Chapter VI: Part III: Instructions for the Preparation of the Tests Employed (4)

But if it were necessary to plunge the hydrometer in a hundred different solutions in order to produce the scale, it is easy to conceive that that would be extremely troublesome, especially for hydrometers which are employed in commerce, and which do not need to be so extremely accurate. When the density of the mixtures or solutions is a mean between those of the substances which enter into them, you may content yourself with marking the zero and one other fixed point, (_a_ and _b_, pl. 4, fig. 20.) Then, as the stalk of the hydrometer is evidently of equal diameter in all its extent, you can divide the space which separates the two fixed points into a certain number of equal parts. One of these, being taken for unity, represents a particular quantity of the substance which you have added to a determined weight of distilled water. By means of this unity you can carry the scale up and down the stalk of the instrument. It is thus, that, to obtain a Baumé’s hydrometer, after having obtained the zero by immersion in distilled water, you plunge the instrument into a solution containing a hundred parts of water and fifteen of common salt, to have the 15th degree, or containing a hundred water and thirty salt, to have the 30th degree. Upon dividing the interval into fifteen or thirty equal parts, according as you have employed one or the other solution, you obtain the value of the degree, which you can carry upwards or downwards as far as you wish.

Among the substances for which hydrometers are required in commerce, are some which it is impossible to obtain free from water--such are alcohol, the acids, &c. In this case it is necessary to employ the substances in their purest state, and deprived of as much water as possible.

The employment of hydrometers is very extensive: they are used to estimate the strength of lyes, of soap solutions, of wines, milk, &c. There is, in short, no branch of commerce in which these instruments are not required for the purpose of ascertaining the goodness of the articles which are bought and sold. The employment of hydrometers would be still more general, if they could be made to give immediately the absolute specific gravity of the liquids into which they might be plunged, the specific gravity of water being considered as unity. It is possible to graduate a thermometer of this description by proceeding as follows:--

Make choice of a hydrometer of which the exterior part of the stalk is very regular. Introduce the band of paper on which the scale is to be written, and then ballast the instrument. Make a mark where the surface of the distilled water touches the stalk. Remove the hydrometer from the water, wipe it perfectly dry, and weigh it very accurately with a sensible balance. Then pour into it a quantity of mercury equal to its own weight; plunge it again into the water, and again mark the point where the stalk touches the surface of the water. Pour the mercury out of the instrument, transfer the two marks to the scale, and divide this fixed distance into fifty equal parts. Having by this operation obtained the value of the degree, you carry it upwards and downwards, to augment the scale. If you take the first point near the reservoir, the hydrometer will be proper to indicate the density of liquids which are heavier than water; if you take it towards the middle of the tube, the contrary will be the case.

If you destine the hydrometer for liquids much heavier than water--such as acids, for example--you might, after having determined the first point, add to the original ballast as much mercury as is equal to the weight of the whole instrument; then the point where the stalk would touch the surface of the water, and which would be represented by 100, would be very high, and the second point, which would be found below, would be represented by 200. On dividing the space into a hundred equal parts, you would have the value of the degree, which could be carried up and down for the extension of the scale.

The specific gravities being in the inverse ratio of the volumes plunged into the liquid, the numbers of the scale which mark the specific gravities diminish from below; so that, on marking the lowest point 100, you have, on proceeding upwards, the successive degrees 0·99, 0·98, 0·97, 0·96, &c.

The hydrometers with two, three, and four branches, are graduated by having their tubes divided into a hundred or a thousand equal parts. The divisions on each branch must correspond with those on the other branches.

* * * * *

GRADUATION OF BAROMETERS.--The graduation of this instrument consists in dividing a piece of metal, wood, or ivory, into inches and parts of inches. The divided rod is then employed to measure the height of the mercury in the tube. As the rule is moveable, the operation presents no sort of difficulty: all that is necessary is to make the zero of the scale coincide with the inferior level of the mercury; the point which corresponds with the superior level of the mercury, seen in the tube, indicates the height of the barometric column. It is in this manner that the cistern barometer is graduated.

But if the barometer is one of those in which the surface of the mercury is variable, such as the barometer of Gay-Lussac, it is necessary to have recourse to a different process of graduation. If the two branches of the instrument are very regular, and of equal diameter, you first measure with precision the height of the column of mercury, then divide it in the middle, and fix the scale, which must be graduated in such a manner that the mark of fifteen inches corresponds exactly with the middle point. This mode of graduation serves to indicate merely the apparent height of the barometric column. If you desire that the scale should immediately indicate the real height, you must fix the zero at the middle of the column, and then double the figure which marks each degree.

When you do not wish to write the real height, you make two divisions, of which one proceeds upwards, the other downwards. You do not, in this case, double the value of each division, but in observations made with such a barometer scale you add the degree marked by the two surfaces, in order to find the real height.

It is in an analogous manner that you graduate the gauges or short barometers which are employed to measure the density of air under the recipient of the air-pump. You take the height of the mercury in the gauge, and fix at the middle of the column the zero of a double scale, of which one division proceeds upwards, the other downwards; or, instead of this, if you choose to have only one scale, and that an ascending scale, you double the value of every degree.

The zero of the barometric scale can be fixed below the inferior surface of the mercury; but then, to have the real height, it is necessary to measure precisely the height of the mercury in the two branches of the instrument, and to deduct the smaller from the larger.

_Dial (or Wheel) Barometer._--The disposition which should be given to this instrument is precisely the same as that of the _Dial Thermometer_, described in a preceding section. You make a small iron weight float on the inferior surface of the mercury, and fix to this weight a silk thread, which is stretched by a counterpoise, and rolls over a very moveable pulley. The axis of this pulley carries a needle, which turns backwards or forwards according as the column of mercury augments or diminishes. You arrange the whole in such a manner that the extreme variations of this column cannot make the needle describe more than one circumference; with this view you give the pulley a diameter of nearly an inch.

The dial barometer being rather an object of luxury than an instrument of precision, you graduate it by inscribing the following words, at full length, on the scale. In pl. 4, fig. 16, for example, you write,

At the point _a_ Tempest.
... _b_ Much rain.
... _c_ Rain or Wind.
... _d_ Temperate.
... _e_ Fine Weather.
... _f_ Fixed Fair.
... _g_ Very Dry.

You write nothing at the inferior division.

* * * * *

GRADUATION OF THE MANOMETER.--The graduation of this instrument consists in dividing the tube where the air is to be compressed, into a given number of parts of equal capacity; but as, in general, such tubes are employed as are nearly capillary and very regular, the operation is reduced to a linear division, where every degree occupies an equal space.

* * * * *

GRADUATION OF THERMOMETERS. _Construction of Standard Thermometers._--Having constructed your instrument with a very regular tube, or one which has been divided into parts of equal capacity, and having filled it with the proper liquid, according to the instructions given in a preceding section, the graduation is to be effected as follows. Procure very pure ice, break it into small pieces, and fill a vessel with it. When the ice begins to melt, plunge the thermometer into the middle of it, in such a manner that, without touching the sides of the vessel, the whole thermometer, or at least that part of it which contains the liquid, may be covered with ice. Allow the instrument to remain in this state until, in spite of the gradual melting of the ice, the surface of the column of liquid remains at a fixed point, and neither falls nor rises. Mark this point very carefully on the stalk of the thermometer, either with a thread or a little drop of sealing-wax, or with the trace of a diamond or a flint. This is the _freezing point_, the _zero_ of the centigrade scale, the thirty-second degree of Fahrenheit’s scale.

As for the second fixed point, it is marked during an experiment with boiling water, performed as follows:--You employ a vessel of tin plate sufficiently high to enclose the whole thermometer; you pour into this vessel distilled water, till it is about an inch deep, and then you heat it. The vessel is surmounted by a cover pierced with two holes, one of which is intended to receive the stalk of the thermometer, the other to allow the steam to escape. When, on continuing the ebullition, you observe that the mercury ceases to rise in the tube, you mark the point at which it has stopped, just as you marked the first point. The last mark indicates the _boiling point_; the one hundredth degree of the centigrade scale, the two hundred and twelfth degree of Fahrenheit’s scale. You transfer to paper the distance which is found between the first point and the second point determined, and you divide this distance into one hundred equal parts, or degrees, for the centigrade thermometer, into eighty parts for the thermometer of Réaumur, and into one hundred and eighty for that of Fahrenheit. If the tube of the instrument is very regular in the bore, the degrees should be equal in length; if, on the contrary, you have been obliged to divide it into parts of equal capacity, you find how many of these parts or little spaces it is necessary to take to constitute one of the above degrees. You find this by dividing their whole number by 100, or 80, or 180, according to the degrees of the scale which you intend to make use of. Thus, if you find between the two points fixed by melting ice and boiling water, three hundred divisions of equal capacity, it is necessary to include _three_ of these divisions in every _degree_ of the centigrade scale.

The vessel employed to take the boiling point must be of metal, and its surface should be perfectly clean and well polished, and have no rough points. If sand, or other matters, were permitted to repose on the vessel, and to form asperities, the water would enter into ebullition at an inferior temperature.

This operation should, moreover, be performed under an atmospherical pressure, which is indicated by the barometer when the mercury stands at twenty-nine inches and a half. But as this pressure is different according to the elevation of the place of operation, and, indeed, suffers continual variations even in the same place, it follows that the temperature of boiling water is subject to continual changes, and that, in the graduation of the thermometer, it is indispensably necessary to take notice of the height of the barometer at the very moment that the point denoting the degree of boiling-water is fixed upon. You succeed in making the necessary corrections by the help of the following table, which is founded on the experiments of Sir G. Shuckburg and of the Committee of the Royal Society.

[See the Table on the opposite page.]

_Common Thermometers._--Having, by the method which we have just described, obtained a _Standard Thermometer_, you may procure with facility as many ordinary thermometers as you desire. It is proper to employ the most regular tubes which you can obtain, and when the instruments are ready to be graduated, you must bring them into comparison with your standard thermometer. You place them together into a liquid of which you gradually raise the temperature, and you mark several points on the scale of the new thermometer, the intervals between which are subsequently divided into as many degrees as are marked on the scale of the standard thermometer. Thus, for example, you mark the 10° and 15°, and afterwards divide the interval into five equal parts. This gives you the length of a degree on the stalk of the new instrument. The more you multiply these fixed points, the more you insure the precision of the thermometer. When you have taken a certain number of points, you measure the remainder with the compasses.

* * * * *

+-----------------------------------+------------------------+
|Height of the Barometer in Inches. | |
+-----------------+-----------------+ Correction in |
|When the boiling |When the boiling | 1000ths of the |
| point is found | point is found | interval between |
| by immersing | by immersing | the freezing |
| the Instrument | the Instrument | and boiling points |
| in _Steam_. | in _Water_. | of Water. |
+-----------------+-----------------+------------------------+
| ... | 30.60 | 10 } |
| ... | 30.50 | 9 } |
| 30.71 | 30.41 | 8 } |
| 30.50 | 30.29 | 7 } |
| 30.48 | 30.18 | 6 } |
| 30.37 | 30.07 | 5 } Lower. |
| 30.25 | 30.95 | 4 } |
| 30.14 | 30.84 | 3 } |
| 30.03 | 30.73 | 2 } |
| 29.91 | 30.61 | 1 } |
| 29.80 | 30.50 | 0 } |
| | | |
| 29.69 | 29.39 | 1 } |
| 29.58 | 29.28 | 2 } |
| 29.47 | 29.17 | 3 } |
| 29.36 | 29.06 | 4 } |
| 29.25 | 28.95 | 5 } Higher. |
| 29.14 | 28.84 | 6 } |
| 29.03 | 28.73 | 7 } |
| 28.92 | 28.62 | 8 } |
| 28.81 | 28.51 | 9 } |
| 28.70 | | 10 } |
| | |The boiling point to be |
| | |marked so much higher or|
| | |lower than the stand of |
| | |the mercury during the |
| | |experiment. |
+-----------------+-----------------+------------------------+

The zero, 0°, of the thermometer of Fahrenheit, is taken by means of a mixture of snow and common salt, and its maximum point is, like that of the preceding thermometer, taken by means of boiling water; but this interval is divided into 212 degrees; so that the scale marks 32° where the centigrade and Réaumur’s scales mark 0°.

The thermometer of Delisle has but one fixed point, which is the heat of boiling water; this is the zero of the instrument. The inferior degrees are 0,0001 (one ten-thousandth part) of the capacity of the bulb and stalk of the thermometer. It marks 150° at 0° of the centigrade, or 32° of Fahrenheit’s thermometer.

The dial, the maximum and the minimum thermometers, are graduated according to the same principles as the common thermometers.

You can, with a mercurial thermometer, make the centigrade scale rise to 300 or 400 degrees above zero; but with an alcohol thermometer, you must never go beyond the heat of boiling water. On the contrary, the inferior degrees of the alcohol thermometer can be carried to the very lowest point, while those of the mercurial thermometer should be stopped at thirty or thirty-five degrees below the zero of the centigrade scale, as the mercury then approaches very near the point of its congelation. In all cases, the degrees of thermometer scales are indicated by the sign - when they are below zero, and by the sign + when they are above it; the -is always marked, but the + generally omitted. See pl. 4, fig. 6.

We may observe here that it is proper from time to time to plunge the standard thermometer into melting ice, for the purpose of verifying its exactness. It has been found that thermometers constructed with a vacuum above the column of mercury gradually become inaccurate, the 0° ascending, until it corresponds with + 1° or + 2°. This singular effect is attributable to the constant pressure of the atmosphere, which, being supported merely by the resistance of the very thin sides of the thermometer, finally presses them together, and diminishes the capacity of the reservoir. It is partly for the sake of avoiding this inconvenience that we consider it good not to make an entire vacuum above the mercury, but to leave a portion of air in the tube, and at the same time to form a little reservoir at the summit of the instrument.

_Differential Thermometer._--To graduate this instrument, you first maintain the two bulbs at an equal temperature, by which you determine the first fixed point, which is zero. Then, enveloping one of the two bulbs with melting snow, and elevating the other by means of a vessel with warm water, to a known temperature--to 20° Centigrade, for example--you fix a certain space, which you afterwards divide into 20 equal parts or degrees. The scale is continued by carrying successively to each side the known value of a degree.

* * * * *

GRADUATION OF RUMFORD’S THERMOSCOPE.--This instrument is graduated by dividing the tube which separates the two bulbs into equal parts, the number of which is arbitrary, though, in general, the thermoscope tube is divided into nine or eleven parts. There is always an odd number of degrees, and you manage so that the odd degree is found in the middle of the tube. It carries the mark of zero at each end, and the figures 1, 2, 3, &c. proceed from each end of this middle degree, and form two corresponding scales.

* * * * *

GRADUATION OF MARIOTTE’S TUBE.--You divide the little branch which is sealed at the end into a certain number of parts of equal capacity, and the large branch into inches and parts of inches. It is necessary to take care that the zero of the two ascending scales correspond, and are situated above the inferior bend formed by the two branches of the instrument.

THE END.

W. WILSON, PRINTER, 57, SKINNER-STREET, LONDON.

End of Project Gutenberg's The Art of Glass-Blowing, by T. P. Danger