PROPERTIES OF MATTER 33 



The view preferred at present introduces a new idea altogether. 

 As cells freeze, the solid formed is essentially pure ice, while the 

 dissolved materials become more and more concentrated as more ice 

 forms. Finally these materials begin to come out of solution as solids, 

 and certain of them, especially some of the proteins discussed later, 

 change in the solid state to forms that do not redissolve on melting of 

 the ice. These proteins are required in many cellular operations, and 

 many must be dissolved in order to function. Hence the cells cease 

 to operate. Perhaps this concept should be combined with all the 

 others in explaining death by freezing, or perhaps death results from 

 poisons that accumulate in the cells at low temperatures. Only further 

 experimentation can answer the problem. 



Some microorganisms can be repeatedly frozen and thawed and still 

 live. Others are quite sensitive to such treatment. Multicellular ani- 

 mals are usually killed by freezing and so are many plants. Yet the 

 woody plants of cold climates freeze repeatedly during a single winter 

 and still survive. On the other hand they may be quite sensitive in 

 the middle of their growing seasons. In recent experiments it has 

 been possible to freeze hamsters solid and thaw them out again 

 without serious injury. So, in addition to the causes of death by 

 freezing, the precise effect of the treatment and the nature of the 

 variations between species must still be explained. 



SOLUTIONS 



Solutions are mixed systems with one or more substances uniformly 

 distributed in another. In addition, the distribution is commonly at 

 the level of individual molecules or ions, rather than involving aggre- 

 gates. Solutions fall into three categories based on physical state. 



When two gases are mixed, they intermingle at the molecular level, 

 forming a solution whose physical properties are typically those of 

 gaseous systems. Thus any gases not reacting chemically form solu- 

 tions that behave according to the ordinary gas laws. Hence solutions 

 in the gas state require no special consideration. 



Gases, liquids, or solids may be suitably dispersed in solids to yield 

 solutions in the solid state. Solutions of this type may prove to be 

 very important in biology, but they are not well understood and are 

 not readily susceptible to quantitative description. 



Finally, the dispersion of solid, liquid, or gas in liquid gives the 

 widely studied and biologically important liquid system whose physical 

 and chemical properties differ from those of the individual compo- 



