262 THE FUTURE OF ARID LANDS 
Inquisitive search for usable fresh water over the years has 
resulted in many workable processes for desalting saline waters. 
Most of the known procedures for desalting have been classified (1) 
broadly into three major groups according to the type of process 
used, namely, physical, chemical, and electrical. Within each of 
these general classifications are many specific methods of water 
conditioning. These include vaporization, crystallization, sub- 
limation, adsorption, ultrasonics, osmosis, ion exchange, electro- 
ion migration, and numerous other processes and phenomena. 
Of particular interest at the present time is the method of 
electro-ion migration utilizing membranes which selectively 
remove cations or anions of the dissolved salts in saline waters. 
This process has been the subject of much experimentation. 
New synthetic membranes being developed give hope of increas- 
ing the efficiency of this method to a practical range. 
Although it was at first thought that distillation had been 
advanced about as far as possible, that is not true, and much 
research is now being carried on under the Salt Water Conversion 
Program on various distillation processes. Those being studied 
critically are vapor compression stills, vacuum distillation, mul- 
tiple effect evaporators, critical pressure separation and various 
combinations of these schemes. As a result of these studies, 
valuable data have been acquired on potential means for reducing 
cost. One of the most recent developments in this field is Hick- 
man’s modification of conventional vapor compression, which 
takes advantage of fundamental principles of boiling to increase 
the rate of evaporation. This modification, although still in the 
research stage, gives hope of greatly increasing the capacity over 
conventional evaporators. 
Research in these and other methods is revealing the funda- 
mental principles of operation and is pointing out the advantages 
and disadvantages of many of them from a practical standpoint. 
The time limitation precludes even superficial discussion here of 
the merits of these processes. They are mentioned merely to 
indicate potential avenues for further developments as applicable 
to specific needs in various areas based on local conditions and 
available energy sources. 
