XIX. For the sake of illustrating the proposed analogy, it will be proper that I should here explain the conditions under which contractions are produced by the application of two frogs to each other, and compare them with the phænomena of the Leyden flasks. I shall therefore show, in a few words, the different methods in which several frogs are made to contract at the same time, and in which Leyden jars are discharged. Place on a glass plate two frogs, one of which has its spinal marrow armed, and let a communication be established between its muscles and the spinal marrow of another frog, by means of a small metallic chain. If the arc be formed from the armed spinal marrow of the one frog to the muscles of the other, contractions will be produced in both. Let us now apply to Leyden flasks the arrangement followed with the frogs. If two electric Leyden jars stand on a glass plate, when an arc is applied, some electricity will be elicited; but an absolute explosion will never be produced. If one extremity of a metallic wire, however, be brought in contact with the inside of one of the Leyden jars, and the other with the exterior coating of the other, on applying an arc to the other two coatings, which have no communication with the metallic wire that has been added, an explosion will take place, and both the jars will be discharged. But the above contractions may be produced in a manner still simpler. If the spinal marrow of one frog be united to the muscles of another, as soon as an arc is formed from the spinal marrow of the armed frog to the remote muscles of the other, strong contractions will be produced in both. The analogy between animal and artificial electricity, which is the object of our research, will always be apparent in this experiment. Two charged Leyden flasks, suspended in such a manner from the conductor of the machine that the exterior coating of the one is connected with the interior coating of the other, form a very happy representation of the frogs; for, the same arc being applied, and in the same manner, both to the frogs and the Leyden flasks, when an explosion is produced by the latter contractions will take place in the former.