When several batteries are required, they should be disposed in the same order as if they all constituted one trough, (observing through the whole series to keep the zinc surfaces constantly opposed to the copper ones,) and connected together by some metallic substance, such as a piece of sheet lead, or tin-foil, about half the width of the trough. Batteries combined in this way should all be, as nearly as possible, of the same power. For if a bad battery be united to five good ones, each of the same number of plates, the effect of the whole will be equal only to six times that of the bad one—as in electrical batteries, if three jars of different sizes be charged together, the whole charge will be equal to only three times that of the smallest jar. CHAP. IV. _The Method of performing Galvanic Experiments with Frogs; with some conclusions drawn from them._ Every sensible heart must be shocked with the idea of torturing defenceless animals, merely to gratify an idle curiosity. The chapter which we shall now lay before the reader is founded entirely on the assertions of other writers upon this subject; to which, however, we have not the least doubt that the fullest credit is due. But we have not chosen to prove the veracity of their statements by our own experiments, believing that any small additional knowledge we might possibly have obtained in this way, would have been purchased at too great a price—the sacrifice of feeling and humanity. Take a living frog, and after amputating the hind legs, (for they are the best, on account of the number of joints) let the largest nerve, called the _crural nerve_, be laid bare, and surrounded with a slip of tin-foil, or a piece of sheet lead—then lay a piece of zinc, or other metal different from that on the nerve, in contact with the neighbouring muscles; form a communication by another piece of zinc, or other good conductor, between the metal in contact with the muscle and the _armed_ part of the nerve, and violent contractions will be produced in the limb. There is another method of producing these convulsions, which has been preferred on account of its simplicity.—It is by forming a communication between a nerve, armed as above, and an adjoining muscle, by a piece of zinc, without the assistance of a communicator.—This was one of the first methods of Galvanizing frogs, before the invention of the pile and trough. But since these discoveries, frogs have been made to show more violent convulsions. We now proceed to relate some of the conclusions which have been drawn from the experiments on frogs. 1. The contractions produced in the limb of a frog are stronger the farther the metal is placed from the origin of the nerve. 2. When the metal has remained for some time on a particular part of the nerve the motion will cease; but it may be renewed by changing the position of the metal, and carrying it lower on the nerve.