PROPERTIES OF MATTER 27 



compared to that of the solvent. On the other hand, if the dissolved 

 molecules are moderately small and interact but weakly with each 

 other and with the molecules of the liquid, viscosity is reduced. Such 

 is the case with solutions of ethyl alcohol or oxygen in water, for 

 example. In any given kind of solution the viscosity will be related to 

 concentration, but it is not yet possible to predict viscosity from 

 theoretical considerations alone. Hence concentration can be deter- 

 mined from viscosity only by comparison with an experimental stand- 

 ard series. 



The Liquid State in Nature 



Pure liquids are non-existent in nature, but solutions of the liquid 

 type are very widespread. Even rain drops, about as close a natural 

 approach to a pure liquid as there is, are solutions of atmospheric 

 gases in water. Liquid solutions, mainly aqueous, are fundamental to 

 biology. The environments of most cells are liquid since many species 

 are suspended in solutions and many others are bathed in liquids 

 circidating and diffusing about in the interiors of multicellular forms. 



Liquids possess certain attributes of special biological importance. 

 They are dense enough to provide support against gravitational forces. 

 Yet liquids have viscosities low enough to allow the movement of 

 motile forms. Thus liquid provides an especially suitable environment 

 for life, so much so that life on earth is presumed to have originated 

 in a liquid system. 



Biological Role of Water 



Oxygen is the most abundant element of the earth's crust. Hydro- 

 gen, though much less abundant on a weight basis, is also exceedingly 

 plentiful on the basis of the number of equivalents, owing to the low 

 atomic weight of this element. Although both oxygen and hydrogen 

 are distributed among a great many kinds of compounds, together they 

 make up water, one of the most common substances of the earth's 

 crust. In fact water is so abundant and so widely distributed that it 

 is universally available on the surface of the earth. It is the only 

 liquid naturally occurring in such enormous quantity, including even 

 petroleum. Moreover, the characteristic physical properties of water 

 favor a very wide distribution. The freezing and boiling points are 

 such that the normal temperature fluctuations of the earth lead to 

 vaporization of water, wide distribution of the vapor, and condensa- 

 tion over the surface leading to rain or snow everywhere. 



This ubiquitous nature of water, especially in liquid form, suits it 

 admirably for a major biological role. Indeed it is difficult to imagine 



