From Dr. R.S. Scorer of Britain's Imperial College of Science comes the latest theory about the cause of lightning flashes. Dr. Scorer believes the cause is hail falling through super-cooled clouds. Ice particles bouncing off the falling hail acquired a positive charge and rise to the top of the cloud while the hail carries a negative charge to the bottom of the cloud. According to Dr. Scorer, Benjamin Franklin first proved thunderclouds are charged with electricity. Later investigation showed that the tops of the clouds have a great positive charge and the bottoms a great negative charge. When the charges become great enough to break down the insulating properties of the air, lightning flashes carry the electricity within the cloud, or from cloud to cloud, or from cloud to earth. But the question remained: how do the charges develop within the cloud? To seek the cause, Dr. Scorer and his colleagues at the college first duplicated thundercloud currents in a liquid tank. They found that mixture takes place only at the tops of the clouds. Next a study of thunderclouds over the North Atlantic showed that lightning occurs only when the air temperature around the cloud is below freezing. Particles at the top of the cloud begin to freeze but those in the remainder of the cloud stay unfrozen although below freezing temperature. To measure moisture the scientists sent planes equipped with external refrigerated rods into the clouds. The idea was that moisture would freeze on the rods and could later be measured. The men found, however, that some of the moisture particles bounced off the rods. His accidental discovery set the scientists on a new course of action. In the laboratory, S.E. Reynolds whirled a refrigerated rod through ice particles and found that the particles which bounced off the rod acquired a positive charge. This was the missing link. Without hail and super-cooled clouds, he concluded, there could be no lightning. This passage is about a______.