174 SECRETS OF EARTH AND SEA 



spirit, or other liquid) of a thermometer as it expands 

 from the shrunken bulk which it occupies when placed 

 in freezing water to the full length which it attains when 

 the water is heated to boiling point. This is called the 

 centigrade scale, or scale of a hundred degrees. But, 

 as we know by the records of travellers in the Arctic 

 regions and by the experiments made in laboratories, 

 there are " degrees " of coldness or diminution of heat 

 which are much below that of freezing water, and can 

 be measured by the further shrinking of the column of 

 liquid in the thermometer, so that we record " degrees " 

 "below zero centigrade," each of the same length as 

 those above it and corresponding to the same " quantum " 

 of decrease or increment of heat. As we pass from the 

 temperature at which water is solid to that much lower 

 or diminished state of hotness at which mercury becomes 

 solid, the shrinking column of the thermometer (in which 

 a liquid is used not rendered solid by this amount of 

 cooling) falls through 39 degrees of the centigrade size, 

 so that we say that mercury freezes at minus 39 or at 

 39 degrees below zero of the centigrade scale. The con- 

 clusion has now been reached that the absolute zero or 

 cessation of all heat in a body is represented by a fall 

 of no less than 273 degrees below zero on the centigrade 

 scale. Hydrogen gas becomes a. liquid at 252 degrees 

 below zero centigrade, and a solid at 264 degrees. If we 

 start our counting of those degrees or increments of heat, 

 of which there are 100 between the freezing and boiling 

 points of water, at the absolute zero or condition of total 

 absence of heat, we must say that hydrogen " melts " — 

 that is, passes from the solid to the liquid state — at 

 1 1 degrees (absolute), and boils at about 20 degrees 

 (absolute), whilst water does not melt until 273 degrees 

 (absolute) of temperature are reached, and boils at 

 373 degrees above the absolute zero. 



