“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.