Conversations on Chemistry Part 6

MRS. B.

Undoubtedly; and this I can immediately show you by a very simple experiment. This piece of iron now exactly fits the frame, or ring, made to receive it; but if heated red hot, it will no longer do so, for its dimensions will be so much increased by the caloric that has penetrated into it, that it will be much too large for the frame.

The iron is now red hot; by applying it to the frame, we shall see how much it is dilated.

EMILY.

Considerably so indeed! I knew that heat had this effect on bodies, but I did not imagine that it could be made so conspicuous.

MRS. B.

By means of this instrument (called a Pyrometer) we may estimate, in the most exact manner, the various dilatations of any solid body by heat.

The body we are now going to submit to trial is this small iron bar; I fix it to this apparatus, (PLATE I. Fig. 1.) and then heat it by lighting the three lamps beneath it: when the bar expands, it increases in length as well as thickness; and, as one end communicates with this wheel-work, whilst the other end is fixed and immoveable, no sooner does it begin to dilate than it presses against the wheel-work, and sets in motion the index, which points out the degrees of dilatation on the dial-plate.

[Ill.u.s.tration: Plate I. Vol. I. p. 38.

Fig. 1. Pyrometer.

A.A Bar of Metal.

1.2.3 Lamps burning.

B.B Wheel work.

C Index.

Fig. 2 A.A Gla.s.s tubes with bulbs.

B.B Gla.s.ses of water in which they are immersed.]

EMILY.

This is, indeed, a very curious instrument; but I do not understand the use of the wheels: would it not be more simple, and answer the purpose equally well, if the bar, in dilating, pressed against the index, and put it in motion without the intervention of the wheels?

MRS. B.

The use of the wheels is merely to multiply the motion, and therefore render the effect of the caloric more obvious; for if the index moved no more than the bar increased in length, its motion would scarcely be perceptible; but by means of the wheels it moves in a much greater proportion, which therefore renders the variations far more conspicuous.

By submitting different bodies to the test of the pyrometer, it is found that they are far from dilating in the same proportion. Different metals expand in different degrees, and other kinds of solid bodies vary still more in this respect. But this different susceptibility of dilatation is still more remarkable in fluids than in solid bodies, as I shall show you. I have here two gla.s.s tubes, terminated at one end by large bulbs.

We shall fill the bulbs, the one with spirit of wine, the other with water. I have coloured both liquids, in order that the effect may be more conspicuous. The spirit of wine, you see, dilates by the warmth of my hand as I hold the bulb.

EMILY.

It certainly does, for I see it is rising into the tube. But water, it seems, is not so easily affected by heat; for scarcely any change is produced on it by the warmth of the hand.

MRS. B.

True; we shall now plunge the bulbs into hot water, (PLATE I. Fig. 2.) and you will see both liquids rise in the tubes; but the spirit of wine will ascend highest.

CAROLINE.

How rapidly it expands! Now it has nearly reached the top of the tube, though the water has hardly begun to rise.

EMILY.

The water now begins to dilate. Are not these gla.s.s tubes, with liquids rising within them, very like thermometers?

MRS. B.

A thermometer is constructed exactly on the same principle, and these tubes require only a scale to answer the purpose of thermometers: but they would be rather awkward in their dimensions. The tubes and bulbs of thermometers, though of various sizes, are in general much smaller than these; the tube too is hermetically closed, and the air excluded from it. The fluid most generally used in thermometers is mercury, commonly called quicksilver, the dilatations and contractions of which correspond more exactly to the additions, and subtractions, of caloric, than those of any other fluid.

CAROLINE.

Yet I have often seen coloured spirit of wine used in thermometers.

MRS. B.

The expansions and contractions of that liquid are not quite so uniform as those of mercury; but in cases in which it is not requisite to ascertain the temperature with great precision, spirit of wine will answer the purpose equally well, and indeed in some respects better, as the expansion of the latter is greater, and therefore more conspicuous.

This fluid is used likewise in situations and experiments in which mercury would be frozen; for mercury becomes a solid body, like a piece of lead or any other metal, at a certain degree of cold: but no degree of cold has ever been known to freeze spirit of wine.

A thermometer, therefore, consists of a tube with a bulb, such as you see here, containing a fluid whose degrees of dilatation and contraction are indicated by a scale to which the tube is fixed. The degree which indicates the boiling point, simply means that, when the fluid is sufficiently dilated to rise to this point, the heat is such that water exposed to the same temperature will boil. When, on the other hand, the fluid is so much condensed as to sink to the freezing point, we know that water will freeze at that temperature. The extreme points of the scales are not the same in all thermometers, nor are the degrees always divided in the same manner. In different countries philosophers have chosen to adopt different scales and divisions. The two thermometers most used are those of Fahrenheit, and of Reaumur; the first is generally preferred by the English, the latter by the French.

EMILY.

The variety of scale must be very inconvenient, and I should think liable to occasion confusion, when French and English experiments are compared.

MRS. B.

The inconvenience is but very trifling, because the different gradations of the scales do not affect the principle upon which thermometers are constructed. When we know, for instance, that Fahrenheit's scale is divided into 212 degrees, in which 32 corresponds with the freezing point, and 212 with the point of boiling water: and that Reaumur's is divided only into 80 degrees, in which 0 denotes the freezing point, and 80 that of boiling water, it is easy to compare the two scales together, and reduce the one into the other. But, for greater convenience, thermometers are sometimes constructed with both these scales, one on either side of the tube; so that the correspondence of the different degrees of the two scales is thus instantly seen. Here is one of these scales, (PLATE II. Fig. 1.) by which you can at once perceive that each degree of Reaumur's corresponds to 2 of Fahrenheit's division. But I believe the French have, of late, given the preference to what they call the centigrade scale, in which the s.p.a.ce between the freezing and the boiling point is divided into 100 degrees.

[Ill.u.s.tration: Plate II. Vol. I. p. 42.

Fig. 1. Thermometer.

Fahrenheit's Scale.

Reaumur's Scale.

Boiling point of Water Freezing point of Water

Fig. 2. Differential Thermometer.]

CAROLINE.

That seems to me the most reasonable division, and I cannot guess why the freezing point is called 32, or what advantage is derived from it.

MRS. B.

There really is no advantage in it; and it originated in a mistaken opinion of the instrument-maker, Fahrenheit, who first constructed these thermometers. He mixed snow and salt together, and produced by that means a degree of cold which he concluded was the greatest possible, and therefore made his scale begin from that point. Between that and boiling water he made 212 degrees, and the freezing point was found to be at 32.

EMILY.

Are spirit of wine, and mercury, the only liquids used in the construction of thermometers?

MRS. B.

I believe they are the only liquids now in use, though some others, such as linseed oil, would make tolerable thermometers: but for experiments in which a very quick and delicate test of the changes of temperature is required, air is the fluid sometimes employed. The bulb of air thermometers is filled with common air only, and its expansion and contraction are indicated by a small drop of any coloured liquor, which is suspended within the tube, and moves up and down, according as the air within the bulb and tube expands or contracts. But in general, air thermometers, however sensible to changes of temperature, are by no means accurate in their indications.