NATURAL PHILOSOPHY MATTER, MOTION, AND HEAT. 



The most convenient substances for the construc- 

 tion of thermometers are found to be mercury and 

 alcohol, or spirit of wine. For ordinary tempera- 

 tures, mercury is preferable ; but when too much 

 heat is withdrawn, it freezes or becomes solid, and 

 therefore, for very low temperatures, alcohol is 

 used, which cannot be congealed by any known 

 cold. The mercury or alcohol is inclosed in a 

 glass tube with a hollow bulb at one end, the 

 other being closed. It is then graduated by first 

 plunging it in melting ice, and marking on the 

 glass, or on an ivory scale attached to it, the point 

 at which the mercury comes to stand. This mark 

 is the freezing-point of water, for water freezing 

 and ice melting have the same temperature. The 

 thermometer is next placed in boiling water, when 

 the mercury rises to a certain height, and then 

 continues steady. A second mark is here made, 

 which is the boiling-point. 



The space between these two points is then 

 divided into a number of equal parts, called 

 degrees, and parts of the same length are set off 

 above and below the boiling and freezing points, 

 as far as required. 



In the thermometer used in this country, the 

 space between the freezing and boiling points is 

 divided into 180 equal 

 parts, and we begin 

 counting at 32 below 

 the freezing-point. A 

 cipher is placed there, 

 and it is called the zero 

 or nothing - point of 

 the thermometer. The 

 freezing-point of water 

 thus comes to be marked 

 by the number 32, and 

 the boiling-point, which 

 is 1 80 higher, by 212. 

 In the thermometer 

 chiefly used on the 

 continent, the space 

 between the freezing 

 and boiling points of 

 water is divided into 

 100 equal parts, and 

 the graduation begins 

 Fig. 19. at the freezing-point, 



which is marked o, 



or zero. According to this thermometer, which 

 is called the centigrade, water freezes at o, and 

 boils at 100. 



The centigrade thermometer is now much em- 

 ployed in scientific researches in this country. To 

 prevent any confusion arising from its being mis- 

 taken for the thermometer first described, which is 

 called, from its original maker, Fahrenheit's, or the 

 Fahrenheit thermometer, the letter F is placed 

 after temperatures indicated by his thermometer, 

 and the letter C after those denoted by the cen- 

 tigrade. 



From its zero-point each thermometer counts 

 downwards as well as upwards ; and to distinguish 

 the degrees below zero from those above it, the 

 former are distinguished by prefixing to them the 

 minus sign . Thus, mercury is said to freeze 

 at 40 F. ; that is, at 40 below Fahrenheit's 

 zero. 



Another scale much used in Germany is that 

 called Reaumur's, in which the freezing-point is 

 marked o, and the boiling-point 80. The degrees 



on this scale are thus larger than those of Fahren- 

 heit, or even of the centigrade : 9 F. = 4 R. or 

 5 C. ; and by means of these proportions, a tem- 

 perature stated in one scale may be reduced to 

 either of the others, care being taken to allow for 

 Fahrenheit's scale commencing, not at the freezing- 

 point, as the others do, but 32^ below it 



Fahrenheit chose the temperature of 32 below 

 freezing as the zero-point of his scale, because it 

 was the lowest that had then been observed, and 

 was considered to indicate the complete absence 

 of heat ; but it is now known that there are natural 

 temperatures at least 90 below this ; and by 

 artificial mixtures, a cold has been produced of 

 -146 F. 



SPECIFIC HEAT. 



Different substances require different quantities 

 of heat to raise them to the same temperature. 

 This is expressed by saying that each possesses a 

 specific capacity for heat, or, more shortly, a specific 

 heat. The fact can be proved in a variety of ways. 

 Thus, if we cause equal quantities of bodies which 

 have all been raised to the same temperature, to 

 melt ice, we shall find that a much greater weight 

 of it will be melted by one body than by another. 

 Thus, mercury at 212 will melt much less ice than 

 an equal quantity of water at the same tempera- 

 ture will, for the mercury has much less heat to 

 give out, so as to produce liquefaction, than the 

 water has. 



Specific heats are generally stated with reference 

 to equal weights, rather than to equal measures, 

 of bodies. Thus, a pound of water in rising to a 

 given temperature, absorbs thirty times more heat 

 than a pound of mercury in rising to the same 

 temperature ; so that the capacity of water for 

 heat exceeds that of mercury thirty times ; and if 

 we call the specific heat of mercury i, that of 

 water will be 30. 



The mechanical theory of heat affords a satis- 

 factory explanation of this difference in bodies. 

 Chemistry establishes that the atoms of different 

 substances differ greatly in weight ; an atom of 

 oxygen, for example, has sixteen times the weight 

 of an atom of hydrogen. Now, experiment seems 

 to prove that when two substances ar at the same 

 temperature, the ultimate particles of both have 

 each the same amount of the motal energy we 

 call heat, the smaller particles making up for want 

 of weight by greater speed. But, as it requires 

 sixteen times more atoms of hydrogen to make a 

 pound than it does of oxygen, a pound of hydrogen 

 must thus contain sixteen times as much heat as 

 a pound of oxygen. It follows that to raise the 

 temperature of a pound of hydrogen through, say, 

 10, requires sixteen times as much heat added as 

 is required in the case of oxygen. Thus, the 

 specific heat increases as the atomic weight 

 diminishes. 



The great specific heat of water has a most 

 important relation to the welfare of the living 

 creatures on the globe. The sea, and other great 

 beds of water, which spread over so large a por- 

 tion of the earth, cannot in the hot seasons of the 

 year become rapidly raised in temperature ; in the 

 cold seasons of the year, on the other hand, they 

 cool slowly, and, moreover, in cooling, evolve 

 much heat, which equalises the temperature of the 

 air as well as that of the land. 



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