It was in this cautious spirit that Baily acted in his experiments on the density of the earth. The accuracy of his results depended upon the elimination of all disturbing influences, so that the oscillation of his torsion balance should measure gravity alone. Hence he varied the apparatus in many ways, changing the small balls subject to attraction, changing the connecting rod, and the means of suspension. He observed the effect of disturbances, such as the presence of visitors, the occurrence of violent storms, &c., and as no real alteration was produced in the results, he confidently attributed them to gravity.[344]
Newton would probably have discovered the mode of constructing achromatic lenses, but for the unsuspected effect of some sugar of lead which he is supposed to have dissolved in the water of a prism. He tried, by means of a glass prism combined with a water prism, to produce dispersion of light without refraction, and if he had succeeded there would have been an obvious mode of producing refraction without dispersion. His failure is attributed to his adding lead acetate to the water for the purpose of increasing its refractive power, the lead having a high dispersive power which frustrated his purpose.[345] Judging from Newton’s remarks, in the Philosophical Transactions, it would appear as if he had not, without many unsuccessful trials, despaired of the construction of achromatic glasses.[346]
The Academicians of Cimento, in their early and ingenious experiments upon the vacuum, were often misled by the mechanical imperfections of their apparatus. They concluded that the air had nothing to do with the production of sounds, evidently because their vacuum was not sufficiently perfect. Otto von Guericke fell into a like mistake in the use of his newly-constructed air-pump, doubtless from the unsuspected presence of air sufficiently dense to convey the sound of the bell.
It is hardly requisite to point out that the doctrine of spontaneous generation is due to the unsuspected presence of germs, even after the most careful efforts to exclude them, and in the case of many diseases, both of animals and plants, germs which we have no means as yet of detecting are doubtless the active cause. It has long been a subject of dispute, again, whether the plants which spring from newly turned land grow from seeds long buried in that land, or from seeds brought by the wind. Argument is unphilosophical when direct trial can readily be applied; for by turning up some old ground, and covering a portion of it with a glass case, the conveyance of seeds by the wind can be entirely prevented, and if the same plants appear within and without the case, it will become clear that the seeds are in the earth. By gross oversight some experimenters have thought before now that crops of rye had sprung up where oats had been sown.
Blind or Test Experiments.
Every conclusive experiment necessarily consists in the comparison of results between two different combinations of circumstances. To give a fair probability that A is the cause of X, we must maintain invariable all surrounding objects and conditions, and we must then show that where A is X is, and where A is not X is not. This cannot really be accomplished in a single trial. If, for instance, a chemist places a certain suspected substance in Marsh’s test apparatus, and finds that it gives a small deposit of metallic arsenic, he cannot be sure that the arsenic really proceeds from the suspected substance; the impurity of the zinc or sulphuric acid may have been the cause of its appearance. It is therefore the practice of chemists to make what they call a blind experiment, that is to try whether arsenic appears in the absence of the suspected substance. The same precaution ought to be taken in all important analytical operations. Indeed, it is not merely a precaution, it is an essential part of any experiment. If the blind trial be not made, the chemist merely assumes that he knows what would happen. Whenever we assert that because A and X are found together A is the cause of X, we assume that if A were absent X would be absent. But wherever it is possible, we ought not to take this as a mere assumption, or even as a matter of inference. Experience is ultimately the basis of all our inferences, but if we can bring immediate experience to bear upon the point in question we should not trust to anything more remote and liable to error. When Faraday examined the magnetic properties of the bearing apparatus, in the absence of the substance to be experimented on, he really made a blind experiment (p. [431]).
We ought, also, to test the accuracy of a method of experiment whenever we can, by introducing known amounts of the substance or force to be detected. A new analytical process for the quantitative estimation of an element should be tested by performing it upon a mixture compounded so as to contain a known quantity of that element. The accuracy of the gold assay process greatly depends upon the precaution of assaying alloys of gold of exactly known composition.[347] Gabriel Plattes’ works give evidence of much scientific spirit, and when discussing the supposed merits of the divining rod for the discovery of subterranean treasure, he sensibly suggests that the rod should be tried in places where veins of metal are known to exist.[348]
Negative Results of Experiment.
When we pay proper regard to the imperfection of all measuring instruments and the possible minuteness of effects, we shall see much reason for interpreting with caution the negative results of experiments. We may fail to discover the existence of an expected effect, not because that effect is really non-existent, but because it is of a magnitude inappreciable to our senses, or confounded with other effects of much greater amount. As there is no limit on à priori grounds to the smallness of a phenomenon, we can never, by a single experiment, prove the non-existence of a supposed effect. We are always at liberty to assume that a certain amount of effect might have been detected by greater delicacy of measurement. We cannot safely affirm that the moon has no atmosphere at all. We may doubtless show that the atmosphere, if present, is less dense than the air in the so-called vacuum of an air-pump, as did Du Sejour. It is equally impossible to prove that gravity occupies no time in transmission. Laplace indeed ascertained that the velocity of propagation of the influence was at least fifty million times greater than that of light;[349] but it does not really follow that it is instantaneous; and were there any means of detecting the action of one star upon another exceedingly distant star, we might possibly find an appreciable interval occupied in the transmission of the gravitating impulse. Newton could not demonstrate the absence of all resistance to matter moving through empty space; but he ascertained by an experiment with the pendulum (p. [443]), that if such resistance existed, it was in amount less than one five-thousandth part of the external resistance of the air.[350]
A curious instance of false negative inference is furnished by experiments on light. Euler rejected the corpuscular theory on the ground that particles of matter moving with the immense velocity of light would possess momentum, of which there was no evidence. Bennet had attempted to detect the momentum of light by concentrating the rays of the sun upon a delicately balanced body. Observing no result, it was considered to be proved that light had no momentum. Mr. Crookes, however, having suspended thin vanes, blacked on one side, in a nearly vacuous globe, found that they move under the influence of light. It is now allowed that this effect can be explained in accordance with the undulatory theory of light, and the molecular theory of gases. It comes to this—that Bennet failed to detect an effect which he might have detected with a better method of experimenting; but if he had found it, the phenomenon would have confirmed, not the corpuscular theory of light, as was expected, but the rival undulatory theory. The conclusion drawn from Bennet’s experiment was falsely drawn, but it was nevertheless true in matter.