According to the researches of M. Plateau, all the phenomena of the influence of vibrations on jets of liquid are referable to the conflict between the vibrations and the _forces of figure_ (“_forces figuratrices_”). If the physical fact is admitted—and it seems to be indisputable—that a liquid cylinder attains a _limit of stability_ when the proportion between its length and its diameter is in the ratio of twenty-two to seven, it is almost a _physical necessity_ that the jet should assume the constitution indicated by the observations of Savart. It likewise seems highly probable that a liquid jet, while in a transition stage to discontinuous drops, should be exceedingly sensitive to the influence of all kinds of vibrations. It must be confessed, however, that Plateau’s beautiful and coherent theory does not appear to embrace Savart’s last experiment, in which the musical tones were produced by a jet of water issuing under the surface of the same liquid. It is rather difficult to imagine what agency the “forces of figure” could have, under such circumstances, in the production of the phenomenon. This curious experiment tends to corroborate Savart’s original idea, that the vibrations which produce the sounds must take place in the glass reservoir itself, and that the cause must be inherent in the phenomenon of the flow. To apply the principles of Plateau’s theory to gaseous jets, we are compelled to abandon the idea of the _non-existence of molecular cohesion in gases_. But is there not abundant evidence to show that cohesion _does exist_ among the particles of gaseous masses? Does not the deviation from rigorous accuracy, both in the law of Mariotte and Gay-Lussac—especially in the case of condensable gases, as shown by the admirable experiments of M. Regnault—clearly prove that the hypothesis of the non-existence of cohesion in aëriform bodies is fallacious? Do not the expanding rings which ascend when a bubble of phosphuretted hydrogen takes fire in the air indicate the existence of some cohesive force in the gaseous product of combustion (aqueous vapor), whose outlines are marked by the opaque phosphoric acid? In short, does not the very _form_ of the flame of a “fish-tail” burner demonstrate that cohesion _must exist_ among the particles of the issuing gas? It is well known that in this burner the single jet which issues is formed by the union of _two oblique jets_ immediately before the gas is emitted. The result is a perpendicular _sheet of flame_. How is such a result produced by the mutual action of two jets, unless the force of cohesion is brought into play? Is it not obvious that such a fanlike flame must be produced by the same causes as those varied and beautiful forms of aqueous sheets, developed by the mutual action of jets of water, so strikingly exhibited in the experiments of Savart and of Magnus?