26 Introduction. 



degree, and this being true the freezing and boiling points 

 of various substances may be taken as standards of tem- 

 perature. 



Water being a common substance which changes its state 

 at convenient and common rates of molecular motion has 

 been selected to fix two degrees of temperature called the 

 freezing and boiling points of water. When a thermom- 

 eter scale is to be graduated its bulb is placed under the in- 

 fluence of melting or freezing water, and the place at which 

 the moving point comes to rest marked ; then it is placed 

 under the conditions of boiling water and the new point 

 also marked. The space between these two points on the 

 scale is then divided into 80, 100 or 180 divisions, accord- 

 ing to the system which it is designed to follow. Since this 

 range in molecular vibration is divided into 180 degrees on 

 the Fahrenheit scale its degrees are the shortest, while 

 those of the Reaumer scale are the longest because the same 

 range is divided into but 80 divisions. 



The Centigrade and the Fahrenheit scales are the two 

 commonly used in this country, the degree of the former 

 being equal to I of the latter.' 



34. Accuracy of Thermometers. The bulbs of most ther- 

 mometers shrink after they are blown and if they have not 

 been permitted to stand for a number of years to season 

 before fixing the zero and boiling points of the scale, these 

 points will change and the thermometer will give incorrect 

 readings in time and the cheapBr grades of thermometers 

 are liable to be subject to this error. 



The accuracy of the freezing point may be approxi- 

 mately tested by surrounding the bulb with snow or 

 crushed ice out of which the melted water may drain, al- 

 lowing the thermometer to remain until the temperature 

 becomes stationary. 



The accuracy of the boiling point may also be approxi- 

 mately determined by holding the bulb in rapidly boiling 

 soft water. 



