Page:Popular Science Monthly Volume 15.djvu/113

 fresh advances were quickly made in the knowledge of groups of compound bodies.

But there is another way in which the investigation of residual phenomena may aid, and has largely aided, the advance of scientific knowledge.

Phenomena, regarded as residual, have not unfrequently been shown to be completely explicable in terms of a known law; and thus fresh light has been thrown upon the modifying influence exerted on the action of the law by the conditions under which the law acts.

The orbit of Lexell's comet was accurately determined; nevertheless, the comet failed to appear at the proper time. Here, surely, was a phenomenon which could not be explained by the law of gravitation alone: hypotheses, plausible and probable in themselves, were broached to account for the apparently exceptional phenomenon. But subsequent investigation showed that that appearance of the comet, from observations of which the orbit had been calculated, was due to the disturbing influence of one of the members of the solar system (probably of Jupiter) whereby the comet had been dragged within the limits of our vision, but that this visit to earthly spheres was altogether abnormal: the phenomenon presented by the visit of the comet was entirely explicable in terms of the law of gravitation.

What could be more opposed to our ordinary notions concerning the effects of heat than the fact that water should be frozen in a red hot vessel? But this phenomenon, apparently inexplicable in terms of any known law, upon exact investigation finds demonstrable explanation without recourse being had to the action of an unknown agent. The experiment is carried out by pouring liquid sulphur dioxide—a liquid which boils at a temperature lower than that of the freezing point of water—into a red-hot platinum crucible, immediately adding a little water, and quickly turning out the ice which is produced.

Experiment shows that when a liquid is suddenly brought into contact with a highly heated smooth surface, vapor is evolved which surrounds the mass of liquid as it were with a screen through which the heat, radiated from the hot surface underneath, passes but slowly; the liquid thus rests upon a cushion of its own vapor, and does not touch the hot surface beneath. The temperature of a mass of liquid in this (spheroidal) condition is lower than that at which the liquid boils. Now, as liquid sulphur dioxide boils at a temperature lower than that at which water freezes, and as immediately the liquid touches the heated platinum crucible it is partially vaporized, and the residual liquid is then floated, so to speak, upon the stratum of gas so produced, it follows that, so long as this condition is maintained, the liquid contents of the crucible are at a very low temperature; hence the temperature of the water coming into contact with this cold liquid is greatly reduced, and the water is frozen.

Exact investigation of this phenomenon, therefore, adds much to