different degrees of intensity in different bodies: the metals and solids exhibit this attraction most forcibly; the atoms of liquids which move freely over each other, are evidently not under the same control; and gases, which cannot be converted into liquids by the coldest climate, are evidently influenced by this law only in a slight degree. In the article on carbonic acid, it will be seen, that artificial cold, of an intense degree, can liquify some of the gases. 13. In the majority of cases it is heat which determines the intensity of attraction of cohesion. Water, for example, which exists in three states, steam, water, and ice, will prove the truth of this statement. When cooled down to 32 degrees, termed the freezing point on the thermometer of Fahrenheit, (vide 26,) attraction of cohesion is brought so forcibly into operation, that it becomes a solid, termed ice. On the contrary, when heat is applied, until the temperature reaches 212 degrees, the boiling point, this attraction is overcome to such an extent, that its atoms fly from each other to form steam, which occupies about 1728 times the space of the original quantity of water. 14. As we remove the pressure of the atmosphere by an air-pump from the surface of the fluid, a less portion of heat will be required to convert water into steam; or, in other words, water boils at a less temperature than 212 degrees, if atmospheric pressure be removed. The same effect is produced on the tops of high mountains, in which situations atmospheric pressure is less than at the level of the sea. 15. Tenacity, which prevents bodies from being torn, depends upon attraction of cohesion being exerted in an intense degree: some idea of its effects may be formed from the following illustrations :-a wire of iron, the most tenacious of the metals, the one-tenth of an inch in diameter, will support a weight of 450lbs.; silver wire, same diameter, 370lbs.; and lead wire only 29 lbs. QUESTIONS. 32. What is the derivation of attraction of cohesion ? 33. What does it imply? 34. Does it exist in the same intensity in all bodies? 35. Why does the intensity of attraction of cohesion vary in ice, water, and steam? 36. What space does steam occupy, compared with the volume of the original quantity of water? 37. Why does water boil with less heat under the exhausted receiver of an air-pump? 38. What does tenacity result from? 16. When the atoms of a body are few, and are at liberty to be moved freely, as is the case with liquids, we find that the cohesive force appears to collect them around a centre, thus forming a sphere; witness the dew-drop, or the globule of mercury. In obedience to this law, the earth and the planetary system assumed the form of spheres, which, when set in rapid motion by the Divine will, brought centrifugal force into operation, and, tending to fly from their respective centres, were converted into spheroids, which differs from spheres, in having one diameter longer than the other. The equatorial diameter of the earth, for example, is longer than the polar. (In a work on chemistry, it would be unwise to enter into the subject of centrifugal force it should, however, be explained to the pupil by the parent or preceptor.) QUESTIONS. 39. Why do fluids tend to assume the form of spheres ? 40. Why is not the earth a sphere? 41. How does a sphere differ from a spheroid? 17. If the pupil can form an idea of the nature of attraction of cohesion from what has been said, it will, perhaps, not be difficult to follow up the investigation, by considering the subject of attraction of gravitation, which may be thus distinguished from the preceding. Attraction of cohesion tends to form a mass of the same nature, by collecting its atoms round a centre: this power only acts at insensible distances, that is, when the atoms are not apparently separated. Attraction of gravitation, on the contrary, which exists in large masses, as the sun, earth, and planetary system, operates upon other masses of a different nature, at immense distances. Without this invisible and stupendous power, termed by astronomers centripetal force, (deriv. centrum, a centre, and peto, to seek,) the planets would fly from the sun, and the satellites from the planets, by cen |