vol.

that A is the cause of X, I must maintain invariable all surrounding objects and conditions, and I must then show that where A is X is, and where A is not X is not. Now this cannot really be accomplished in a single trial. If, for instance, a chemist places a certain suspected substance in the Marsh's test apparatus, and finds that it gives a small deposit of metallic arsenic, he cannot be sure that the arsenic really proceeded from the suspected substance; for the impurity of the zinc or sulphuric acid might 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 imply and assume that if A were absent X would be absent. But wherever it is possible, we ought clearly not to leave 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 with care 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 aparatus, in the absence of the substance to be exmented on, he really made a blind experiment (see 1.41).

t also, whenever we can, to test the sufficiency f any method of experiment by introducing of the substance or force to be detected. cal process for the quantitative estishould 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 m. 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, and, we might add, known not to exist".

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 an amount inappreciable to our senses, or confounded with other effects of much greater amount. As in fact there is no limit on à priori grounds to the smallness of a phenomenon, we can never, on the grounds of 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

m Watts, 'Dictionary of Chemistry,' vol. ii. pp. 936, 937.

n Discovery of Subterraneal Treasure,' London, 1639, p. 48.

million times greater than that of light; 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 space, or the adamantine basis of light; but he ascertained by one of the most beautiful experiments with the pendulum, elsewhere more fully described (vol. ii. p. 55), that if such resistance existed, it was in amount less than one five-thousandth part of the external resistance of the airp.

Innumerable incidents in the history of science tend to show that phenomena, which one generation has failed to detect, may become accurately known to a succeeding generation. The compressibility of water which the Academicians of Florence could not prove, because at a low pressure the effect was too small to perceive, and at a High pressure the water oozed through their silver vessel, snow become the subject of exact measurements and

ise calculation. Independently of Newton, Hooke ertained very remarkable notions concerning the nature of gravitation. In this and other subjects he showed, indeed a genius for experimental investigation which would have placed him in the first rank in any other age than that of Newton. He correctly conceived that the force of y would decrease as we receded from the centre of th, and he boldly attempted to prove it by experiFaxing exactly counterpoised two weights in the of a balance, or rather one weight against another and a long piece of fine cord, he removed his

balance to the top of the Dome of St. Paul's, and tried whether the balance remained in equilibrium after one weight was allowed to hang down to a depth of 240 feet. No difference could be perceived when the weights were at the same and at different levels, but Hooke rightly held that the failure arose from the insufficient difference of height. He says, 'Yet I am apt to think some difference might be discovered in greater heights". The radius of the earth being about 20,922,000 feet, we can now readily calculate from the known law of gravity that a height of 240 would not make a greater difference than one part in 40,000 of the weight. Such a difference would doubtless be inappreciable in the balances of that day, though it could readily be detected by balances now frequently constructed. Again, the mutual gravitation of bodies at the earth's surface is so small that Newton appears to have made no attempts to demonstrate its existence experimentally, merely remarking that it was too small to fall under the observation of our senses. It has since been successfully detected and measured by Cavendish, Baily and others.

The smallness of the quantities which we can now observe is often very astonishing. A balance will weigh to one millionth part of the load or less. Sir Joseph Whitworth can measure to the one millionth part of an inch. A rise of temperature of the 88ooth part of a degree centigrade has been detected by Dr. Joule. The spectroscope can reveal the presence of the one 180,000,000th part of a grain of soda, and the sense of smell can probably feel the presence of a far less quantity of odorous matter t. We must nevertheless remember that effects of indefinitely

t Keill's Introduction to Natural Philosophy.' 3rd ed., London, 1733, pp. 48-54.

less amount than these must exist, and we should state our negative result with corresponding caution. We can only disprove the existence of a quantitative phenomenon by showing deductively, from the laws of nature, that if present it would amount to a perceptible quantity. As in the case of other negative arguments (vol. ii. p. 19) we must demonstrate that the effect would appear, where it is by experiment found not to appear.

Limits of Experiment.

It will be obvious that there are many operations of nature which we are quite incapable of imitating in our experiments. Our object is to study the conditions under which a certain effect is produced; but one of those conditions may involve a great length of time. There are instances on record of experiments extending over five or ten years, and even over a large part of a lifetime; but such intervals of time are almost nothing to the time during which nature may have been at work. The contents of a mineral vein in Cornwall may have been undergoing gradual change for a million years or more. All metamorphic rocks have doubtless endured high temperature and enormous pressure for almost inconceivable periods of time, so that chemical geology is generally

Arguments have been continually brought against Darwin's theory, founded upon the absence of any clear instance of the production of a new species. During an historical period of perhaps four thousand years, no animal, it is said, has been so much domesticated as to become different in species. It might as well be argued, as it seems to me, that no geological changes are taking as because no new mountain has risen in Great Britain Our actual experience of