the mineralogist usually employ mere observation when they examine animals, plants, and minerals, as they are met with in their natural condition.

In experiment, on the contrary, we vary at our will the combinations of things and circumstances, and then observe the result. It is thus that the chemist discovers the composition of water by using an electric current to separate its two constituents, oxygen and hydrogen. The mineralogist may employ experiment when he melts two or three substances together to ascertain how a particular mineral may have been produced. Even the botanist and zoologist are not confined to passive observation; for by removing animals or plants to different climates and different soils, and by what is called domestication, they may try how far the natural forms and species are capable of alteration.

It is obvious that experiment is the most potent and direct mode of obtaining facts where it can be applied. We might have to wait years or centuries to meet accidentally with facts which we can readily produce at any moment in a laboratory; and it is probable that most of the chemical substances now known, and many excessively useful products, would never have been discovered at all by waiting till nature presented them spontaneously to our observation. Many forces and changes too may go on in nature constantly, but in so slight a degree as to escape our senses, and render some experimental means necessary for their detection. Electricity doubtless operates in every particle of matter, perhaps at every moment of time; and even the ancients could not but notice its action in the loadstone, in lightning, in the Aurora Borealis, or in a piece of rubbed amber (electrum). But in lightning electricity was too intense and dangerous; in the other cases it was too feeble to be properly understood. The science of electricity and magnetism could

only advance by getting regular supplies of electricity from the common electric machine or the galvanic battery, and by making powerful electro-magnets. Most if not all the effects which electricity produces must go on in nature, but altogether too obscurely for observation.

Experiment, again, is rendered indispensable by the fact that on the surface of the earth we usually meet substances under certain uniform conditions, so that we could never learn by observation what would be the nature of such substances under other conditions. Thus carbonic acid is only met in the form of a gas, proceeding from the combustion of carbon; but when exposed to extreme pressure and cold, it is condensed into a liquid, and may even be converted into a snow-like solid substance. Many other gases have in like manner been liquefied or solidified; and there is reason to believe that every substance is capable of taking all the three forms of solid, liquid and gas, if only the conditions of temperature and pressure can be sufficiently varied. Mere observation of nature would have led us, on the contrary, to suppose that nearly all substances were fixed in one condition only, and could not be converted from solid into liquid and from liquid into gas.

It must not be supposed however that we can draw any precise line between observation and experiment, and say where the one ends and the other begins. The difference is rather one of degree than of kind; and all we can say is that the more we vary the conditions artificially the more we employ experiment. I have said that meteorology is a science of nearly pure observation, but if we purposely ascend mountains to observe the rarefaction and cooling of the atmosphere by elevation, or if we make balloon ascents for the same purpose, like Gay Lussac and Glaisher, we so vary the mode of observation as almost to render it experimental. Astronomers again

may almost be said to experiment instead of merely observing when they simultaneously employ instruments as far to the north, and as far to the south, upon the earth's surface as possible, in order to observe the apparent difference of place of Venus when crossing the sun in a transit, so as thus to compare the distances of Venus and the sun with the dimensions of the earth.

Sir John Herschel has excellently described the difference in question in his Discourse on the Study of Natural Philosophy*. "Essentially they are much alike, and differ rather in degree than in kind; so that perhaps the terms passive and active observation might better express their distinction; but it is, nevertheless, highly important to mark the different states of mind in inquiries carried on by their respective aids, as well as their different effects in promoting the progress of science. In the former, we sit still and listen to a tale, told us, perhaps obscurely, piecemeal, and at long intervals of time, with our attention more or less awake. It is only by after rumination that we gather its full import; and often, when the opportunity is gone by, we have to regret that our attention was not more particularly directed to some point which, at the time, appeared of little moment, but of which we at length appreciate the importance. In the latter, on the other hand, we cross-examine our witness, and by comparing one part of his evidence with the other, while he is yet before us, and reasoning upon it in his presence, are enabled to put pointed and searching questions, the answer to which may at once enable us to make up our minds. Accordingly it has been found invariably, that in those departments of physics where the phenomena are beyond our control, or into which experimental enquiry, from other causes, has not been carried, the pro

gress of knowledge has been slow, uncertain and irregular; while in such as admit of experiment, and in which mankind have agreed to its adoption, it has been rapid, sure, and steady."

Not uncommonly, however, nature has, so to speak, made experiments upon a scale and for a duration with which we cannot possibly compete. Thus we do not need to try the soil and situation which suits any given plant best; we have but to look about and notice the habitat or situation in which it is naturally found in the most flourishing condition, and that, we may be sure, indicates the result of ages of natural experiment. The distances of the fixed stars would probably have been for ever unknown to us did not the earth by describing an orbit with a diameter of 182,000,000 miles make a sort of experimental base for observation, so that we can see the stars in very slightly altered positions, and thus judge their distances compared with the earth's orbit. Eclipses, transits, occultations and remarkable conjunctures of the planets, are also kinds of natural experiments which have often been recorded in early times, and thus afford data of the utmost value.

Logic can give little or no aid in making an acute or accurate observer. There are no definite rules which can be laid down upon the subject. To observe well is an art which can only be acquired by practice and training; and it is one of the greatest advantages of the pursuit of the Natural Sciences that the faculty of clear and steady observation is thereby cultivated. Logic can however give us this caution, which has been well pointed out by Mr Mill-to discriminate accurately between what we really do observe and what we only infer from the facts observed. So long as we only record and describe what our senses

• See Lockyer's Elementary Lessons in Astronomy, Nos. XLVI, XLVII.

have actually witnessed, we cannot commit an error; but the moment we presume or infer anything we are liable to mistake. For instance, we examine the sun's surface with a telescope and observe that it is intensely bright except where there are small breaks or circular openings in the surface with a dark interior. We are irresistibly led to the conclusion that the inside of the sun is colder and darker than the outside, and record as a fact that we saw the dark interior of the sun through certain openings in its luminous atmosphere. Such a record, however, would involve mistaken inference, for we saw nothing but dark spots, and we should not have done more in observation than record the shape, size, appearance and change of such spots. Whether they are dark clouds above the luminous surface, glimpses of the dark interior, or, as is now almost certainly inferred, something entirely different from either, can only be proved by a comparison of many unprejudiced observations.

The reader cannot too often bear in mind the caution against confusing facts observed with inferences from those facts. It is not too much to say that nine-tenths of what we seem to see and hear is inferred, not really felt. Every sense possesses what are called acquired perceptions, that is, the power of judging unconsciously, by long experience, of many things which cannot be the objects of direct perception. The eye cannot see distance, yet we constantly imagine and say that we see things at such and such distances, unconscious that it is the result of judgment. As Mr Mill remarks, it is too much to say "I saw my brother." All I positively know is that I saw some one who closely resembled my brother as far as could be observed. It is by judgment only I can assert he was my brother, and that judgment may possibly be wrong.

Nothing is more important in observation and experi