sired it is very doubtful whether the economy of the steam engine can be superseded by any one of them. It is true that there is no expenditure for fuel, but the interest on the extra cost of the plant and the maintenance thereof, as well as the additional space required, may more than offset this gain; and the fact that so little is done in the way of utilizing them would seem to show that up to the present time their value has failed to make any great impression upon engineers who have looked into the subject. It does not follow from this that they will never come into use on a more extensive scale than at present, but it does follow that the dreams of those who believe that they will eventually supersede all forms of prime movers that consume fuel will never be realized. Through the increased value of fuel or the reduced cost of construction of the apparatus, or both, they may become competitors to a greater or less extent, but more than this can not be expected.
Considering, now, the effects of the solution of the problem of obtaining electricity direct from coal, it can be said that it is far more likely to revolutionize the affairs of the world than the utilization of the natural forms of energy; but it must also be said that we are not justified, in view of what is now known in relation to the subject, in assuming that it will ever realize the predictions of the oversanguine prophets. If we could solve the problem according to our ideal, all that is expected of it would be accomplished; but such a solution is highly improbable, if not actually impossible. Our ideal battery would be as simple as a boiler, and be provided with a place where coal could be fed in and another through which the residue could be removed. In a boiler, the pressure of the steam, as well as the quantity generated, can be increased by simply increasing the size of the fire box, but this simplicity could not be obtained even in our ideal battery, because the electromotive force would remain the same no matter how much the size of the cell might be increased. To obtain an electromotive force high enough for practical purposes it would be necessary to use a large number of cells, and, to feed these without too much trouble, it would be necessary to devise an automatic feeder capable of operating with a degree of perfection hardly obtainable without the aid of human intelligence.
It may be permissible to dream of such perfection, but we are not justified in assuming that it is possible. Electricity can be obtained from chemical action only when the material acted upon is in the electric circuit. If two metals are placed in a solution that can decompose one of them, an electric current will flow in a wire the ends of which are attached to the two metals. If two solutions capable of acting upon each other are separated by a porous partition, and into each a plate attached to a wire is im-
