# Popular Science Monthly/Volume 15/May 1879/Residual Phenomena

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* RESIDUAL PHENOMENA.
By Professor PATTISON MUIR.
IN his "Preliminary Discourse on the Study of Natural Philosophy," Sir John Herschel remarks upon the importance of examining those phenomena of nature which are not wholly explicable in terms of any well-established theory. Instances of such residual phenomena, as Sir John Herschel terms them, are given in the discourse.
Newton's theory of comets, viz., that these bodies obey the law of gravitation while revolving in oblique orbits round the sun, appeared to account for the facts which had been noticed concerning the comet of Halley; but the period calculated for Encke's comet, on this hypothesis, was found to be rather longer than the actual, observed period, and, moreover, the duration of the observed period showed a small but regular diminution. Hence, Newton's theory, taken alone, was not sufficient to account for the facts. But, inasmuch as Newton's law of gravitation rested upon a sure and well-established foundation, the fact observed concerning Encke's comet could not be regarded as disproving the law; hence these facts were to be explained by tracing them to the action of some agent either of known or of, as yet, unknown nature.
The regularly diminishing period of Encke's comet remained a residual phenomenon, not contradicting the law of gravitation, but awaiting full explanation.
A residual phenomenon is, then, a phenomenon which is not fully explained by any established theory; but at the same time it is not a phenomenon which is absolutely contradictory to any such theory, for, if this were the case, the theory in question must perforce be abandoned.
Advances are made in natural science by a judicious use of hypotheses. Facts are accurately observed, or are gained by exact experiment, and are compared with facts; inferences are drawn, and are compared with other inferences, until a good working hypothesis is attained. From this hypothesis deductions are made which must necessarily prove true if the hypothesis be correct; the truth or falsity of the alleged facts is tested by an appeal to Nature; and so wider hypotheses are gained, each in turn being tested and tried by an appeal to facts, until, finally, that generalization is reached which includes in its expression so many and so varied phenomena that to it is given the name of a "law of Nature."
But notwithstanding the sure and tried foundations upon which each law of nature rests, phenomena ever and anon become apparent which refuse to be completely explained by any of these laws. Upon more careful examination, it may be found that such phenomena have been erroneously observed, and they may be brought under the application ​of a known law, acting perhaps in a peculiar and even unprecedented manner. In such cases the phenomena cease to be residual phenomena.
But, on the other hand, some of the observed phenomena may resist every attempt made to explain them; they may refuse to retire from the list of established facts, and at the same time refuse to find their full explanation in terms of any well-established law. But, while so doing, these phenomena may also not be opposed to the law; they may not be contradictory to, but simply not wholly explainable by, any known law of nature.
Instances of the valuable results which have been obtained by the exact investigation of residual phenomena are numerous in every branch of natural science. One of the most striking is furnished by Newton's investigation of the atmospheric velocity of sound.
Newton showed that the velocity of sound in air might be calculated from certain theoretical considerations; a rough measurement of the actual velocity gave him a number differing very considerably from that which his theory required. Later and more exact experiments failed to explain the discrepancy, but in 1816 Laplace gave an explanation of the seemingly exceptional phenomenon, which not only sustained the theory of Newton, but also paved the way to the modern doctrine of the equivalency of heat and mechanical work. In the residual phenomenon which was left unexplained by Newton lay the germ of one of the greatest advances made by science in recent years.
Another striking instance of the value of residual phenomena is to be found in the history of chemical science.
From his experiments upon combustion, Lavoisier concluded that the peculiar properties of acids are due to the presence of the element oxygen in these bodies. But an undoubtedly acid substance was known (muriatic acid) from which no oxygen could be obtained. Here was a residual phenomenon—a phenomenon not absolutely contradictory of the law, that that group of bodies called acids is characterized by the presence of oxygen, but certainly a phenomenon demanding accurate investigation. Closer examination might have shown that the acid supposed to contain no oxygen was not really free from that element, or it might have led to the adoption of a higher generalization concerning the nature of the group "acids," or, lastly, it might have necessitated an entire alteration in the terms of Lavoisier's so-called law.
Chemists, however, for many years contented themselves with asserting that, as Lavoisier had pronounced oxygen to be the acidifying principle, and as muriatic acid was undoubtedly a true acid, this body must contain oxygen. But Sir Humphry Davy showed that an accurate examination of the residual phenomenon presented by muriatic acid led to a more extended and more exact knowledge of the nature of acids, and necessitated a change in the prevalent views concerning these bodies. The views of Lavoisier were found to express a truth, but not the whole truth; fresh incitement was given to research, and ​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 ​our knowledge of the laws which govern the vaporization of liquids, and shows us these laws at work under peculiar conditions, while at the same time it brings the apparently exceptional phenomenon under the domain of a known law. Once more, the examination of residual phenomena may be, and has often been, of immense service to science, in freeing naturalists from the tyranny of an established theory which has for long been regarded as of necessity affording a full explanation of the entire series of facts to which it is applied.
The tyranny of orthodoxy is not unknown in science. The overthrow of that tyranny is one result of the investigation of residual phenomena.
During the greater part of the eighteenth century the theory of Phlogiston was all-prevalent in chemistry. According to this theory, when a body burns, it gives out a something called Phlogiston, the escape of this mystical something being the cause of the phenomena which attend the combustion.
This theory accounted in a fairly satisfactory manner for the greater number of the observed facts. One little fact, however, was scarcely explicable by the Phlogistic theory. So far as rough measurement went, the weight of the burned body appeared to be greater than that of the body previous to combustion. This residual fact was long overlooked, but the genius of Lavoisier forbade him to pass over so important a circumstance. By repeated and exact experiment, Lavoisier established the correctness of the residual phenomenon, and he showed that the phenomenon was inexplicable in terms of the commonly accepted theory.
Modern research has taught us that the fact firmly established by Lavoisier is not absolutely contradictory of a modified Phlogistic theory; but Lavoisier's work necessitated a thorough revisal of the prevalent theory of combustion, and prepared the way for great advances which have at last enabled us to reconcile his theory with that of the Phlogisteans in modified form. Had Lavoisier consented to overlook the seemingly little fact that a body after burning is heavier than it was before, chemical science would probably have been for many years compelled to submit to the thralldom of the Phlogistic theory, which, in its then accepted form, barred the path of true advance.
When Galileo's telescope discovered to the gaze of the astronomer the satellites of Jupiter, did not those in authority protest most vehemently against the residual phenomenon? Why? Because they saw that this phenomenon could not be made to fit into the accepted cosmical theories of the day: not only was it inexplicable in terms of these theories, but it was absolutely opposed to them. Galileo, however, persisted, the phenomenon was more fully investigated, and the science of astronomy was placed upon a sure basis; the reign of mere authority in scientific matters was brought to an end, and Nature was installed as the supreme adjudicator in all matters of scientific inquiry.
​But the examination of residual phenomena may also help to free investigators from that tyranny which is exerted by a number of concordant results, all seemingly pointing to but one conclusion.
If experiment after experiment points to one conclusion, and if all, with the exception of perhaps a single residual fact, is in favor of this conclusion, it is hard to resist the temptation to ignore that fact, and adopt what, but for it, is apparently the true conclusion. But this method is not the scientific method. The fact must be examined. It may be that the outstanding fact is finally reduced within the sphere of the previously adopted hypothesis, or it may be that a new hypothesis is suggested which explains this and all the other phenomena.
The great Swedish chemist Berzelius carefully examined the properties of the compounds of a newly discovered element; he determined the chemical and physical characteristics of this element, to which he gave the name of Vanadium. The facts ascertained by the experiments of Berzelius formed a concordant series; so far as these experiments extended, everything appeared to be in keeping with the conclusions arrived at by him. But it was afterward noticed that the crystalline form of certain compounds of the metal vanadium was different from that required by the commonly accepted and, as it appeared, well-established theories concerning the connection between crystalline form and chemical structure. The examination, by Roscoe, of the residual phenomena presented by the crystalline forms of the vanadium compounds led to the astonishing discovery that the so-called metallic vanadium of Berzelius was really not an elementary body, but a compound of the true metal vanadium with oxygen. This peculiar oxide presents most of the physical properties of a metal; indeed, so metal-like is this oxide that the presence in it of oxygen was entirely overlooked, even by so careful a worker as Berzelius.
The researches of Roscoe threw a new light upon the chemical history of vanadium, and at the same time confirmed in a marked manner the law connecting chemical structure with crystalline form.
But, lastly, the study of residual phenomena may aid in freeing our minds from that fascinating, but surely erroneous, idea which a mere superficial acquaintance with natural science tends so much to strengthen, viz., that Nature is, and indeed must be, extremely simple.
The simplicity of Nature is a favorite theme with a certain class of would-be philosophers: it is a doctrine easily accepted, but a doctrine which has led to pernicious results.
Extreme instances of the overruling power of this idea may be found in the fascination exerted over minds, even of the highest order, by numerical analogies, that are really baseless. The seven colors of the spectrum were supposed, even by the great master himself, to have some mysterious connection with the seven tones of music. The number of the satellites of Jupiter added to the single satellite of the earth leaves but one satellite for Saturn, if the perfect number six is to be ​made up; hence Huygens concluded that Saturn could have but one satellite.
When chemistry emerged as a distinct branch of science from the superstitions and conceits which had so long overshadowed her, the line of demarkation between chemical and mechanical action was made clear and unmistakable. On this side were ranged all phenomena purely mechanical; on that, all phenomena purely chemical. Nature's laws must be simple. One great fact was predicated of each class of phenomena—the distinction was a simple distinction. But as Nature's facts were more thoroughly searched into, phenomena were remarked which tended to discredit the extreme simplicity of the division into chemical and mechanical actions; those phenomena were passed by as too trivial for serious notice. But the residual phenomena at last forced themselves upon the attention of chemists; and one great result of the examination of these phenomena has been the discovery that the simple classification into chemical phenomena on that side and mechanical on this was too simple—was, in fact, an artificial classification; that there is no sharp line of demarkation in Nature, but that a series of facts exists which bridges over the gulf formerly supposed to be fixed between the two sets of phenomena.
The earlier study of biological science tended to show a great simplicity in the vital processes occurring among all living things; but the more advanced study of the same science has altogether overthrown the simplicity of the earlier scheme. Certain animals, and classes of animals, seem deliberately to adopt strange expedients for reproducing their kind, as if to warn us against such hasty generalizations. How should we have imagined the possibility of fertilization for successive generations, of hermaphroditism, or of reproduction by fissure, etc., being found among the methods which Nature adopts for replenishing the earth, had we contented ourselves with an examination of the comparatively simple methods of ordinary sexual reproduction?
The importance of residual phenomena is undoubtedly great; the difficulties which attend the study of these phenomena are likewise great.
A phenomenon, supposed to be residual, may be found on closer examination to be fully explained by some known law, acting either under ordinary or under modified conditions. Before, therefore, attempting to find a new hypothesis which shall explain the residual phenomenon, it is necessary to determine the fact of the phenomenon being truly residual. Of course, if an explanation be found for the seemingly inexplicable phenomenon without the necessity of introducing a new hypothesis, a distinct step has been made in scientific advance. If, however, the phenomenon refuse to be explained by any known law, a new hypothesis must be found, or the old must be modified so as to admit of an explanation being given for the hitherto inexplicable fact.
Of the new hypotheses which present themselves to the mind, which ​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.
​That the molecules of the elements, i. e., the smallest individual parts which exhibit the properties of the elements, consist of yet smaller parts, or atoms, is undoubted. The generalization holds, with few exceptions, that the elementary molecules contain each two atoms. The exceptions are exhibited by the elements phosphorus, arsenic, cadmium, and mercury, the two former being possessed of molecular weights four times as great as their atomic weights, while the molecular weights of the two latter are equal to their atomic weights. No conclusive explanation has as yet been given of this fact; it remains a true residual phenomenon.
Again, the atoms of the elements are possessed each of a certain definite binding power. Each is capable of uniting with a fixed maximum number of other atoms, but this binding power is not always completely exercised. Why does this power vary? How is its action modified by the conditions under which it is exercised? Can the known facts concerning the action of this binding power, or valency as it is called, be brought within the scope of any definite and workable hypothesis? These questions are to be solved by the researches of the chemists of the future.
Once more, the properties of certain elements vary considerably with variations in the conditions of those elements. Oxygen, when exposed to the action of the electric discharge, is not split up into any form of matter other than itself, nor does it combine with any other form of matter, nevertheless its properties are largely modified. The molecular weight of ozone—the new form of oxygen produced by the action of the electric discharge—is known to be one and a half time greater than that of ordinary oxygen. But, nevertheless, no complete explanation of the facts, of which this special fact is a representative, has yet been given. Allotropy remains a residual phenomenon in chemical science.
Many animal instincts, e. g., the curious instinct which prompts the cuckoo to lay a single egg in a nest not her own, connected as this instinct undoubtedly is with the similar but less perfectly developed instinct of the American Molothrus bonariensis, have not as yet been completely brought within the sphere of any wide generalization.
Why should the use of its sting inflict injury, if not death, upon the bee?
Why do variations in structure or function arise suddenly in various animals?
These questions, and many questions similar to these, await their full explanation.
Science advances by slow but sure steps; she carefully propounds hypotheses, and carefully marks off those phenomena which these hypotheses leave unexplained. She is aware that the phenomena occurring in that immense sphere assigned to her are not always to be explained by one, but often by many hypotheses. Phenomenon is ​modified by phenomenon. Law reacts upon law. All she knows is lawful, but all is not yet intelligible. With patience and sure faith she advances to the goal; the road is long, but the reward is great.—Fraser's Magazine.