=57. Methods of ascertaining the exact Boiling Temperature.=--The normal boiling temperature of water all nations have tacitly agreed to fix under a normal barometric pressure of 29922 inches of mercury, having the temperature of melting ice, in the lat.i.tude of 45, and at the sea-level.

If the atmospheric pressure at the time or place of graduating a thermometer does not equal this, the boiling temperature will be higher or lower according as the pressure is greater or less. Hence a reading must be taken from a reliable barometer, which must also be corrected for errors and temperature, and reduced for lat.i.tude, in order to compare the actual atmospheric pressure at the time with the a.s.sumed normal pressure.

Tables of vapour tension, as they are termed, have been computed from accurate experimental investigations and theory,--giving the temperatures of the vapour of water for all probable pressures; Regnault's, the most recent, is considered the most accurate; and his investigations are based upon the standard pressure given above, and are for the same lat.i.tude. His Table, therefore, will give the temperature on the thermometric scale corresponding to the pressure.

The Commissioners appointed by the British Government to construct standard weights and measures, decided that the normal boiling-point, 212, on the thermometer should represent the temperature of steam generated under an atmospheric pressure equal in inches of mercury, at the temperature of freezing water, to 29922 + (cos. 2 lat.i.tude 0766) + (00000179 height in feet above the sea-level). Hence, at London, lat.

5130' N., we deduce 29905 as the barometric pressure representing the normal boiling point of water,--the trifling correction due to height being neglected. If then, in the lat.i.tude of London, the barometric pressure, at the time of fixing the boiling point, be not 29905 inches, that point will be higher or lower, according to the difference of the pressure from the normal. Near the sea-level about 059 inch of such difference is equivalent to 1 Fahrenheit in the boiling point.

Suppose, then, the atmospheric pressure at London to be 30785 inches, the following calculation gives the corresponding boiling temperature for Fahrenheit's scale:--

Observed pressure 30785 Normal " 29905 ------ Difference 880 =======

As 059 is to 088, so is 1 to 15.

That is, the water boils at 15 above its normal temperature; so that, in this case, the normal temperature to be placed on the scale, viz. 212, must be 15 lower than the mark made on the tube at the height at which the mercury stood under the influence of the boiling water.

The temperature of the vapour of boiling water may be found, at any time and place, as follows:--Multiply the atmospheric pressure by the factor due to the lat.i.tude, given in the annexed Table V., and with the result seek the temperature in Table VI.

TABLE V. TABLE VI.

+----------------------------------------------------------------+

Lat.i.tude.

Factor.

Temperature

Tension.

Temperature

Tension.

of Vapour.

of Vapour.

---------+---------+++-----------+--------++-----------+--------

Degrees.

Degrees.

Inches.

Degrees.

Inches.

0

099735

179

14934

197

22036

5

099739

180

15271

198

22501

10

099751

181

15614

199

22974

15

099770

182

15963

200

23456

20

099797

183

16318

201

23946

25

099830

184

16680

202

24445

30

099868

185

17049

203

24952

35

099910

186

17425

204

25468

40

099954

187

17808

205

25993

45

100000

188

18197

206

26527

50

100046

189

18594

207

27070

55

100090

190

18998

208

27623

60

100132

191

19409

209

28185

65

100170

192

19828

210

28756

70

100203

193

20254

211

29335

75

100230

194

20688

212

29922

80

100249

195

21129

213

30515

196

21578

214

31115

+----------------------------------------------------------------+

_How to use the Tables._--When the _temperature_ is known to decimals of a degree, take out the tension for the degree, and multiply the difference between it and the next tension by the decimals of the temperature, and add the product to the tension, for the degree.

Required the tension corresponding to 19784.

197 = 22036 465 84 = 391 198 = 22501 197 = 22036 ------ ------ Difference 465 19784 = 22427 ====== ======

When the _tension_ is given, take the difference between it and the next less tension in the Table, and divide this difference by the difference between the next less and next greater tensions. The quotient will be the decimals to add to the degree opposite the next less tension.

Thus, for 23214 inches, required the temperature.

Given 23214 Next greater 23456 22974 Next less 22974 ------ ------ 240 Difference 482 240 And ---- = 5 482 Temperature opposite next less 1990 ----- Temperature required 1995 =====

A similar method of interpolation in taking out numerical quant.i.ties is applicable to almost all tables; and should be practised with all those given in this work.

_Example._--Thus, in Liverpool, lat. 53 30' N., the barometer reading 29876 inches, its attached thermometer 55, and the correction of the instrument being + 015 (including index error, capillarity and capacity), what temperature should be a.s.signed for the boiling point marked on the thermometer?

Observed barometer 29876 Correction + 015 ------ 29891 Correction for temperature - 074 ------ Reduced reading 29817 Factor from Table V. 100077 ------- 208719 208719 29817 ----------- Equivalent for lat. 45 2983995909 ===========

In Table VI., 2984 gives temperature 21186.

=58. Displacement of the Freezing Point.=--Either the prolonged effect of the atmospheric pressure upon the thin gla.s.s of the bulbs of thermometers, or the gradual restoration of the equilibrium of the particles of the gla.s.s after having been greatly disturbed by the operation of boiling the mercury, seems to be the cause of the freezing points of standard thermometers reading from a few tenths to a degree higher in the course of some years, as has been repeatedly observed. To obviate this small error, it is our practice to place the tubes aside for about six months before fixing the freezing point, in order to give time for the gla.s.s to regain its former state of aggregation. The making of accurate thermometers is a task attended with many difficulties, the princ.i.p.al one being the liability of the zero or freezing point varying constantly, so much so, that a thermometer that is perfectly correct to-day, if immersed in boiling water, will be no longer accurate; at least, it will take some time before it again settles into its normal state. Then, again, if a thermometer is recently blown, filled, and graduated immediately, or, at least, before some months have elapsed, though every care may have been taken with the production of the instrument, it will require some correction; so that the instrument, however carefully made, should from time to time be plunged into finely-pounded ice, in order to verify the freezing point.

=59. The Scale.=--The two fixed points having been determined, it is necessary to apply the scale. The thermometers in general use in the United Kingdom, the British Colonies, and North America are constructed with Fahrenheit's scale. Fahrenheit was a philosophical instrument maker of Amsterdam, who, about the year 1724, invented the scale which has given his name to the thermometer. The freezing point is marked 32, the boiling point 212, so that the intermediate s.p.a.ce is divided into 180 equal parts, called degrees. "The principle which dictated this _peculiar division_ of the scale is as follows:--When the instrument stood at the greatest cold of Iceland, or 0 degree, it was computed to contain 11124 equal parts of quicksilver, which, when plunged in melting snow, expanded to 11156 parts; hence the intermediate s.p.a.ce was divided into 32 equal portions, and 32 was taken as the freezing point of water: when the thermometer was plunged in boiling water, the quicksilver was expanded to 11336; and therefore 212 was marked as the boiling point of that fluid.

In _practice_, Fahrenheit determined the divisions of his scale from two fixed points, the freezing and boiling of water. _The theory_ of the division, if we may so speak, was derived from the lowest cold observed in Iceland, and the expansions of a given portion of mercury" (_Professor Trail_).

The divisions of the scale can be carried beyond the fixed points, if requisite, by equal graduations. Fahrenheit's scale is very convenient in some respects. The meteorological observer is seldom troubled with negative signs, as the zero of the scale is much below freezing. Again, the divisions are more numerous, and consequently smaller, than on other scales in use; and the further subdivision into tenths of degrees, seems to give all the minuteness usually required.

_Celcius_, a Swede, in 1742, proposed zero for the freezing point, and 100 for the boiling point, all temperatures below zero being distinguishable by the sign (--) minus. This scale is known as the _centigrade_, and is in use in France, Sweden, and the southern part of Europe. It has the advantage of the decimal notation, with the embarra.s.sment of the negative sign.

_Reaumur_, a Frenchman, proposed zero for the freezing point, and 80 for the boiling point, an arrangement inferior to the centigrade. It is, however, in use in Spain, Switzerland, and Germany.

It is merely a simple arithmetical operation to change the indications of any one of these scales into the equivalents on the others. To facilitate such conversions, tables are convenient, when a large number of observations are under discussion; and they can be easily formed or obtained.

In the absence of such tables, the following formulae will insure accuracy of method, and save thinking, when occasional conversions are wanted to be made:--F. stands for Fahrenheit, C. for Centigrade, and R. for Reaumur.

Given. Required. Solution.

F. C. = (F.-32) 5/9 F. R. = (F.-32) 4/9 C. F. = 9/5 C. + 32 C. R. = 4/5 C.

R. F. = 9/5 R. + 32 R. C. = 5/4 R.

_Example._--Convert 25 of Fahrenheit's scale into the corresponding temperature on the Centigrade scale.

Here C. = (25 - 32) 5/9 C. = -35/9 = -39

or nearly 4 _below_ zero of the Centigrade scale. The algebraical sign must be carefully attended-to in the calculations.

=60. The method of testing Thermometers= for meteorological purposes is very simple. Such thermometers are seldom required to read above 120. In these the freezing point having been determined, the divisions of the scale are ascertained by careful comparisons, with a standard thermometer, in water of the requisite temperature. "For the freezing point, the bulbs, and a considerable portion of the tubes of the thermometers, are immersed in pounded ice. For the higher temperatures, the thermometers are placed in a cylindrical gla.s.s vessel containing water of the required heat: the scales of the thermometers intended to be tested, together with the Standard with which they are to be compared, are read through the gla.s.s.

In this way the scale readings may be tested at any required degree of temperature, and the usual practice is to test them at every ten degrees from 32 to 92 of Fahrenheit."--_FitzRoy._

=61. Porcelain Scale Plates.=--Thermometer scales of bra.s.s, wood, or ivory, either by atmospheric influence or dipping in sea-water, are very liable to become soiled and discoloured, so much so that after a very little time the divisions are rendered nearly invisible. To obviate this inconvenience, Messrs. Negretti and Zambra were the first to introduce into extensive use thermometer and barometer scale-plates made of porcelain, having the divisions and figures engraved thereon by means of fluoric acid, and permanently burnt-in and blackened, so as always to present a clear legible scale. That these scales have been found superior to all others, may be inferred from the fact that all the thermometers now supplied to the various government departments are provided with such scales.

They can be adapted to replace any of the old forms of bra.s.s or zinc scales, the divisions and figures of which have become obliterated or indistinct.

=62. Enamelled Tubes.=--Nearly all thermometer tubes are now made with enamelled backs. This contrivance of enamelling the backs of the tubes enables the makers to use finer threads of mercury than had before been found practicable; for were it not for the great contrast between the dark thread of mercury and the white enamel on the gla.s.s, many of the thermometers now in use would be positively illegible. The enamelling of thermometers is an invention of Messrs. Negretti and Zambra. It is necessary to state this, as many persons, from interested motives, are anxious to ignore to whom the credit of the invention is due.

=63. Thermometers of extreme Sensitiveness.=--Thermometers for delicate experiments are no novelty. Thermometers have been made with very delicate bulbs to contain a very small quant.i.ty of mercury. Such instruments have also been made with spiral or coiled tubular bulbs, but the thickness of gla.s.s required to keep these coils or spirals in shape, and in fact to prevent their falling to pieces, served to nullify the effect sought to be produced, viz. instantaneous action; and where a small thin bulb was employed, the indicating column was generally so fine that it was positively invisible except by the aid of a powerful lens. Messrs.

Negretti and Zambra have now introduced a new form of thermometer, which combines sensitiveness and quickness of action, together with a good visible column. The bulb of this thermometer is of the gridiron form. Care has been taken in constructing the bulb, so that the objections attending spirals and other forms have been overcome; for whilst the reservoir or bulb is made of gla.s.s so thin that it is only by a spirit lamp and not a gla.s.s blower's blowpipe that it can be formed, yet it is still so rigid (owing to its peculiar configuration) that no variations in its indications can be detected, whether it be held in a horizontal, vertical, or oblique position, nor will any error be detected if it be stood on its own bulb. They have made thermometers with bulbs or reservoirs formed of about nine inches of excessively thin cylindrical gla.s.s, whose outer diameter is not more than a twentieth of an inch; so that, owing to the large surface presented, the indications are positively instantaneous.

This form of thermometer was constructed expressly to meet the requirements of scientific balloon ascents, to enable thermometrical readings to be taken at the precise elevation. It was contemplated to procure a metallic thermometer, but on the production of this perfect instrument the idea was abandoned.

64. VARIETIES OF THERMOMETERS.

Fig. 37 is an ill.u.s.tration of boxwood scale thermometers for general use and common purposes.

Fig. 38, Negretti and Zambra's Travelling Thermometer; it is fixed in a plated metal (silver or otherwise) case, similar to a pencil-case, and has the scale divided upon its stem.

Fig. 39, Thermometer mounted on a slab of gla.s.s, upon which the scale is etched, the back being either oak, mahogany, or ebony.

Fig. 40, Portable Thermometer, in a bronzed bra.s.s or German silver revolving case.

Fig. 41, Pocket Thermometer, on ivory or metallic scale, in morocco or papier-mache case.