For about seventy-five years after the discovery of Galvani chemical action was our only method of generating currents of electricity, and it is largely owing to the inadequacy of this method of production that so few uses for electricity were discovered previous to the perfection of the dynamo about a third of a century ago. Two things have conspired to bring about this _age of electricity_. (1) The dynamo reduced the cost of production from five dollars to ten cents per kilowatt hour. (2) Mankind grew extravagant, greatly increased the number of things which it considered necessary, and at length became both able and willing to spend more for the things which it demanded. The so-called voltaic cell is of scarcely more than academic interest now. The school which, as a rule, follows half a century behind practical life, has taught and still teaches the philosophy of the galvanic cell with great particularity. It is now being urged to undertake the teaching of the dynamo. Meanwhile the dynamo has almost driven out of existence all electric battery cells except the storage cell and the so-called "dry cell," and each year the dynamo is encroaching more and more upon the territory of the dry cell. In the present day, when a passenger upon a street car pushes a button to stop the car, he uses, not a voltaic cell, but a 500-volt dynamo current to ring a small buzzer, and it costs the company not one-hundredth part as much as it would to furnish him a battery equipment to do the same thing. Small dynamos and magnetos are displacing dry battery cells in the sparking equipment of motor boats and automobiles. We lifted a dry battery cell out of its pasteboard case and found that it was contained in a metal cup of sheet zinc. The top of this was sealed over airtight with pitch, the purpose of which is to prevent this "dry" cell from drying up. We dug away the hardened pitch and found a black powder which was distinctly moist. In case the pitch becomes cracked or a hole appears in the zinc cup, the moisture passes out and the cell ceases to act as a generator of electric current. The zinc cup had a lining of pasteboard on the sides and the bottom, similar to the pasteboard which enveloped the outside, only the lining was quite moist. A corrugated rod of carbon about an inch in diameter occupied the middle of the cup, and the space around it was packed full of a mixture of ammonium chloride, manganese dioxide, and other substances like plaster, etc., which differ with different cells. A dry cell which has been long in use is quite apt to show stains upon its pasteboard case. These are caused by holes which appear in the zinc. The production of electric current by the cell is dependent wholly upon a chemical action between the zinc and the ammonium chloride which results in the destruction of both. This chemical action cannot go on without moisture.