LIQUID nitrogen seems a good place to start if you want to cool something down. If Peter Dearman, a British inventor, is correct, it might be even better than it looks. Mr Dearman is a man with a bee in his bonnet. He has dreamed, since he was 15, of making a useful motor powered by liquid nitrogen. Now, at 64, he thinks he has done it—not, as he had originally imagined, to run a car, but to run a refrigerator that promises to be more efficient and less polluting than conventional alternatives.