shall be chosen? That which is clear and definite, and from which results can be deduced in a form which permits of their being tested by experiment.
If such an hypothesis be found, it then becomes necessary to ask, Does this hypothesis explain facts other than those included in the special residual phenomenon under consideration? An hypothesis which explains, or seems to explain, an isolated phenomenon, but which does not include other phenomena within its grasp, or which does not leap to the discovery of hitherto unknown facts, may be a true hypothesis, but it is certainly one which must be accepted with caution, and only provisionally until a better be found.
Finally, the new hypothesis must be in keeping with the well-established laws of nature. An hypothesis which contradicts any of these can not be accepted, although it may explain the special phenomenon to give a reason for which it has been called into existence.
The recent history of natural science furnishes many examples of the use of residual phenomena. Let me mention two only: one, in which an hypothesis has been suggested, proved, and adopted; another, in which the value of the hypothesis suggested is not yet finally determined.
It is well known that plants derive their support from the air and the soil; that support consists partly of mineral, partly of vegetable matter. But the curious fact was noticed that the leaves of certain plants frequently had adhering to them remains of insects or even entire insects. Following up this fact, Mr. Darwin and others have established the generalization that members of more than one species of plants derive their nourishment mainly from animal matter, and that these plants thrive better upon such food than upon the ordinary kinds of plant-food. Thus another thread has been added to the bond which visibly connects the animal and vegetable kingdoms.
The chemical elements have long been regarded as truly elementary bodies, that is, as bodies from which no form of matter other than themselves can be obtained. But phenomena presented by the spectra of certain of these elements seem almost inexplicable by the commonly accepted view. Mr. Lockyer has carefully examined many of the so called elementary spectra, at temperatures varying from that of a gas flame to that of the star Sirius, and, in order to explain the phenomena noticed, he has provisionally adopted the hypothesis that the so-called elements are really compound bodies. This hypothesis, whether eventually confirmed or refuted, suggests a large field for research to the chemist and to the physicist, from which neither can fail to reap most valuable results.
The observed residual phenomena of nature which yet await solution are many and varied; every branch of scientific work presents its own list. Let me glance at a few, and they shall be chiefly chosen from those phenomena which are investigated by the science of chemistry.
