Because they are extremely scarce or non-existent in their natural state in the United States, a few elements or other commodities have been the object of intensive government efforts to either directly produce them on a full scale, or to encourage private production by acquiring and transferring to private firms certain of the assets requisite to production. These are nitrogen, helium gas, fuels, rubber, synthetic liquid and abaca (a plant the fiber of which is used in making hemp).[281] Thus the Board of Directors of the Tennessee Valley Authority were given power to exercise the right of eminent domain[282] and to make and sell fixed nitrogen and fertilizers with the specific injunction that it maintain in stand-by condition suitable facilities for the production of explosives in the event of war or a national emergency.[283] The plant might be used for the fixation of nitrogen for agricultural purposes or leased, as long as conversion to war production could be made quickly. The TVA of course was authorized to produce and sell electric power,[284] but the government reserved the right in case of war or national emergency declared by Congress to preempt TVA-produced electricity as well as nitrogen.[285] The Helium Gas Conservation Act of 1937 authorized the Secretary of the Interior, through the Bureau of Mines, to acquire lands, and acquire or construct such plants as were necessary to establish a federal monopoly of helium. In 1942 the Secretary of Agriculture was authorized to construct or operate factories for the growth and processing of guayule and other rubber-bearing plants.[286] In 1947 Congress proclaimed the continued existence of a short supply of rubber, a highly strategic and critical material needed for the common defense and which cannot, in its natural state, be grown in the United States. It reaffirmed the policy that there shall be maintained at all times in the interest of the national security and common defense, in addition to stock piles of natural rubber, a technologically advanced and rapidly expandable domestic rubber-producing industry. To this end, the powers of the United States to manufacture and sell synthetic rubber were to continue in force and the government would retain at least the minimum copolymer plant capacity to produce “not less than six hundred thousand long tons per year.”[287] A year later, in March 1948, a policy of reliance upon the development of a free, competitive synthetic-rubber industry and the termination of government production was enunciated, the President to exercise certain powers of control to insure the existence of an extensive government demand for domestic synthetic-rubber.[288]