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tically. In this case the object is to place them in that order in which each substance can have only one proper place, and in which it can be most easily found. Most plans of arranging them have the disadvantage of affording several places in the series in which a single substance might, with nearly equal consistency, be placed, and therefore making it uncertain in which of those it may be found; but an order, similar in principle to that adopted in the inorganic portion of the celebrated work on chemistry, by Leopold Gmelin, obviates that disadvantage. It is more difficult to classify bodies in the concrete sciences than in the simple ones, because each mode of classification is of nearly equal scientific importance.

As classification is of such great value, the completion of the various tables of constants in each of the physical sciences and chemistry would be an important advance. An immense amount of original investigation remains to be made in this direction. The subject of classification is ably treated in Whewell's Philosophy of the Inductive Sciences,' Book VIII. p. 449.

CHAPTER XXIV.

DIFFICULTIES OF SCIENTIFIC RESEARCH.

NOTWITHSTANDING the vast amount of knowledge which remains to be discovered, it is extremely difficult to find a scientific truth which is both new and important. One great difficulty consists in selecting a good subject for investigation. Many researches are difficult in consequence of their abstruseness, and others on account of

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their complexity; and many also in consequence of the number of interfering circumstances which cannot be excluded; the latter is particularly the case in experiments upon living structures. Many are difficult in consequence of extraneous circumstances, such as rarity of the requisite substances or conditions necessary for the experiments or observations; some through slowness of process, or great length of time required; some owing to great magnitude or cost; others in consequence of minuteness of the substances, or feebleness of the actions, the want of powerful and delicate instruments, or tests, &c.

The difficulties of research arise largely from our ignorance. Many of the phenomena we are acquainted with and which seem rare, are probably only the more conspicuous instances of common effects. A vast number of phenomena continually before us are unnoticed, owing to their excessive smallness of amount, extreme slowness of action, &c. We are as yet acquainted only with the comparatively common and more conspicuous phenomena of matter, the structures and actions of masses; and are almost entirely unacquainted with the actual sizes, forms, positions, weights, and directions of movement, of the molecules of the substances we use; and as the phenomena of masses are but collective results of those of the molecules, and as those molecules are all of them often profoundly disturbed by every change of mechanical force, light, heat, electricity, magnetism, or chemical power, and some of these forces are always acting upon them, it is evident we continually operate in almost complete ignorance of some of the most fundamental conditions of our experiments.

We occupy, as human beings on this globe, the position of creatures of almost infinitely feeble power, helplessly subject to the influence of an immense number of circum

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stances of which we are ignorant, believing a vast amount of error and contradiction, surrounded on all sides by a nearly unlimited number of phenomena requiring an almost infinitely great degree of intelligence to understand them completely. And we appear to be destined, through an immeasurable time in the future, to be compelled to pass through an almost infinite amount of toil, in order to discover them.

Difficulty in all cases is greater or less according to the extent of our powers; that which is difficult to one man is more or less so to another; but with all men the difficulties are amazingly increased by the limited extent of our faculties. Our organs of vision do not enable us to decompose light, to distinguish simple colours from compound ones, or even to perceive one-third of the length of the entire solar spectrum, the ultra-red and ultra-violet rays being to us quite invisible; we also cannot see distinctly in feeble light. We have no sense at all for the detection of magnetism. Our mental faculties are also extremely limited, and are more frequently undeveloped than our senses. Many persons who possess scientific knowledge have but little manipulative ability; and of those who have been accustomed to teach science and repeat the experiments of others, but few have the power of imagining new and likely hypotheses, and a still smaller number possess the combination of abilities requisite to constitute a successful and original investigator. addition to all this, the impressions we derive from our senses are often fallacious, and the inferences we draw from our impressions are frequently erroneous, and our every act and thought is extremely liable to be tainted with mistake and error.

In

The difficulty of research is amazingly increased either by the magnitude, the minuteness, the complexity, or

abstruseness of many natural phenomena. The powers of man's mind, when compared with many of the problems presented by nature, appear as nothing to infinity, and in consequence of this we are obliged to employ every kind of artificial assistance. But these aids have also only a limited power. Telescopes will not enable us to see an infinite distance, nor microscopes to see atoms or even molecules. Spectroscopes, polariscopes, and photometers have also a limit of action, and for the aid of the senses of smell and taste we have no appliances. With the assistance of presses we can only produce a finite degree of pressure; and with the aid of air-pumps we can only rarefy gases to a limited degree. By means of a balance a difference of weight of 1 in 10 millions can hardly be detected, and a smaller quantity than one-hundredth of a milligram cannot at present be weighed. The highest amounts of power and accuracy yet attained are as follows:-Mousson estimated that he had subjected water to a pressure of 1,300 atmospheres, or 8 tons per square inch; Whitworth has measured a difference of one-millionth of an inch in length; and Joule has detected a difference in temperature of one 8,800th of a centigrade degree. By means of a mirror and electric spark Wheatstone measured one 72,000th of a second of time. By means of his chronoscope Noble has detected a period of time as small as 1 millionth of a second; and with the aid of the pendulum Airy observed a difference of time of 1 in 10 millions, or 2.1 seconds per day. By means of the microscope we can see an object a 50,000th to 100,000th part of an inch in length, but fail to see one of much smaller dimensions. By the aid of the spectroscope, Bunsen detected as small a quantity as one 180-millionth of a grain of sodium. Nessler's reagent renders manifest 1 part of ammonia in more than 100 million parts of water; and the sense of

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smell in recognising the odour of musk is probably even much more delicate than this.

There are limits to our experiments in every direction. Notwithstanding the apparently extreme delicacy of our various means of detecting and measuring substances and forces, they fall extremely short of what is necessary in order to detect many molecular phenomena; and our artificial aids may be said to be altogether inadequate. Nearly every method of detecting and measuring substances and forces remains to be further and immensely refined. According to Sorby, we have already, in consequence of the properties of light, nearly reached the limits of the powers of the microscope; and to be able to see the ultimate molecules of organic bodies would require us to use a magnifying power of from 500 to 2,000 times greater than those we now possess.1

It is chiefly by great refinements of scientific methods. that we can hope to detect minute residual phenomena, and discover those less obvious truths which are the most universal, and which will probably disclose to us the great principles of nature co-ordinating the many lesser truths we at present know.

A great advance in degree of refinement of scientific method is sometimes a result of an apparently very trivial circumstance, such for example as the use of platinum vessels in enabling more accurate chemical analyses to be made. If we could cause every substance to form its equivalent of ammonia, as we do at present with nitrous and nitric acid, we should at once obtain in an indirect manner an extremely delicate test for every substance. A method of indefinitely increasing the magnitude of minute effects is indicated in the principle of action of Holtz's

1Address to the Royal Microscopical Society, Feb. 2, 1876.