36 ELEMENTS OF LABORATORY WORK 



touch one another, but radiation takes place between bodies 

 when separated even by great distances. In any system of 

 material bodies, whatever may be their natures, relative posi- 

 tions, and temperatures, the final condition of the system is 

 one in which they are all at the same temperature. The 

 change may proceed by conduction or radiation, or by both. 

 It is to be noted that we are directly conscious of variety of 

 temperature, just as we are directly conscious of variety of 

 motion. 



30. Standard Temperatures. 1. Observe that a thermo- 

 meter shows the temperature of ice to remain constant while 

 melting. Several observations are to be made. The density of 

 the mercury in a thermometer does not alter until all the ice 

 is melted. 



2. Observe similarly that the temperature of water, when 

 it boils under unaltered conditions, remains constant. 



3. Note that these two statements cease to be true if the 

 thermometer is very large compared with the quantity of ice 

 or water used. In each case the introduction of the thermo- 

 meter, a body of a different temperature, affects the thermal 

 condition of the system ; but after a time the thermal condition 

 becomes constant again, and is unaltered so long as the water 

 boils or the ice is melting. 



Two standard temperatures are thus found, and other 

 temperatures may be compared with them. The ordinary 

 thermometer is constructed of a thin closed glass tube with a 

 bulb at one end. This bulb, and a portion of the narrow even 

 bore of the tube, contains pure mercury. The rest of the tube 

 is empty, so as to allow free movement of the mercury. We 

 perceive that an alteration of temperature causes the mercury 

 to expand more than the glass containing it. The use of the 

 instrument is founded on this inequality of expansion. 



The variations in the level of the mercury column, due to 

 changes of density, are observed by the help of a scale. This 

 scale is constructed by marking as zero the position of the 

 column when the thermometer is placed in melting ice ; and, 

 when the thermometer is in boiling water, marking the posi- 

 tion of the column 100. Between these points the scale is 



