NOTE ON THE

METRIC SYSTEM OF WEIGHTS & MEASURES

THE weights and measures used in this book are for the most part those based upon the French or Metric System. In this system the standard of length is the length of a certain bar of platinum preserved in Paris. This is called a metre, and is somewhat more than a yard (39.37 inches). It is divided into tenths, hundredths, and thousandths, which are called decimetres, centimetres, and millimetres respectively. Both in measuring and in calculating this decimal method of division is much more convenient than our way of dividing up a yard into 3 feet, each equal to 12 inches and so on. We shall use as our unit of length the centimetre. The relation between the centimetre and the inch can be seen from the accompanying figure.

The unit of area is a square centimetre, and the unit of volume a cubic centimetre (commonly abbreviated into 'I c.c.'). The volume of a cubic decimetre is called a litre since I decimetre 10 centimetres, it follows that a litre (or cubic decimetre) contains 1000 c.c. A litre is somewhat more than a pint and three-quarters.

The standard of weight-or rather of mass--is called a kilogramme, and is a little more than 2 lbs. It is divided into 1000 parts called grammes. We shall use the gramme as our unit of mass.

The British standard pound bears no simple relation to the units of length and volume used in this country; but the original standard kilogramme was so constructed as to have a mass equal to that of a cubic decimetre (or litre) of water at a temperature of 4° Centigrade. Thus I c.c. (one-thousandth of a cubic decimetre) of water at 4° C. weighs exactly one gramme (one-thousandth of a kilogramme).

A little experience will show the student how convenient it is to have a definite and simple relation between the unit of volume and the unit of mass he will also find that by using a decimal system we avoid a number of troublesome calculations, such as those required for reducing miles to inches and pounds to ounces. The system is now generally used in scientific books, and by scientific men in all countries: hence it is

Inches

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Millimetres and Centimetres

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advisable that the student should acquire some acquaintance with it at as early a stage as possible.

HEAT

CHAPTER I

TEMPERATURE

1. Hot and Cold Bodies. When we stand in front of a fire on a cold day we experience a pleasant sensation of warmth. The same sensation is produced, in a more or less pleasant degree, when we touch objects in the neighbourhood of the fireplace which have been warmed by the fire. We attribute the sensation, in the first case directly, and in the second case indirectly, to the heat given out by the fire, and objects which produce this feeling when we touch them are Isaid to be hot. Objects which have not been warmed by the fire will generally produce a sensation of an opposite kind, and we say that they feel cold. In ordinary language we frequently use the same word to denote the effect produced and the cause which produces it: thus we speak of the sensation of heat. Whatever heat may be, we may for the present regard it as something that produces in us the sensation of warmth. Let us now consider more closely what we mean by the terms 'hot' and 'cold,' and how we can best find out which of two bodies is hotter than the other.

You will perhaps say that this can easily be found out by touching each of them with the hand and finding out which of the two feels the hotter. Now try this in a room heated artificially, say a hothouse which is heated by steam or hotwater pipes. The pipes themselves, and perhaps the object nearest to them, will feel very hot, and we may naturally assume

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that they are really hotter than other objects. But these latter produce sensations which differ among themselves in a surprising manner. The wooden shelves on which the flowerpots are arranged do not appear much warmer than the hand itself, whereas the iron brackets on which they are supported feel uncomfortably hot; a piece of slate or iron in a far corner of the room may even appear hotter to the hand than a wooden shelf just above the heating apparatus or a strip of matting wound round the pipes, so that nearness to the source of heat cannot be the cause of the difference in behaviour. Before concluding that the iron and slate are really warmer than the wood and matting we ought to extend our observations and examine them more closely, so as to see whether there is any source of error.

Try a cold room next, say an ordinary dwelling-room on a cold day and without any fire. Touch the various articles in the room as you walk round and note the sensations which they produce in the hand. The table and chairs appear somewhat cold, the mantelpiece (if of slate or marble) colder, and the fireplace and fire-irons colder still. The carpet and tablecloth scarcely produce any sensation of cold at all, and if you bury your hand in a thick soft hearthrug it will appear warm to the touch after a while. The naked soles of the feet, which are ordinarily preserved from extremes of heat and cold by stockings and leather soles, are even more sensitive than the hands. You can readily test this by drawing a hearthrug near to a slate or tiled hearth and standing bare-foot partly on each the rug scarcely produces any sensation of cold, but the hearthstone is uncomfortably cold, and the floor between slightly cold. And in this case we have no good reason for believing that the stone is really colder than the wood, or the wood than the rug.

2. Conduction of Heat.-If you take a poker and push the point of it well into a fire, you will find that after a while the handle begins to get warm; the heat travels along the poker by a process which is called conduction. You can observe the same thing in a teaspoon partly immersed in hot water, and if you take two different teaspoons-a common one made of Britannia metal and a silver one—and put them in the same cup of hot water, you will find that the handle of the

silver spoon gets hot more quickly than the other. The silver conducts heat more rapidly than the alloy, or is said to be a better conductor. Metals are generally good conductors of heat, and stone, slate, and marble are fairly good; wood is a bad conductor, and wool, flannel, and fur still worse. Now when you take hold of a warm iron rod heat at once begins to flow from the hot body (the iron) to the colder body (your hand) which is in contact with it; and, since iron is a good conductor of heat, there is also a rapid flow of heat from all parts of the rod toward that part which is cooled by contact with the hand. Thus there is a rapid transference of heat from the iron to your hand; your hand gets warm quickly and you say that the iron 'feels hot.' An equally hot wooden rod would not 'feel' nearly as hot to the hand: wood is such a bad conductor that the part of the rod first cooled by contact with the hand would practically remain cool. And so when you touch a cold body; if it is a good conductor, iron or stone, heat flows from your hand and is rapidly conducted away into colder parts of the body. But if it is a bad conductor, such as flannel, most of the heat taken from your hand remains in the part of the flannel which is in contact with the hand, and so it soon begins to feel warm. Thus the heat-sensation which you experience when you touch a body with your hand depends not only upon whether the body is hot or cold, but also to some extent upon its conducting power. It also depends upon the previous state of the hand, as can be shown by the following experiment.

EXPT. 1.-Take three large basins and fill the first with ice or a freezing mixture of ice and salt. Into the second pour tap-water or water slightly warmed, and into the third water as hot as the hand can bear. Plunge your left hand into the ice and your right into the hot water: after holding them there for a few minutes put both hands in the middle basin. The water in it will, by contrast, appear cold to your right hand, while, at the same time, it feels warm to the left hand.

3. Temperature.-In scientific language a hot body is said to have a high temperature, while a cold body is said to have a low temperature. It is important that you should understand exactly what this term temperature means. It is not the same thing as heat. Nor is it a quality of any particular body, for