POPULAR SCIENCE 431 



There may also arise mechanical injuries to the tissue, such as 

 may result if the tissue, like pure water, should expand on 

 freezing. 



It would not be a matter of much moment in practice if 

 the changes which took place during freezing were absolutely 

 reversed on thawing, for then the last state of the tissue would 

 be the same as the first. As a matter of fact, it is known from 

 everyday experience that this is not the case. Richardson 

 states that with slow thawing the changes are made reversible, 

 but recent German work denies the possibility of this^taking 

 place, at any rate in the case of fish. 



Since the disturbance in the water relation of the tissues 

 takes place principally during the period represented by the 

 horizontal portion of the freezing curve, it would appear 

 probable that shortening this period would correspondingly 

 lessen the disturbance produced in the water relation, or, 

 stated in other words, it is to be expected that with increased 

 rate of freezing and thawing there will result a corresponding 

 decrease in the disturbance to the water relations in the tissues. 

 The rate of freezing of a body depends on a number of 

 factors. Naturally it depends on the size of the body frozen 

 and on its shape ; the larger the body the longer the time taken 

 to freeze it. And here it may be pointed out that when a 

 body is frozen by immersion in a colder medium, every part of 

 the body does not have the same temperature at the same 

 time. The outer parts of the cooled object fall rapidly in tem- 

 perature and soon become frozen, whereas, further inside the 

 body, the time taken for the freezing process to take place 

 increases, being longest in the middle of the thickest part. 

 Thus the rate at which the inner parts of the cooled object 

 freeze depends on the " heat conductivity " of the body, i.e. 

 on the rate at which heat can pass through and so be drawn 

 from the inner regions. 



It will also depend on the specific heat of the cooled object, 

 that is, on the quantity of heat which must be withdrawn to 

 lower the temperature of unit mass of the substance through 

 one degree. Among the different food substances which it is 

 desirable to preserve by freezing, there is not much variation 

 either in specific heat or conductivity, for the tissues in ques- 

 tion are so largely water that both the specific heat and con- 

 ductivity cannot be far removed from those of water. There 

 are, however, three other factors on which the rates of cooling 

 and freezing depend ; these are the temperature of the cooling 

 medium, the extent to which the latter is kept in motion, and its 

 heat-conductivity. As regards the temperature of the cooling 

 medium the lower this is the more rapid will be the rate of 

 cooling. This is the fact summarised in Newton's well-known 



