MODE OF PLANNING AN EXPERIMENT.

309

of such materials as are most ready at hand. The unessential parts of such a model may generally be of the roughest kind, but the essential conditions, as far as we know or can guess them, should be fulfilled as perfectly as possible. For example, if a feeble rotatory effect is sought for, the friction of the moving parts should be reduced to a minimum, and the motive power exalted to the highest degree. Circumstances which, we feel assured, do not affect the results, such as crudeness of some parts of the apparatus, and in some cases even impurity of the materials, may be entirely disregarded; but this point can only be determined by actual knowledge, and should not be acted upon unless we are quite certain. Eminent investigators have usually constructed the apparatus used in their first experiments on a subject in a rough sort of way; the model of the first safety-lamp made by Sir Humphry Davy was formed by wrapping a piece of wiregauze round his thumb, and is now in the possession of the Royal Society. Undue fondness for beautiful apparatus is, in the adult student, rather a sign of an amateur than of an original mind. When an investigation is completed, an apparatus, perfect in appearance and durability as well as in action, may then with propriety be constructed.

As new phenomena, when first discovered, are often very minute, the proportions of the different parts and conditions of the experiments should be very carefully considered at the outset, or as soon as possible; for want of this, early experiments often fail, which, if properly arranged, would have succeeded. Where the effects are extremely minute, they are magnified or multiplied by means of various contrivances, which differ in almost every different case, and these contrivances are described in books relating to the various sciences. The pendulum is a well-known means of magnifying, by multiplication,

minute differences of time; the mirror, for magnifying the signs of minute movements, as in the needles of magnetometers and galvanometers; the electric condenser, for multiplying the effect of minute electric charges; and so on.

Experiments are usually arranged upon a suitable degree of magnitude, such that the desired effect may be conspicuous, and interfering circumstances as small as possible, for the latter are often greater than the expected result. An ordinary chemical analysis, for example, is usually made upon from 20 to 100 grains of the substance, in order to keep within a moderate compass the unavoidable errors of the process. When the expected effect is small, the substance or apparatus for producing it should be large, and the means of detecting it should be very delicate.

Experiments are, in some cases, made for the purpose of discovering a new phenomenon, and in others for investigating an already known truth. In the latter instance, if the phenomenon appears to be of an altogether novel kind, the experiments have at the outset to be made in a less systematic way, until some guiding idea of its probable nature is obtained; but, even in such a case, the employment of a table or series of leading ideas for raising hypotheses is of advantage. In a series of experiments, especially in those made for the purpose of determining causes, we usually vary only one circumstance or condition. at a time, and draw from it a single or small number of conclusions. In other cases, however, by means of a single experiment we are sometimes enabled to make a whole series of determinations at once.

All experiments have limits of time; some, however, require very long periods. Sir William Thomson has in progress some experiments of diffusion of an aqueous soluSee p. 370.

HOW TO CONDUCT AN EXPERIMENT.

311

tion of sulphate of copper in very tall vertical glass tubes, which are calculated to require several hundred years to complete them. A multitude of experiments in the subjects of crystallisation and liquid diffusion might easily be devised and commenced, which would require thousands of years for completion.

Before commencing an experiment, a plan should be formed of the mode of conducting it, i.e. of the order in which the various changes should be made, and of the notes to be taken. A memorandum should also be made of the different substances or apparatus employed, their conditions, forms, weights, sizes, positions, relations to each other, temperature, or other circumstances which may appear essential, in order that their effects may be studied, and that the experiment may be exactly repeated, if necessary, at any future time. In complex experiments, or in those with dangerous substances, a rehearsal is oftentimes necessary before the actual experiment, in order that every attention may be paid to the more critical points in the actual trial. I frequently adopted this plan whilst investigating the properties of the extremely dangerous substance, anhydrous hydrofluoric acid. Having made all the necessary preparations, a preliminary trial is then made; and, having thus found that the apparatus and materials will act, one of the first points to be determined is whether or not any of the obvious or well-known causes of interference are influencing it. If, for example, a galvanometer is used, and its needle moves, that instrument should be disconnected, and the experiment repeated without it. If the needle now moves, an interference exists, and the galvanometer must be removed to such a distance (sometimes as much as 30 or 40 feet) from the experimental apparatus that an experiment with the latter no longer affects it, except when the two are connected

together. In nearly all chemical experiments, in order to prevent interferences, the first preliminary condition to be secured is a high degree of purity of the substances. Even when a new or uninvestigated phenomenon does really exist, the exclusion of interferences takes a long time, and then another long period is consumed in developing the phenomenon to a degree of magnitude suitable for investigation; and very few of the experiments made up to this point can be published, because they are imperfect.

In actual research, the experiment or apparatus is often arranged in such a way that the order and disposition of its different parts can be readily altered; and whilst the experiments are in progress, the direction, distance, and strength of the forces employed are varied, instead of preparing afresh for every modified trial. By this plan a number of results and the means of drawing many conclusions are obtained with the trouble of only one preparation. For example, if a large voltaic battery is employed, a number of experiments are prepared before charging it, and the battery is so arranged that the magnitude and tension of its current can be easily varied.

The number and extent of the preparations necessary for experiments vary, of course, with the character and magnitude of the research. Nearly always, the amount of trouble required to prepare for experiments is very much greater than that expended upon the actual trials. Waiting for substances and apparatus is also a frequent source of delay, and clearing away the residues of experiments consumes a great deal of time.

For the purposes of experiment, every physical investigator requires a suitable and sufficient supply of materials and apparatus, especially those necessary for generating, governing, detecting, and measuring the different forces of nature. Every chemical experimentalist also needs

NECESSITY OF MANIPULATIVE SKILL.

313

to possess a stock of ordinary and rare chemicals of the highest attainable degree of purity, as well as cruder ones for commoner purposes. Rare minerals and the residues of peculiar manufacturing processes, especially those obtained in the working of rare substances, constitute a valuable addition to the stock of an investigating chemist, because new elementary bodies and the wide-spread existence of the rarer elements have not unfrequently been discovered by examining such substances.

CHAPTER XXXIII.

NECESSITY OF MANIPULATIVE SKILL.

NEXT in importance to the skilful use of a gifted mind in research comes dexterous employment of the human hand. To the mental qualifications of scientific knowledge, imagination, and invention, it is almost indispensable to add aptitude in mechanical matters, and cleverness in experimental manipulation. Great manipulative ability can be acquired only by long practice, which should be commenced at an early age. Fondness for mechanical pursuits in a child has often betokened skill in discovery. Newton, when a youth, was clever in constructing mechanical toys. In scientific study also, as in other abstruse meditations, the mind soon becomes exhausted by intense thinking, but is usually relieved by preparing and making experiments. Study and manipulation in physics and chemistry go hand-in-hand, and in actual research they often alternate; from the results of an experiment just made we draw new conclusions and infer additional hypotheses, and we then make other experiments to test them, and so on.