PYRONOMICS. 85 
be employed. The latte: is most generally advisable, on account of its 
retaining its fluidity at a low degree of temperature, not vaporizing but with 
a considerable degree of heat. In addition to this, its expansion, without 
the ordinary range of temperature, is in direct proportion to the increment 
of heat. 
The Mercurial Thermometer (pl. 19, fig. 7) consists of a narrow 
cylindrical glass tube, with a bulb blown at one end, the whole, except part 
of the tube, filled with mercury. The space above the mercury is a 
vacuum ; the upper end of the tube is hermetically sealed. The filling of 
the thermometer is effected by atmospheric pressure. Thus, the empty 
tube is heated as much as possible, and the open end immersed in a vessel 
of mercury. A partial vacuum being formed on the cooling of the tube by 
contact with the mercury, a certain portion of this liquid is driven into it. 
If a sufficient amount be not yet introduced, the mercury already in the 
tube is made to boil, and, after the empty space is filled with the vapor, the 
tube is again inserted in the vessel of mercury as before. When the tube 
becomes thus completely filled with mercury at an elevated temperature, its 
upper end is hermetically sealed by being brought into the flame of a blow- 
pipe. On the contraction of the mercury by cooling, the empty space left is a 
perfect vacuum. The height of the mercury in the tube is measured by the 
scale or graduated division attached to it. ‘This scale is constructed by 
fixing in the first place two points of temperature corresponding to the 
freezing and the boiling points of water. To obtain the former, immerse 
the thermometer in a quantity of finely pounded ice melting into water, 
and after a short time mark the elevation of the mercury upon the tube by 
making there a fine mark or scratch. For the latter, take a long-necked 
vessel filled with distilled water, and after causing the water to boil, again 
immerse the thermometer tube. The elevation of the mercury, after a 
short time, must be again marked on the tube, as being indicative of the 
boiling point of water. The distance between these two points, the freezing 
and the boiling points of water, being thus obtained, the intervening space 
may be divided into any number of parts. In the scale of Reaumur it is 
divided into eighty, and in that of Celsius or the centigrade thermometer, 
into 100 parts, the zero being at the melting point of ice. Graduations of 
the centigrade thermometer over 360° above zero, and 30° or 40° below 
zero, are hardly available, as these degrees are too close to the boiling and 
freezing points of mercury, near which the expansion and contraction are 
‘not in precise proportion to the variation of temperature. 
Besides these two scales, the first of which (Reaumur’s) is chiefly usea 
in Germany, the second (the centigrade) in France, there is still a 
third (the Fahrenheit) employed in England and America. Fahrenheit, 
seeking to avoid negative quantities, obtained, as he thought, the point of 
maximum cold, by mixing salt and ice together; this he called zero of his 
scale. He divided the interval between this and the boiling point into 212 
equal parts, the freezing point falling at 32°, and thus gained the advantage 
of having fewer fractional quantities in estimations of temperature by his 
instrument. There are, of course, 180 degrees between the freezing ana 
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