516 U. S. BUREAU OF FISHERIES 



not begin to freeze until it is cooled to 31.5° or 30.5° ; but it will not 

 freeze hard and solid even at this temperature, because as part of the 

 water freezes out what is left has a higher concentration than it had 

 at the beginning, and a lower temperature is required for further 

 freezing. Probably not all the water in fish is ever frozen at ordinary 

 freezing temperatures. A fish will be apparently hard at 20°, but at 

 0° it is harder. At 5° F. about 17 per cent of the water remains un- 

 frozen; at 31° F. below zero, about 2.66 per cent remains liquid. 

 Only at about 75° below zero is it all frozen. 



CONDUCTIVITY 



Heat is conducted through any substance, as, for example, when 

 one end of a metal rod is heated the other end becomes warm. This 

 conduction is a matter of the molecules striking against one another 

 and transmitting the motion. Some substances conduct heat better 

 than others. All metals are relatively good conductors, silver and 

 copper being the best. Air and all gases generally are exceedingly 

 poor conductors, especially if they are prevented from circulating. 

 This is true because the molecules are farther apart and collide less 

 frequently. For the same reason, solids generally are better con- 

 ductors than liquids. 



Substances like cork, feathers, wool, sawdust, etc., that hold much 

 air entrapped, are poor conductors of heat and are called insulators. 

 When heat or cold must be confined in a space it must be surrounded 

 by an insulator. Cork, wool, and like materials serve this purpose 

 when they are dry because of the air they contain, but when water 

 enters them and drives out the air the insulating value is impaired, 

 because, as already said, liquid is a better conductor of heat than gas. 



The rate of transfer of heat through an insulating medium is 

 approximately inversely proportional to the thickness of the insulat- 

 ing substance. About half as much heat will flow per minute through 

 a square foot of cork 2 inches thick as through a square foot of cork 

 1 inch thick, and a third as much will flow through a slab 3 inches 

 thick. On the other hand, when it is desirable to cause heat to flow as 

 rapidly as possible from one body to another the path of travel of the 

 heat must be through as good a conductor as possible. For example, 

 in a sharp freezer, where fish are in a metal pan and the pan rests on 

 a metal pipe, the heat travels freely through the points of contact 

 between fish and pan and pan and pipes; but most of the fish is in 

 contact with air, which is an exceedingly poor conductor, hence the 

 comparatively slow freezing attained in this way. If more rapid 

 freezing is to be attained the fish must be brought into intimate 

 contact with a good conductor that is refrigerated. 



The rate of transfer of heat from one place to another is in direct 

 proportion to the difference in the temperature of the two places. 

 For example, a can of water at 32° is immersed in brine at 22°. The 

 difference in the temperatures of the water and the brine is 10°. An- 

 other can of water at 32° is immersed in brine at 12°. The difference 

 in temperature in this case is 20°. The water in the second can will 

 freeze twice as rapidly as the first if the conditions remain constant, 

 because the difference in the temperature of water and brine is twice 

 as great in the second case as in the first. Another can in brine at 2° 

 would freeze three times as fast as the one at 22°. This simple fact, 



