merely psychological conceptions of heat and temperature, which we have been considering hitherto. Our sensation of temperature cannot help us in this, as sensations cannot be compared with one another quantitatively. We cannot perform a measurement of heat or of temperature until we have found some physical property of a body which varies uniquely and continuously with its thermal condition, and which can be measured in time and space by one of the ordinary scientific methods. Fortunately experiment has Shown us that there are a great many physical properties of bodies which vary uniquely in this way with their thermal condition, and which may all, therefore, be used for the measurement of heat or of temperature. Nearly all the physical properties of a body are altered when it is heated. One of the easiest properties to measure is the volume. A body occupies more space when it is hot than when it is cold. We say, therefore, that heat causes expansion, and cold causes contraction. The property of expansion on heating is common to all bodies with very few exceptions (one exception is water between 0 and and is generally used as the basis of temperature measurements. The expansion on heating is most marked in the case of gases, and was noticed first in them; as early as 100 we find this property of gases made use of by Hero of Alexandria in some ingenious experiments. In the case of liquids and solids the expansion is much less noticeable; in the latter case it is even somewhat difficult to determine. Gases and liquids are most suitable for the measurement of temperature, and for the construction of temperature measuring instruments, or thermometers, as they are called. The gas thermometer was discovered by Liquid thermometers were introduced later. Fig. 1 is a diagram of Galileo's air thermometer, Fig. 2 of a liquid thermometer as used at the present day. About the Publisher Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.com This book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully; any imperfections that remain are intentionally left to preserve the state of such historical works."

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