74 THE GENESEE FAKMEK. 



gives the right result as to the quantity of metal present ; but there is a well grounded 

 suspicion that the coin contains too much copper or silver. In what way, short of a 

 chemical analysis, can the fact be ascertained ? If the coin contains a lighter metal than 

 gold, (and copper and silver are lighter,) to give the proper weight in the atmosphere 

 it must be as much larger in bulk as there is an excess of silver or copper. Hence, if 

 the coin be suspended in water by a fine silk thread attached to the bottom of the pan 

 connected with one end of the balance, and weights be placed on the opposite pan or 

 scale, less weights will be required to support the coin the larger its volume as compared 

 with its weight in the atmosphere — the support of the water lifting it upward equal to 

 the water displaced. By weighing a coin in the air and again in pure water, if it give 

 the right weight in both instances, it can hardly fail of being genuine. Platinum being 

 a heavier metal than gold, formerly when gold was comparatively dearer than it now is, 

 alloys of gold, cojDper, and platinum are said to have been formed so near the right 

 specific gravity, that nothing short of an analysis would expose the fraud. Since plati- 

 num has risen in price by its extensive use in tipping the ends of telegraph wires, and 

 gold has become unusually abundant, any fraud of this kind would yield no profit if 

 undetected. 



All weighing, whether performed by the chemist or the farmer, have for their object 

 to measure the force of gravity of the body weighed. A variety of experiments may 

 easily be tried to illustrate the influence of the surrounding medium on the gravity of 

 bodies, whether in air, water, or quicksilver. 



Cohesion is a term used to designate that force which keeps the molecules of matter 

 together. It is usually called " the cohesion of aggregation ;" or " attractive aggrega- 

 tion." If there was no force antagonist to this, the whole imiverse would be one solid, 

 immovable mass of matter. The force oi repulsion, however, so strikingly shown in the 

 air that surrounds the globe, serves to prevent such an aggregation of molecules. In 

 splitting logs for rails and staves, and rocks for building purposes, the farmer has occa- 

 sion to overcome the cohesive force of both organized and mineral matter. Cohesion is 

 strongly developed in beating gold into leaves of extreme tenuity. A single grain will 

 readily cover 1400 square inches; and with a microscope the gold on the millionth of 

 a square inch is distinctly visible ; so that without parting with its cohesive attraction, 

 gold may be divided into particles of at least 1-1,400,000,000 of a square inch in size, 

 and retain the color and other properties of the largest mass. 



This force of molecular cohesion acts only at a distance so minute as to escape the 

 most delicate examination. The fragments of a piece of glass or metal just broken, 

 when laid ever so closely together, have no tendency to unite again ; but if the surfaces 

 be pressed together, union may take place, though only in a few points, and imperfectly. 

 Yet when the surfaces of plate glass laid flat on each other, and subjected to consider- 

 able pressure, are allowed to remain for some time, they are found to grow together so 

 completely, that thick masses may be ground as if they had always formed a single 

 piece. Surfaces of lead may be cut so smoothly that, when pressed together, the cohe- 

 sive force is brought into play with considerable power. The viscidity of fluids is 

 thought to arise from the same natural law, as is also the globular form of a drop of 

 rain. When a drop of water spreads over a large surface of wood or other solid, the 

 cohesive force between the particles of water and the solid is stronger than that between 

 the particles of water for each other. In the wetting of dry soils, this difi'usion of water 

 by the molecular attraction of liquids and solids is strikingly exemplified. 



Clairaut found, as the result of his mathematical investigations, that all the phe- 

 nomena of capillary tubes depend on the relation of two forces : 1st, The cohesion of 

 the particles of the fluid for each other. 2d, The attraction of the solid for those of the 

 fluid. When a glass tube is dipped into different liquids, if the force of the attraction 

 of the ghiss is less than half the force of cohesion of the fluid, the fluid will be depressed 



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