CHAPTER XV.

ANALYSIS OF QUANTITATIVE PHENOMENA.

IN the two preceding chapters we have been engaged in considering how a phenomenon may be accurately measured and expressed. So delicate and complex an operation is a measurement which pretends to any considerable degree of exactness, that no small part of the skill and patience of physicists is usually spent upon this operation. Much of this difficulty arises from the fact that it is scarcely ever possible to measure one simple phenomenon at a time. The ultimate object must be to discover the mathematical equation or law connecting a quantitative cause with its quantitative effect; this purpose usually involves, as we shall see, the varying of one condition at a time, the other conditions being maintained constant. The labours of the experimentalist would be comparatively light if he could carry out this rule of varying one circumstance at a time. He would then obtain a series of corresponding values of the variable quantities concerned, from which he might by proper hypothetical treatment obtain the required law of connexion. But in reality it is seldom possible to carry out this direction except in an approximate manner. Before then we proceed to the consideration of the actual process of quantitative induction, it is necessary to review the several devices by which the complication of effects can be disentangled. Every phenomenon measured will usually be the sum difference or product of two or more different effects, and these must be in some way analysed and separately

measured before we possess the materials for a true inductive treatment.

Illustrations of the Complication of Effects.

It is easy to bring forward a multitude of instances to show that a phenomenon is seldom to be observed simple and alone. A more or less elaborate process of analysis is almost always necessary. Thus if an experimentalist wishes to observe and measure the expansion of a liquid by heat, he places it in a thermometer tube and registers the rise of the column of liquid in the narrow tube. But he cannot heat the liquid without also heating the glass, so that the change observed is really the difference between the expansions of the liquid and the glass. More minute. investigation will show the necessity perhaps of allowing for further effects, namely the compression of the liquid or the expansion of the bulb due to the increased pressure of the column as it becomes lengthened.

In a great many cases an observed effect will be apparently at least the simple sum of two separate and independent effects. The heat evolved in the combustion. of oil is partly due to the carbon and partly to the hydrogen. A measurement of the heat yielded by the two jointly, cannot inform us how much proceeds from the one and how much from the other. If by some separate determination we can ascertain how much the hydrogen yields, then by mere subtraction we learn what is due to the carbon; and vice versa. The heat conveyed by a liquid, may be partly conveyed by true conduction, partly by convection. The light dispersed in the interior of a liquid consists both of what is reflected by floating particles and what is due to true fluorescence a; and we must find some mode of determining one portion before we can learn the other.

a Stokes, Philosophical Transactions' (1852), vol. cxlii. p. 529.

The apparent motion of the spots on the sun, is the algebraic sum of the sun's axial rotation, and of the proper motion of the spots upon the sun's face; hence the difficulty of ascertaining by direct observations the period of the sun's rotation.

We cannot obtain the weight of a portion of liquid in a chemical balance without weighing it with the containing vessel. Hence to have the real weight of the liquid operated upon in an experiment, we must have a separate weighing of the vessel, with or without the adhering film of liquid according to circumstances. This is likewise the mode in which a cart and its load are weighed together, the tare or weight of the cart previously ascertained being deducted. The variation. in the height of the barometer is a joint effect, partly due to the real variation of the atmospheric pressure, partly due to the expansion of the mercurial column by heat. The effects may be discriminated, if, instead of one barometer tube we have two tubes placed closely side by side, so as to have. exactly the same temperature. If one of them be closed at the bottom so as to be unaffected by the atmospheric pressure, it will show the changes due to temperature only, and, by subtracting these changes from those shown in the other tube, we get the real oscillations of atmospheric pressure. But this correction, as it is called, of the barometric reading, is better effected by calculation from the readings of an ordinary thermometer.

In a great many other cases a quantitative effect will be the difference of two causes acting in opposite directions. The late Sir John Herschel invented an instrument like a large thermometer which he called the Actinometer b, and M. Pouillet constructed a somewhat similar instrument bAdmiralty Manual of Scientific Enquiry,' edited by Sir John Herschel, 2nd ed. p. 299.

called the Pyrheliometer, for ascertaining the heating power of the sun's rays. In both instruments the heat of the sun was absorbed by a reservoir containing water, and the rise of temperature of the water was exactly observed, either by its own expansion or by the readings of a delicate thermometer immersed in it. The details of the construction and use of these instruments are immaterial to our immediate purpose. Now in exposing the actinometer to the sun, we do not obtain the full effect of the heat absorbed, because the receiving surface is at the same time radiating heat into empty space. The observed increment of temperature is in short the difference between what is received from the sun and lost by radiation. But the latter quantity is capable of ready determination; we have only to shade the instrument from the direct rays of the sun, while leaving it exposed to the rest of the open sky, and we can observe how much it cools in a certain time. The total effect of the sun's rays will obviously be the apparent effect plus the cooling effect in an equal time. By alternate exposure in sun and shade during equal intervals the desired result may be obtained with considerable accuracy o.

Two quantitative effects were beautifully distinguished in an experiment of John Canton, devised in 1761 for the purpose of demonstrating the compressibility of water d He constructed a thermometer with a large bulb full of water and a short capillary tube, the part of which above the water was freed from air. Under these circumstances the water was relieved from the pressure of the atmosphere, but the glass bulb in bearing that pressure was somewhat contracted. He next placed the instrument under the receiver of an airpump, and on exhausting the air, observed the water sink in the tube. Having thus Pouillet, Taylor's Scientific Memoirs,' vol. iv. p. 45. d Jamin, 'Cours de Physique,' vol. i. p. 158.

obtained a measure of the effect of atmospheric pressure on the bulb, he opened the top of the thermometer tube and admitted the air. The level of the water now sank still more, partly from the pressure on the bulb being now compensated, and partly from the compression of the water by the atmospheric pressure. It is obvious that the amount of the latter effect was approximately the difference of the two observed depressions.

Not uncommonly indeed the actual phenomenon which we wish to measure is considerably less than various disturbing effects which enter into the question. Thus the compressibility of mercury is considerably less than the expansion of the vessels in which it is measured under pressure, so that the attention of the experimentalist has chiefly to be concentrated on the change of magnitude of the vessels. Many astronomical phenomena, such as the parallax or proper motions of the fixed stars, are far less than the instrumental imperfections, and the other phenomena of precession, nutation, aberration, &c. Even Flamsteed imagined he had discovered the parallax of the pole star, and time after time astronomers mistook various other phenomena for that minute motion which they were so desirous to discover.

Methods of Eliminating Error.

In any particular experiment it is the object of the experimentalist to measure a single effect only, and he endeavours to obtain that effect free from any interfering effects. If this cannot be, as it seldom or never can really be, he makes the effect as considerable as possible compared with the other effects, which he reduces to a minimum, and treats as noxious errors. Those quantities,

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Baily's Account of the Rev. John Flamsteed,' p. 58.