the very short time needed to score mortality, that in a longer time frame one can observe mortality in those insects also. So we have a completely different mechanism, the dark reaction, which is less toxic and doesn't use light as a catalyst." A third toxic mechanism, occurring only in insects treated at the larval stage, has also been observed recently, Heitz says. Toxicity is manifested by aberrations of development in the various larval instars, through the pupal stage, and on to the adult stage. Often, the adult insects are unable to emerge from their pupal cases. The phenomenon has been observed in face flies and mosquitoes as well as in houseflies. Laboratory toxicology studies first revealed that adult houseflies could easily be killed by dyes and visible light, Heitz relates. "When further studies showed that the larval stage of the housefly was even more easily killed, it set the stage for initial small-scale field tests at a caged layer egg farm." After those tests showed positive control of the flies, large-scale tests began. The tests took place in a 30,000-bird chicken house with 14 manure pits having a total area of about V4 acre. The pits were sprayed weekly with a solution of erythrosin B, at the rate of 4 lb of dye per acre. Samples were taken a week before the spraying began and again immediately before each spray treatment. The samples revealed that the larval housefly population decreased nearly 90% in the course of the test. However, the larvae of the soldier fly—a beneficial species, according to Heitz—weren't harmed. Instead, the soldier fly larval population increased almost 100%. Adults were also counted. "The adult housefly population decreased approximately 75%," Heitz says, "and the soldier fly population fluctuated with no discernible trends." The dye method of fly control offers several other advantages, according to Heitz. For one thing, the dyes should present no threat to the environment; they degrade rapidly, with a half-life of about tv/o hours in direct sunlight. Erythrosin B itself is relatively nontoxic to mammals and so are its degradation products. In addition, Heitz says, the treatment is simple and inexpensive. The operator need only dissolve the dye in water and spray it on the manure. And at the current erythrosin B price of $10 to $12 per lb, it would cost only about $50 per month, plus labor, to treat a 30,000-bird chicken house. At present, Heitz says, estimates run from $100 to $500 per month for fly control in similar layer houses. • 22
C&EN April 23, 1979
Nonbiological sources of natural gas touted Natural gas, buried in the earth, was formed eons ago as organic matter decayed—at least, according to conventional wisdom. But that wisjdom is being questioned, and a suspicion is growing that much more gas than is now estimated may lie within the earth. Gordon J. MacDonald of Mitre Corp., McLean, Va., outlined conclusions of his study on the future of fuel during the ACS/CSJ Chemical Congress in Honolulu. "There might be far more natural gas than we think," he says. Gas not only is a relatively "clean" form of energy, it also is relatively efficient, he notes. "That might help to slow down the greenhouse effect." In the U.S., MacDonald says, about three fourths of the known natural gas deposits are found in fields lacking petroleum. Such observations along with discovery of natural gas deposits associated with volcanic rocks in several locations throughout the world suggest that huge deposits of gas with nonbiological origins lie deep within the earth, he argues. For example, Lake Kivu, on the border of Zaire and Rwanda in Africa, contains a sizable quantity of methane dissolved in its deep waters, according to MacDonald. "The origin of Kivu's methane has remained a mystery," he says. "The only probable explanation," he continues, "is movement of methane having an igneous or mantle origin up a channel at low enough temperatures to remain stable." He also speculates that there might be other nonbiological sources for
MacDonald: more natural gas
natural gas. For instance, meteorites carrying trapped gas might have burrowed into the earth long ago, and that gas might have slowly leaked into deep caverns beneath the surface. "No single observation can be taken as conclusive proof that gas of nonbiological origins exists," MacDonald admits. "Taken together, however, the observations indicate that new directions should be taken in the exploration for natural gas," he says. He recommends searching for faults "associated with reservoir rocks and traps" and also looking near volcanoes where porous fractures could act as reservoirs for gas. If the hypothesis about gas's nonbiological origins is true, "natural gas will continue to contribute significant amounts of energy over the next few decades," MacDonald predicts. •
Plasma improves silicon cell conductance A plasma treatment method for polycrystalline silicon that improves its conductance as much as 100 times and makes it a much better material for solar cells has been developed by David S. Ginley and Carl Seager of Sandia Laboratories, Albuquerque, N.M. The work was described at the ACS/CSJ Chemical Congress in Honolulu. The treatment, called passivation, consists of treating the polycrystalline silicon with a plasma of an electropositive element. So far, hydrogen and magnesium plasmas have been shown to be effective. Treatment with plasmas of electronegative elements, like oxygen, nitrogen, or fluorine, has just the opposite effect. It substantially decreases the conductance of the silicon. The scientists have tested their
method on quite poor silicon solar cells and improved them greatly. Conductances in the 0.01% range can be increased 1 to 2% using hydrogen plasmas, Ginley says. They have not yet tried a state-of-the art cell, with a conversion efficiency before treatment of about 5 to 7%, to see how much improvement their plasma treatment will give. "We're certainly not going to take a 5% cell and bring it up to 500%," Ginley says. "The best we can hope for is to go from 5% to about 7% or so, or from 7% up to 10%." He points out that 10% efficiency is the goal the Department of Energy has set for polycrystalline silicon cells. "If we could demonstrate a 10% cell, the whole field would really take off," Ginley says. "I think you can get to the 10% range with this method."
