 1884 -MetchnikolT enthusiastically chanpioneil it and gave it his name. There are two varieties of the white blood -corpuscle who.se duty it is to destroy bacteria: these are the large, unicellular leucocyte, or macrophagocyte, and the smaller form, the microphagocyte. Both these forms exhibit ainfeboid movements, and possess the at- tractive force called ehemotaxis which exists be- tween amirboid cells r.nd fiKxl particles. Pha- gocytosis is the incorporation of foreign particles by these am<pboid wliite blood-corpuscles. Leu- cocytes actually take up and envelop living pathogenic organisms, as demonstrated by Koch in 1878. The humoral theory, suggested by Grohmann in 1884, was advanced by Buchner, who claimed that not phagocytosis, but the bacte- ricidal action of blood-plasma, is the cause of the destruction of pathogenic bacteria. He showeil that freshly drained blood, blood-plasma, as well as aqueous hunioi de-^t roved such organisms. It was shown in rebuttal that dispersion in a large amount of watery fluid causes the death of micro- organisms, which grow well when allowed to re- main in a close colony. The prevalent andgenerally adopted theory is the antitoxin theory, defended by Behring and Kitasato, Brieger, Tizzoni, Cat- tani, Koux, Vaillard, and others. According to this theory, the metabolism of bacteria, during disease, gives rise in the blood of the sick person, not only to poisonous alkaloids called toxins (ex- isting either in the bodies of the bacteria or pro- duced by them), but also to defensive proteids called anti-toxins, which possess the property of neutralizing the toxins. Hankin divides the anti- toxins (formerly called alexins by Buchner) into four groups. Proteid substances that occur naturally in animals and possess the power of destroying bacteria or their products, he calls sozins. Similar proteids resulting from acqiiired immunity in animals he calls /ihylaxins. Sozins are subdivided into those which have the property of killing bacteria, or niycosozins, and those which neutralize bacterial products, or toxo- so:ins. A phylaxin which destroys bacteria he calls a mi/eophylaxin. and a phylaxin which neu- tralizes their ptomaTns he calls a toxophylaxin. Consult: Roser, Enlziinilunti und IJeilunf) (T^ip- «ig, 1886) ; Chauveau, Traitf de physique biolo- gique (Paris, 1901); Behring, Die lilulsnum- therapie (I^ipzig, 1892) ; Sternberg, Immunity: I'roteetivc Inoculations in Infectious Diseases and Herum Therapy (New York, 18lto). For the method of securing artificial immunity by means of defensive proteids, see Antitoxin' ; Toxins ; SEnusi Tiikhai-y; and Vaccination. IMMUNITY, in feudal law, and in interna tional law. See Keiualism; Invioladiuty. IMOGEN, Ini'A-jOn. The daughter of Cynibe- line and wife of Posthumus, in Shakespeare's f'ymheline. whom her husband is made to believe false by lachimo, but whose fidelity is afterwards proved. IMOGENE, Im'A-jen, The Fair, The heroine of l^cwis's ballad "Alonzo the Brave and the Fair Iniogenc." IMOLA, e'niAlA. .-V walled city, on the San- terno. in the Province of Bologna, Italy, 21 miles southeast of the city of Bologna (Map: Italy, F .3). It has been the seat of a bishop since 4'J2. It has an ancient castle, several asylums, and two theatres. It markets wine and vegetables, ami manufactures leather, majolica, bricks, gun- powder, and fertilizers. Xear by are mineral springs. In Roman times the city was called "Forum Cornelii.' after its founder, L. Cornelius Sulla. Here was born Saint Peter Chrj-sologus, Archbishop of Ravenna, who died in 440, and tho painter Innocenzo da Imola, who died about lo.")0. Poimlation of commune, in 1881, 29.343; in 1901. 33.210. IMOLA, Innocenzo ua (c.1404-c. 1.550). An Italian painter, born at Imola. His rt'al namo was Innocenzo di Pietro Francucci, and he was the pupil of Francia at Bologna, and then studied under Albertinclli in Florence. Afterwards he went to Bologna and painted the frescoes of tho '"Death and Assumption of the Virgin" at San Michele (l.ilOl, al>o the ""Madonna in Glory," and a '"Madonna with Kneeling Donors." both in the gallery of that city. Among his other works are altar-pieces in the cathedral at Faen- Z!, in Berlin, and in Munich. His best jiictures are reminiscent of Raiihacl. IMOSHAGH, iVmAshiig'. The national name of certain ])astoral tribes of Tuaregs in the .Saha- ra Desert, who are independent. See TiAnEG. IMPACT (from OF. impacter. empaeter. to press dose together, from Lat. impingere, to dash against, from tii, in -r pangere, to fasten). If a body in motion meets a second body — either at rest or in motion itself — there is said to be an 'impact' between them. The general principles of mechanics of course niiiy be applied to the changes in the motion. (See JIeciianics.) The geometrical sum of the linear momentum of the two bodies is not chan^'cd by the impact : nor is the sum of their kinetic energies, provided they are perfectly elastic. Consider the two bodies as moving along the same horizontjil line. Let their masses be m, anil m^ and the velocities of their centres of inertia before and after impact be r, and r, and V, and Vj, respectively. Then, for all bodies mit", + niji's = niiV, + mjV.. (This law was given by Wallis, Wren, and Huygcns almost simultaneously, 1008. The first applied it to inelastic Imdies; the last two to elastic ones.) If the bodies are absolutely in- elastic, V, = Vj; if they are perfectly elastic, i m,f,' -f i ».,r,= = J m.Vi' + 1 »i,V,«;
