place, whatever may be its effect upon the compound after union; but that heat has the power of diminishing the affinity with which the atoms of a compound are held together is shown by its decomposing action on some of the metallic oxides, as those of silver and mercury. The manifestations of chemical affinity between two uniting bodies are no doubt often heightened by the application of heat, hi consequence of its diminishing the cohesion or increasing the repulsion between the molecules of each of such uniting bodies. The conversion of water into steam is an example of the neutralization of the force of cohesion by the application of heat, without any appreciable diminution of the force of chemical affinity. The cohesive force of the atoms or the molecules of bodies depends on their distance from one another, decreasing as the distance increases, and disappearing entirely when that becomes sensible; but the law by which the decrease takes place is not known. It has been questioned, however, whether molecular attraction may not follow the law of attraction of gravitation, varying inversely as the squares of the distance; or, in other words, whether the two are not modifications of the same force. Under this assumption the explanation of the cessation of cohesive attraction at sensible distances is furnished by the obvious fact that any sensible distance is almost infinitely so much greater than the distance between the centres of the adjacent atoms or molecules of a solid or liquid, that the difference in the attractive forces becomes practically infinite. The question as to the amount of cohesive force which exists between the molecules of liquids is attended with difficulty. The fact that water will boil in a vacuum at a low temperature might suggest the inference that there exists no positive cohesive force between its particles; but the formation of spherical drops of water is opposed to such a conclusion. Again, when a liquid boils in the open air it is usually considered that the repulsion between the molecules is sufficient to overcome the pressure of the atmosphere, and therefore it would appear as if there were no cohesive attraction between them; but the irregularity with which ebullition takes place under certain circumstances, as when the air which is usually contained in water has been expelled, would seem to confirm the opinion that in reality there are present both attractive and repulsive forces, which alternate in predominance; the attractive force predominating when the molecules are within certain distances, especially when they are comparatively at rest, and the reipulsive force predominating when the distances are deranged by motion, or increased by the intrusion of particles of vapor or air. (See Boiling Point.) Moreover, water may be considerably reduced in temperature below its freezing point without congelation taking place, when a jar given to the vessel, or a pebble or crystal dropped into it, will cause the immediate manifestation of intense cohesive force and the formation of ice. It would therefore appear that the molecules of a liquid may have slight cohesive attraction for each other, even when the temperature is sufficient to cause repulsion between the particles of its vapor; and also that when the freezing point is reached, the cohesion may not be sensibly increased until some disturbing cause operates. The modifications of cohesive attraction are the cause of those different properties of bodies which are called tenacity, hardness, ductility, and elasticity, and will be treated under their appropriate heads, and in the article Strength of Materials.
COHOES, a city of Albany co., New York, on the right bank of the Mohawk river, at its confluence with the Hudson, and on the Erie canal near its junction with the Champlain canal, 8 m. N". of Albany; pop. in 1850, 4,229; in 1860, 8,800; and in 1870, 15,357, of whom 7, 947 were natives and 7, 41 foreigners. Cohoes falls, 70 ft. in perpendicular height, and remarkable for their picturesque beauty, are just above the city. The Albany division of the Rensselaer and Saratoga railroad and the Troy and Schenectady (New York Central) railroad pass through Cohoes, and horse cars run to Troy. Among the public buildings, the Roman Catholic church of St. Bernard, built of brick, with high towers, is particularly worthy of mention. It cost $100,000, is finely decorated within, and is furnished with a chime of bells. The Episcopal church of St. John, with the parsonage connected, is of Schenectady blue stone, and cost $75,000. Each of the stained windows illustrates one of the figures in Revelations; and being formed of hammered or rolled glass, by which the light is reflected rather than transmitted, they possess great brilliancy and depth of color. Cohoes derives its importance from the extent of its manufactures, ample water power being supplied by the falls. The Cohoes company, organized in 1826, and having a capital of $500,000, owns the entire water power of the river from half a mile above the falls to a mile below, the total fall in this distance being 120 ft. A stone dam, 1,443 ft long, was built in 1865 above the falls, at a cost of $180,000. This structure was preceded by a series of wooden dams, the first of which was erected in 1831. The water is used in five successive canals, having falls of 18 to 25 ft.; and again from the level of the state dam below the falls, which was built to supply the canal at this point. The water power is leased by the company at the rate of $20 a year for each horse power, including the necessary land. The Harmony company, having a capital of $2,000,000, owns all the cotton factories in Cohoes, and its mills, comprising six distinct factories, are the largest in the city, the most recent containing 2,700 looms and 130,000 spindles. One of these mills (the first in the United States) has lately been supplied with machinery for
