THE THERMOMETER. 21 



advanced position, marking the highest temperature that has 

 occurred, when the mercury withdraws. The minimum tempe- 

 rature is indicated by the spirit of wine thermometer , which' 

 contains, immersed in the spirit, a small cylinder of ivory, which 

 by a slight inclination of the instrument falls to the surface of 

 the liquid without being able to pass out of it. When the ther- 

 mometer sinks, the ivory is carried back in the spirit ; but when 

 the temperature rises, the alcohol only advances, leaving the ivory 

 where it was. Its extremity most distant from the bulb then 

 indicates the lowest temperature to which the thermometer had 

 been exposed. Before another observation is made, the ivory 

 must be brought again to the surface of the alcohol, by a slight 

 percussion of the instrument. 



Our notions of the range of temperature acquire all their 

 precision from the use of the thermometer. Cold, for instance 

 is allowed a substantial existence as well as heat, in popular 

 language. What is cold ? it is the absence of heat, as dark- 

 ness is the absence of light. The absence of heat, however, 

 is never complete but only partial. Water, after it is frozen 

 into ice, cold as it is in relation to our bodies, has not lost all 

 its heat, for it is easy to cool a thermometer far below the tem- 

 perature of ice, and have it in such a condition as that it shall 

 acquire heat, and be expanded by contact with ice ; thus proving 

 that the ice contains heat. Spirits of wine have not been 

 frozen at the lowest temperature that has hitherto been attained; 

 but even then this liquid possesses heat, and there is no doubt 

 that if a sufficiently large portion of its heat were withdrawn it 

 would freeze like other bodies. The following are interesting 

 circumstances in the range of temperature : 



135 Fahr. Greatest artificial cold. Thilorier. 



121 Solid compound of alcohol and carbonic acid 



melts. 



91 Greatest artificial cold measured by Walker. 

 58 Temperature of planetary space. Fourier. 

 60 Greatest natural cold observed by Ross. 

 55 Greatest natural cold observed by Parry. 

 47 Sulphuric ether congeals. 

 39 Melting point of solid mercury. 

 7 99 A mixture of equal parts of alcohol and water 

 freezes. 



