"Somewhat curious," assented the Inspector; "you see our batteries require a good deal of shelving. All put together, there is in this room about three miles of shelving, completely filled, as you see, with about 22,000 cells or jars. The electricity is generated in these jars. They contain carbon and zinc plates in a solution of bichromate of potash and sulphuric acid and water. We fill them up once every two weeks, and renew the plates occasionally. There is a deal of sulphate of copper used up here, sir, in creating electricity--about six tons in the year. Pure copper accumulates on the plates in the operation, but the zinc wears away." The detective expressed real astonishment and interest in all this, and much more that the Inspector told him. "Poisonous stuff in your jars, I should fancy?" he inquired. "Rather," replied the Inspector. "Does your door ever stand open?" asked the detective. "Sometimes," said the other, with a look of slight surprise. "You never received a visit down here from a mad dog, did you?" asked the man in grey. "Never!" "I only ask the question," continued the other, in a careless tone, "because I once read in the newspapers of a poodle being chased into the Post-Office and never heard of again. It occurred to me that poison might account for it.--A curious-looking thing here; what is it?" He had come to a part of the Battery Room where there was a large frame or case of dark wood, the surface of which was covered with innumerable brass knobs or buttons, which were coupled together by wires. "That is our Battery Test-Box," explained the Inspector. "There are four thousand wires connected with it--two thousand going to the instruments up-stairs, and two thousand connected with the battery-jars. When I complete the circuit by connecting any couple of these buttons, the influence of the current is at once perceived." He took a piece of charcoal, as he spoke, and brought it into contact with two of the knobs. The result was to convert the coal instantly into an intense electric light of dazzling beauty. The point of an ordinary lead pencil applied in the same way became equally brilliant. "That must be a powerful battery," remarked the detective. The Inspector smilingly took two handles from a neighbouring shelf and held them out to his visitor. "Lay hold of these," he said, "and you will feel its powers." The detective did as directed, and received a shock which caused him to fling down the handles with great promptitude and violence. He was too self-possessed a man, however, to seem put out. "Strong!" he said, with a short laugh; "remarkably strong and effective."