STANDARD POINTS. 



arithmetical measure of physical effects, must be to some extent 

 arbitrary. We accordingly find different scales and different 

 thermometric units prevailing in different countries, and even 

 in the same country at different times. 



21. Whatever thermometric unit be adopted, it is necessary 

 that two standard temperatures be selected, to which the mercury 

 can be reduced at the times and places where thermometers may 

 be required to be constructed or verified. The instrument being 

 exposed to these two temperatures, the points at which the mer- 

 curial column stands are marked upon the scale. The space upon 

 the scale between these points is then divided into a certain 

 number of equal parts, which are called degrees, the degree 

 being the thermometric unit. The same divisions are then con- 

 tinued upon the scale above the higher and below the lower 

 standard point, and such divisions may be continued indefinitely. 

 The scale is then complete. 



In this process, the number of equal parts into which the space 

 between the standard points is divided, is altogether arbitrary. 



22. It now remains to number the scale ; and, for this purpose, 

 a zero point must be selected. If there existed a minor limit to 

 temperature, a temperature below which no body could possibly 

 fall, then such a temperature would supply a natural thermometric 

 zero, and the scale might be numbered upwards from it. 



In that case, although the thermometric unit would still remain 

 arbitrary, the zero of the scale would not be so. But no such 

 natural thermometric zero exists. 



There is no natural limit either to the increase or diminution of 

 temperature. The zero, therefore, of the thermometric scale, like 

 the thermometric scale itself, must be arbitrary. 



23. Thermal phenomena present great varieties of standard 

 temperatures, by which thermometric scales may be established, 

 and which may serve equally as terms of temperature for the pur- 

 pose of distinguishing the indications of different thermometers 

 constructed at different times and places. Thus, the temperatures 

 at which all solid bodies fuse, and those at which all liquids con- 

 geal, are fixed. For different bodies these are different, but 

 always the same for the same body. In like manner, the tem- 

 peratures at which all liquids boil under a given pressure are 

 invariable for the same liquids, though different for different 

 liquids. The temperature of the blood in the human species 

 presents another example of a fixed temperature. 



24. Now, any two of these various temperatures naturally fixed 

 might be taken as the thermometric standards, the choice being 

 altogether arbitrary. Thus, it appears that the arithmetical 

 division of the scale, and consequently the thermometric unit, the 



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