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 ADERNO Polo speaks of its wealth and splendor in the middle ages. At the beginning of the 16th century it was so strongly fortified that the Portuguese failed to capture it ; but the Turk- ish domination, from about 1540 to 1630, was injurious ; and the imam of Sana and the sultan of Yemen, who successively ruled Aden for the next three generations, completed the work of the Turks, and left the place a heap of ruins in 1705, when it became independent. In 1838 Capt. Haynes proposed to the sultan of Aden to cede the town to Great Britain, and on his declining the English took forcible possession, Jan. 11, 1839. Since that time the town has gained commercial importance. In 1870 the imports from Great Britain amounted to 110,403, and the exports to 2,633. ADERNO (anc. Adranum), a town of Sicily, in the government and 17 m. N. W. of Catania; pop. in 1861, 12,877. It is situated on a plateau at the S. W. foot of Mt. Etna, and is approached by a steep winding road of 4 m. A large pro- portion of the inhabitants are monks and nuns. There are many remains of the ancient town and ruins of mediaeval buildings; and in the piazza is a Norman castle, now used as a prison. ADET, Pierre Auguste, a French chemist and politician, born at Nevers in 1763, died about 1832. He was sent by the directory in 1795 to the United States as minister plenipotentiary, and presented to congress a tricolor flag on be- half of the French nation. On Oct. 27, 1796, he delivered to the secretary of state the cele- brated decree of the directory complaining that the American government, in its treaty with England, had violated its neutrality and broken the treaty of 1778, and authorizing French ships of war to treat neutral vessels in the same manner that they allowed themselves to be treated by the English. After the delivery of this note Adet announced that he should suspend his functions, and he accordingly re- turned to France, after issuing an inflammatory address to the people of the United States. He subsequently adhered to Napoleon, but his political career remained unimportant. He composed a new system of chemical signs, but it found no favor. ADHESION (Lat. ad, to, and hcerere, to stick), the force by which the particles of different bodies stick together, distinguished from cohe- sion, which is the force that holds the molecules of the same body together. There are six kinds of adhesion: solids to solids, liquids to solids, liquids to liquids, gases to solids, gases to liquids, and gases to gases. 1. Solids to solids. Two glass or metal plates with well ground surfaces, when pressed together, will adhere with such force that the upper one will not only support the lower, but an additional weight will be required to separate them. The amount of this adhesive force has been meas- ured by recording the weights necessary for their separation. The records of the old ex- perimenters on this subject are worthless, be- cause they placed a lubricating fluid, oil or ADHESION 115 fat, between the plates; they found thus the cohesion of the oil or fat, and not the adhesion of the plates. In later times Prechtl in Ger- many has made the most careful experiments in this line ; he took polished metal plates of 1$ inch diameter, suspended the upper one to a balance, brought it to an equilibrium in a hori- zontal position, and attached the lower plate to a support underneath it. Both plates were then brought in contact, so that the flat pol- ished surfaces covered one another perfectly, and the weights required in the scale at the other end of the balance beam to separate the plates were the measures of the adhesion. Prechtrs Adhesion Balance. He found thus the following remarkable law : The adhesion between two plates of the same material is the same as that between one of the plates and any material which possesses a less adhesive force. For instance, to separate two copper plates required a weight of 21 grains ; but the same weight was required to separate one of the copper plates from a plate of bismuth, zinc, tin, lead, &c., notwithstanding the adhe- sive force of bismuth to bismuth, zinc to zinc, &c., was found to be smaller than that of cop- per to copper. Prechtl found also that an at- traction of the plates manifested itself at an appreciable distance before actual contact, and he even measured the amount of this attraction at the distance of ^ of an inch by means of weights in fractions of grains. The suspended plate when brought within this distance was attracted with an accelerated motion till the contact took place with a slight concussion. The idea that the pressure of the air was the chief cause of the adhesion of two such plates, as it is in the case of the well known experi- ment with the Magdeburg hemispheres, was set at rest by Boyle, who suspended the adhe- sive plates charged with weight in the vacuum of an air pump ; the plates were not separated, while the hemispheres held together by the vacuum alone fell apart. The adhesion of solids to solids is also seen in the dust, which will not only adhere to perpendicular but even to inverted surfaces. Granite consists of feld- spar, quartz, and mica, kept together by adhe- sion. A portion of such apparently adhesive force is, however, cohesion. For instance,