THERMOMETRIC SCALES. 33 



boiling-point is called 100 degrees, and temperatures above this 

 are denoted by figures greater than 100 ; for these reasons this 

 scale is called centigrade or "hundred-degree" scale. Degrees 

 expressed on this scale are denoted by the letter C., thus 100 C. 

 (boiling-point), and so on. Another form of scale is unfortunately 

 in use in England ; this is termed the Fahrenheit scale, after the 

 name of its inventor, who for some inexplicable reason considered 

 that because there are 180 geometrical degrees in a semicircle, 

 therefore there should be the same number of steps between the 

 freezing and boiling points ; whilst to complicate the scale still 

 'more, he put his zero-point 32 Fahrenheit degrees below the ice- 

 point ; so that the ice-point becomes 32 degrees Fahrenheit (written 

 32 F.), and consequently the boiling-point 180 + 32 = 212 degrees 

 Fahrenheit (written 212 F.). 



For scientific purposes the centigrade scale is far more exten- 

 sively used than the Fahrenheit, but for determining the tem- 

 perature of rooms, greenhouses, feverish patients, and the weather 

 generally, the Fahrenheit scale is still chiefly used in England. 

 In the following pages, wherever we have occasion to refer to 

 temperature measurements in figures, the centigrade scale will 

 always be employed in preference, on account of its far greater 

 simplicity. 



On some parts of the Continent a third scale known as 

 Reaumur's is used. This is somewhat like the centigrade scale, 

 the ice-point being termed ; but the boiling-point is called 80 

 degrees instead of 100, and similarly throughout the whole range 

 of the scale, so that every 5 centigrade are equal to 4 Reaumur. 

 Fig. 17 represents the relations of the three scales to one another. 



The better kind of .thermometers are constructed so that the 

 engraved scale and the glass tube cannot be separated from one 

 another, whilst the bulb is elongated in such a fashion as not to 

 be wider in diameter than the stem, so that the instrument can, 

 if required, be passed through a perforated cork. Fig. 16 repre- 

 sents one form of chemical thermometer where the scale is a slip 

 of ivory or paper which is placed inside a second or outer tube 

 enclosing the stem of the actual thermometer but not the bulb, 

 being sealed by the glass-blower to the stem just where it joins 

 the bulb. When the graduation is finished the scale is perma- 

 nently fixed to the outer tube by a drop of cement, and the tube 

 sealed up at the upper end. A still better form is where the 

 scale is etched on the glass stem itself (Expt. 271). 



Expt. 29. To Verify the Scale of a Thermometer. Thermo- 

 meters of the cheaper sort are made in quantities at a time by 

 the instrument makers, and far more accurately than might be anti- 



C 



