274 THE FUTURE OF ARID LANDS 
physical nature of the scale-forming agents, thus rendering them 
non-adherent to the metal heating surfaces, or to add sequestering 
agents, maintaining scale-forming solids in solution. 
Much research already has been done in this field, especially 
in England. The United Kingdom has now set up a two-year 
program for additional investigations under the guidance of the 
Department for Scientific and Industrial Research, to be carried 
out at the Admiralty Materials Laboratory. 
Other European countries have been invited to share in the 
costs, the know-how, and other profits. The work will be super- 
vised by a steering group, in which the collaborating countries 
are represented. 
W.P. 8 has quoted the target value for the total cost, including 
capital investment, for reducing sea water (20,000 ppm Cl’) to a 
salt content of 300 ppm Cl’ (by mixing sea water with the dis- 
tilled water) for a capacity of 10 tons per day at $1.30 per 
ton. For a big installation of 1,000 tons per day the cost would be 
30 cents per ton. It should be borne in mind that these figures have 
no absolute value; they give only an idea of the economic level of 
the process, in comparison with other solutions. 
Electrodialysts 
This process has been studied especially in the United States 
(Ionics, Inc., Boston, Mass.) and in the Netherlands (National 
Council for Applied Scientific Research, T.N.O.). 
The process of desalting by direct current, passing through a 
three-compartment apparatus with 2 membranes, e.g., of cello- 
phane, has been known for a long time. It is, however, hardly 
applicable to sea water, since a rinsing liquid is needed of similar 
or lower concentration than that of the fresh water to be obtained. 
However, quite recently a new possibility has been opened as a 
result of the introduction of highly selective membranes, which 
permits the use as rinsing liquids of salt solutions of a considerably 
higher concentration than the dialysate. Moreover, great progress 
has been made by constructional improvement, especially by 
placing a great number of selective membranes in shunt with 
extremely narrow compartments (0.5 millimeter). 
