514 U. S. BUREAU OF FISHERIES 



distances with little interference. Unless confined they will keep 

 on going or expanding and diffusing into space. If the motion 

 is reduced (that is, if the gas is cooled), the molecules travel less 

 freely and for shorter distances, the gas volume contracts, and a 

 liquid (water, in the example chosen) is formed. In this condi- 

 tion the molecules move less freely, but still get about with many 

 collisions and glancing motions. The whole mass is fluid and will 

 assume the shape of its container, while at the exposed surface 

 many of the molecules escape into space — a process known as evapo- 

 ration. If more heat is taken from the water (that is, if the motion 

 of the molecules is still further reduced), the molecules are no 

 longer free to move about from place to place, but each is con- 

 fined to occupy a small space, within which it performs a restricted 

 vibratory motion. In this condition, where the molecules are too 

 much crowded to move freely, the substance is a solid (ice). In 

 solids the molecules often are found to be arranged in definite rows 

 or patterns, in which case the solid is a crystal of definite geometric 

 form. Only at absolute zero (459.2° F. below zero) are the mole- 

 cules absolutely at rest, but this temperature has never been attained. 

 The differences between solid, liquid, and gas are thus purely 

 differences in the amount of motion in the molecules — hot, gas ; cool, 

 liquid; cold, solid. The solid condition is the frozen condition, 

 ordinary iron, for example, being frozen iron. Cold is a purely 

 relative term meaning less heat. Ordinarily we think of things 

 as cold when they contain less heat than we are accustomed to in 

 our sourroundings. 



TEMPERATURE AND HEAT UNITS 



The degree of motion of molecules is expressed in temperature 

 and is measured by the thermometer. Temperature alone tells us 

 nothing about the quantity of heat. This is obvious if we consider 

 that a drop of molten iron may have a temperature of 2,720° F., yet 

 when dropped in a bucket of cold water the water is not noticeably 

 warmed. This is because the quantity of heat in the drop of iron is 

 too small, though the temperature of the drop is very high. The 

 quantity of heat in the English system is measured in British thermal 

 units (B. t. u.) ; a British thermal unit is defined as the amount of 

 heat required to raise the temperature of a pound of water 1° 

 F. If a pound of water is to be warmed from 32° to 100° F., 

 68 B. t, u. of heat would be required ; 2 pounds of water would re- 

 quire 136 B. t. u. to be warmed 68°. Likewise, if 1 pound of water is 

 to be cooled from 100° to 32° it must give off 68 B. t. u. 



FREEZING 



If water has been cooled to 32°, its freezing point, it can not ordi- 

 narily be cooled any further until all of it is frozen, after which the 

 temperature of the frozen water begins to drop again if heat con- 

 tinues to be extracted. The heat that exists in water and all other 

 crystallizable liquids, and which must be extracted to convert it to a 

 solid without change of temperature, is called latent heat. The term 

 is often misapplied to the animal heat in fresh fish. The latent heat 



