Among the most constant circumstances under which we live is the force of gravity, which does not vary, except by a slight fraction of its amount, in any part of the earth’s crust or atmosphere to which we can attain. This force is sufficient to overbear and disguise various actions, for instance, the mutual gravitation of small bodies. It was an interesting experiment of Plateau to neutralise the action of gravity by placing substances in liquids of exactly the same specific gravity. Thus a quantity of oil poured into the middle of a suitable mixture of alcohol and water assumes a spherical shape; on being made to rotate it becomes spheroidal, and then successively separates into a ring and a group of spherules. Thus we have an illustration of the mode in which the planetary system may have been produced,[332] though the extreme difference of scale prevents our arguing with confidence from the experiment to the conditions of the nebular theory.
It is possible that the so-called elements are elementary only to us, because we are restricted to temperatures at which they are fixed. Lavoisier carefully defined an element as a substance which cannot be decomposed by any known means; but it seems almost certain that some series of elements, for instance Iodine, Bromine, and Chlorine, are really compounds of a simpler substance. We must look to the production of intensely high temperatures, yet quite beyond our means, for the decomposition of these so-called elements. Possibly in this age and part of the universe the dissipation of energy has so far proceeded that there are no sources of heat sufficiently intense to effect the decomposition.
Interference of Unsuspected Conditions.
It may happen that we are not aware of all the conditions under which our researches are made. Some substance may be present or some power may be in action, which escapes the most vigilant examination. Not being aware of its existence, we are unable to take proper measures to exclude it, and thus determine the share which it has in the results of our experiments. There can be no doubt that the alchemists were misled and encouraged in their vain attempts by the unsuspected presence of traces of gold and silver in the substances they proposed to transmute. Lead, as drawn from the smelting furnace, almost always contains some silver, and gold is associated with many other metals. Thus small quantities of noble metal would often appear as the result of experiment and raise delusive hopes.
In more than one case the unsuspected presence of common salt in the air has caused great trouble. In the early experiments on electrolysis it was found that when water was decomposed, an acid and an alkali were produced at the poles, together with oxygen and hydrogen. In the absence of any other explanation, some chemists rushed to the conclusion that electricity must have the power of generating acids and alkalies, and one chemist thought he had discovered a new substance called electric acid. But Davy proceeded to a systematic investigation of the circumstances, by varying the conditions. Changing the glass vessel for one of agate or gold, he found that far less alkali was produced; excluding impurities by the use of carefully distilled water, he found that the quantities of acid and alkali were still further diminished; and having thus obtained a clue to the cause, he completed the exclusion of impurities by avoiding contact with his fingers, and by placing the apparatus under an exhausted receiver, no acid or alkali being then detected. It would be difficult to meet with a more elegant case of the detection of a condition previously unsuspected.[333]
It is remarkable that the presence of common salt in the air, proved to exist by Davy, nevertheless continued a stumbling-block in the science of spectrum analysis, and probably prevented men, such as Brewster, Herschel, and Talbot, from anticipating by thirty years the discoveries of Bunsen and Kirchhoff. As I pointed out,[334] the utility of the spectrum was known in the middle of the last century to Thomas Melvill, a talented Scotch physicist, who died at the early age of 27 years.[335] But Melvill was struck in his examination of coloured flames by the extraordinary predominance of homogeneous yellow light, which was due to some circumstance escaping his attention. Wollaston and Fraunhofer were equally struck by the prominence of the yellow line in the spectrum of nearly every kind of light. Talbot expressly recommended the use of the prism for detecting the presence of substances by what we now call spectrum analysis, but he found that all substances, however different the light they yielded in other respects, were identical as regards the production of yellow light. Talbot knew that the salts of soda gave this coloured light, but in spite of Davy’s previous difficulties with salt in electrolysis, it did not occur to him to assert that where the light is, there sodium must be. He suggested water as the most likely source of the yellow light, because of its frequent presence; but even substances which were apparently devoid of water gave the same yellow light.[336] Brewster and Herschel both experimented upon flames almost at the same time as Talbot, and Herschel unequivocally enounced the principle of spectrum analysis.[337] Nevertheless Brewster, after numerous experiments attended with great trouble and disappointment, found that yellow light might be obtained from the combustion of almost any substance. It was not until 1856 that Swan discovered that an almost infinitesimal quantity of sodium chloride, say a millionth part of a grain, was sufficient to tinge a flame of a bright yellow colour. The universal diffusion of the salts of sodium, joined to this unique light-producing power, was thus shown to be the unsuspected condition which had destroyed the confidence of all previous experimenters in the use of the prism. Some references concerning the history of this curious point are given below.[338]
In the science of radiant heat, early inquirers were led to the conclusion that radiation proceeded only from the surface of a solid, or from a very small depth below it. But they happened to experiment upon surfaces covered by coats of varnish, which is highly athermanous or opaque to heat. Had they properly varied the character of the surface, using a highly diathermanous substance like rock salt, they would have obtained very different results.[339]
One of the most extraordinary instances of an erroneous opinion due to overlooking interfering agents is that concerning the increase of rainfall near to the earth’s surface. More than a century ago it was observed that rain-gauges placed upon church steeples, house tops, and other elevated places, gave considerably less rain than if they were on the ground, and it has been recently shown that the variation is most rapid in the close neighbourhood of the ground.[340] All kinds of theories have been started to explain this phenomenon; but I have shown[341] that it is simply due to the interference of wind, which deflects more or less rain from all the gauges which are exposed to it.
The great magnetic power of iron renders it a source of disturbance in magnetic experiments. In building a magnetic observatory great care must therefore be taken that no iron is employed in the construction, and that no masses of iron are near at hand. In some cases magnetic observations have been seriously disturbed by the existence of masses of iron ore in the neighbourhood. In Faraday’s experiments upon feebly magnetic or diamagnetic substances he took the greatest precautions against the presence of disturbing substances in the copper wire, wax, paper, and other articles used in suspending the test objects. It was his custom to try the effect of the magnet upon the apparatus in the absence of the object of experiment, and without this preliminary trial no confidence could be placed in the results.[342] Tyndall has also employed the same mode for testing the freedom of electro-magnetic coils from iron, and was thus enabled to obtain them devoid of any cause of disturbance.[343] It is worthy of notice that in the very infancy of the science of magnetism, the acute experimentalist Gilbert correctly accounted for the opinion existing in his day that magnets would attract silver, by pointing out that the silver contained iron.
Even when we are not aware by previous experience of the probable presence of a special disturbing agent, we ought not to assume the absence of unsuspected interference. If an experiment is of really high importance, so that any considerable branch of science rests upon it, we ought to try it again and again, in as varied conditions as possible. We should intentionally disturb the apparatus in various ways, so as if possible to hit by accident upon any weak point. Especially when our results are more regular than we have fair grounds for anticipating, ought we to suspect some peculiarity in the apparatus which causes it to measure some other phenomenon than that in question, just as Foucault’s pendulum almost always indicates the movement of the axes of its own elliptic path instead of the rotation of the globe.