QUESTIONS. 61. What is the scientific term for heat, with its derivation? 62. State some of the sources from which we obtain heat. 63. Give some illustrations of the disengagement of heat by friction. 64. When we strike steel against flint, does the spark proceed from the flint? 65. In this operation why do we obtain such heat? 25. We have now to consider some of the general effects of heat, one of the most important being expansion. Experiment 9.-Fit an iron cylinder accurately into an aperture made in sheet iron; when the cylinder is heated to redness, it will be found that it has become so much larger by the heat separating its particles from each other, the effect being termed expansion, that it will no longer pass through the aperture. If the heated metal be now plunged into water, it immediately returns to its original dimensions, the process being termed contraction. The contraction and expansion of metals are frequently brought into operation in the arts: for example, in making large vessels for brewers, the iron hoops are made somewhat smaller than the circumference of the vessel; on being heated, they increase to such an extent, that they drop into their places, and by suddenly cooling them, the contraction that ensues binds the parts in the most forcible and uniform manner. By a similar contrivance, the wooden parts of carriage wheels are connected together, the iron hoop or tire being made smaller than the wooden circle. Some idea can be formed of the enormous force with which metals contract on cooling, by the following circumstance :--two side walls of the Conservatoire des Arts et Metiers, a museum for models in Paris, having been pressed out by the weight of the floors, it was proposed by Monsieur Molard to save this national building by the laws to which we have just alluded. Several holes were made in the walls opposite to each other, through these strong iron bars were passed, heat was then applied to them in the apartment, circular plates having been previously screwed on the ends that projected outside of the walls, by the expansion of the bars they were lengthened; the circular plates were necessarily pushed farther from the walls, on being screwed up, the bars were cooled, contraction came into operation, and the yielding walls approached each other; by frequently repeating the same experiment, they were restored to their original position. QUESTIONS. 66. When we heated the iron cylinder, why did it no longer pass through the aperture? 67. How does contraction differ from expansion ? 68. What use is made of this fact, in putting iron hoops on wheels and large vats? 69. Explain the process by which the museum for models in Paris was saved from falling. 26. The expansion and contraction of mercury enables us to ascertain with great precision the various temperatures of surrounding bodies. The instrument is termed thermometer, (Greek, Oepun, heat, μεтρov, a measure :) it consists of a glass tube, terminated by a bulb, or hollow ball, filled with mercury, the opposite end being completely closed, by melting the tube over a flame, it is then said to be hermetically sealed. As the mercury expands by heat, it rises in the tube, and the abstraction of heat producing cold, causes it to fall; a scale is placed at the side, with the divisions termed degrees. The British thermometer, originally constructed by Fahrenheit, a Prussian, is marked or graduated from 0, termed zero, or nought, to 212 degrees, termed the boiling point, at the level of the sea, (vide 14.) Fahrenheit produced his zero by plunging the instrument in a mixture of snow and salt, which produces an intense degree of cold; he erroneously supposed that the cold produced by this frigorific mixture, (Latin, frigor, cold, and facio, to make,) was the greatest that man could obtain; this is not the fact; he therefore termed the starting point zero. At the 32nd division, or degree, he found that water was converted into ice, this he termed the freezing point; subsequently, placing it under the human tongue, the metal rose to 98 degrees, this is termed blood heat; about 76 degrees is summer heat in Great Britain; 60 degrees the medium. The next step was to apply heat artificially obtained on plunging the thermometer in boiling water, the mercury continued to rise rapidly : |