144 



THE THERMOMETER. 



during these states of transition might with equal convenience be taken as the 

 standards for the fixed points of thermometers ; but water, being a substance 

 always attainable and easily reduced to a pure state, has been selected by com- 

 mon consent, in preference to other bodies. 



The same unanimity has not prevailed respecting the division of the scale. 

 It would have been a matter of great convenience, had all nations agreed to di- 

 vide the interval between the boiling and freezing points of thermometers into 

 the same number of equal parts ; but such a convention was scarcely to be ex- 

 pected. When Fahrenheit adopted the fixed points suggested by Newton, it 

 was supposed that the greatest degree of cold which was attainable was that 

 of a mixture of snow and common salt, or snow and sal ammoniac. A ther- 

 mometer, when plunged in such a mixture, was observed to fall considerably 

 below the point at which it stood in melting ice, and at which temperature Fah- 

 renheit determined to commence his scale of numeration upward. The inter- 

 val between this and the temperature of melting ice is divided into 32 equal 

 parts or degrees ; so that upon this scale the temperature produced by mixing 

 snow and common salt is 0, while the temperature of melting ice is 32. He 

 continued these equal divisions upward, and found that when the thermometer 

 was immersed in the steam of boiling water, the barometer standing at about 

 30 inches, the mercury in the thermometer stood at 212. Thus the interval 

 between the freezing and boiling points was 180. Temperatures have since 

 been experienced much lower than that obtained by the mixture of snow and 

 common salt, and hence it has been necessary to continue the scale below the 

 of Fahrenheit. Degrees below this point are called negative degrees, as 

 already explained. 



The scale as adopted by Fahrenheit has continued in general use in this 

 country to the present day ; and in all English works on science, as well as ia 

 the arts, manufactures, and medical practice, the thermometer used is Fah- 

 renheit's thermometer, and the freezing and boiling points are 32 and 212. 

 The thermometer generally used in France before the revolution, and still used 

 in many parts of Europe, was constructed by Reaumur early in the 18th cen- 

 tury. The liquid used by him was spirit of wine ; but, subsequently, mercury 

 was substituted for this by De Luc. The fixed points on this instrument were 

 likewise the freezing and boiling points of water, the scale proceeding up- 

 ward. The interval between the fixed points was divided into 80 equal parts, 

 called degrees. Thus, the freezing point of water was 0, and its boiling 

 point 80. The degrees in this thermometer were longer than those in Fah- 

 renheit, in the proportion of 2\ to 1. To convert a temperature indicated upon 

 Reaumur into the corresponding temperature upon Fahrenheit, it would, there- 

 fore, be necessary to multiply the degrees upon Reaumur by 2J, and to add to 

 the product 32, to allow for the distance of the points at which the scale com- 

 mences. On the other hand, to reduce Fahrenheit's degree to Reaumur, it 

 would be necessary to subtract 32, and to diminish the remainder in the pro- 

 portion of 2| to 1 . 



About the middle of the eighteenth century, Celsius, a Swedish astronomer, 

 constructed thermometers, in which he commenced the scale, like Reaumur, 

 at the freezing point of water, and divided the interval between the freezing and 

 boiling points into 100. This thermometer was adopted, after the revolution, ; 

 in France, under the name of the cunti.vra.de thermometer. It harmonized with ' 

 the uniform decimal system of weights and measures, adopted in that conn- I 

 try, and has been since that time in general use there. 100 of the centigrade - 

 are equal in length to 180 of Fahrenheit* To convert the temperature on the \ 

 centigrade into the corresponding temperature on Fahrenheit, it would \hen be < 

 necessary, first, to increase the number of degrees in the proportion of 100 to J 



