195.]

WET-BULB HYGROMETER.

407

sure of the vapour above it: the temperature of this ether and of the ball in contact with it is lowered, and deposition of dew commences on the surface of the black ball, in the form of a ring, which coincides with the level of the ether. The moment that this occurs, the temperature marked by the included thermometer, d, is observed. It is, however, possible, if the reduction of temperature has been rapid, that the loss of heat may not be perfectly uniform throughout the ether in the black bulb, in consequence of which the temperature indicated by the thermometer, d, may be a little too high; it is therefore well to observe the temperature of d a second time, at the moment when the ring of dew disappears, during the return of the instrument towards the temperature of the surrounding air. This observation will now probably be slightly too low, but the mean of the two will accurately furnish the temperature of the dew-point. The temperature of the atmosphere at the time is indicated by the thermometer k.

In making an observation, the hygrometer should be placed at an open window, and a small cardboard screen should be interposed between the two bulbs, to prevent the vapour of the ether from extending to the atmosphere around the blackened bulb. With proper care, the instrument will yield results of great accuracy. An excellent hygrometer, on a similar principle, but of less portable construction, has been used by Regnault.

(195) Wet-bulb Hygrometer.-Other methods have been proposed for determining the quantity of moisture present in the atmosphere. Of these hygrometers, one only, the wet-bulb hygrometer, need be noticed here it is shown in fig. 150.

In simplicity of action it leaves nothing to be desired, as it consists merely of two similar thermometers, s, s, placed side by side on the same stand, мf; the bulbs, b, b, of both are covered with muslin, and one of them is kept constantly moist by means of the capillary action of a few fibres of cotton, e, which connect it with a small vessel, a c, containing water. The rate of evaporation, and consequently the depression of temperature of the moistened bulb, will be greater in proportion as the atmosphere is further from its point of saturation; and tables have been given for determining the degree of saturation for all differences of temperature within the ordinary atmospheric range.

FIG. 150.

The elaborate researches of Regnault (Ann. Chim. Phys. 1845 [3], xv. 201) have shown that the indications of this hygrometer require a variety of corrections which cannot be correctly estimated. The formula. which on the whole corresponds best with observation is that of Apjohn: f'=f- &× fo• In this formula f' is the pressure of steam at the dew-point, f is the pressure of steam at the observed temperature of the air, d is the difference in temperature of the two

d

88

408

PRECIPITATION OF MOISTURE.

[195.

thermometers, 88 is a constant coefficient for the specific heats of air and steam, p is the observed height of the barometer, 30 is the mean height of the barometer in English inches.

From an extensive series of comparisons made at the Greenwich Observatory between the wet-bulb hygrometer and Daniell's instrument, Glaisher concludes that the dew-point may be ascertained by multiplying the difference between the temperature of the dry and the wet-bulb by a number depending upon the temperature of the air at the time of observation, and subtracting the product from this last-mentioned temperature. The numbers which he gives are contained in the following Table*:

Since the pressure of aqueous vapour diminishes according to the terms of a geometric progression, whilst the temperature falls in arithmetic progression, the pressure of the vapour contained in the atmosphere at any given time is reduced by a fall of temperature more rapidly than in direct proportion to the fall; it therefore necessarily happens, that if a current of heated air, saturated with aqueous vapour, meet a current of cold air, also saturated with vapour, the intermingled portions of air at the mean temperature of the two can only retain a part of the vapour in the invisible condition, and the formation of a cloud or mist is the consequence. For example, suppose two equal volumes of air, one at 16° C. and the other at 4°, each saturated with vapour,

When the dew-point was calculated by Apjohn's formula from the indications of the wet-bulb thermometer, the extreme differences from the true dewpoint, furnished by Daniell's instrument, were found in two years at Greenwich to be 39 between 65° and 70°, and +3°6 between 75° and 80°; whilst the extreme differences by Glaisher's factors are -3°7 between 75° and 80°, and +5°-6 between 75° and So°.-(Daniell's Meteorology, ii. 100. Noble, Proceed. Roy. Soc. 1855, vii. 528.)

See also

196.]

LIQUEFACTION OF GASES.

409

to be intermingled-the temperature of the intermingled air would be 10°. Now the pressure of aqueous vapour at 16° is 13.mm.536; at 4° it is 6-mm-097. The mean of these quantities is .mm.816, but the actual pressure of vapour at 10° is only 9-mm.165; consequently an amount of vapour represented by a pressure of 9.mm.816-9mm. 165, or omm.651, will be precipitated in the form of a cloud. It was upon this principle that Hutton accounted for the formation of rain, and so far as it goes, the theory is satisfactory: there are, however, other important causes concerned, but the subject cannot be appropriately discussed further in this work.

(196) Liquefaction and Solidification of Gases.-Vapours were formerly considered to be essentially different in their nature from gases; but numerous experiments, particularly those by Faraday (Phil. Trans. 1823, 160, 189, and 1845, 155), and Andrews (Phil. Trans. 1869, 575), have shown that the difference between gases and vapours is merely one of degree. Under Faraday's skilful manipulation, numerous gases have been reduced to the liquid state, and not a few have even been obtained in the form of solids. Some few of the gases have still resisted the best devised attempts to liquefy them; but it can hardly be doubted that all gases may be regarded as the vapours of liquids of an extremely high degree of volatility; the liquids resulting from the condensation of gases boiling at temperatures far below the ordinary atmospheric range: vapours, on the contrary, may be considered as the gases of liquids of comparatively low volatility.

Some of the gases are liquefiable with much greater facility than others; for instance, a mere reduction of the temperature to 4° (-20°C.) suffices to reduce sulphurous anhydride at the ordinary atmospheric pressure to the liquid form. Many gases, if generated in strong tubes, under the pressure of their own particles, lose their gaseous state. In this way carbonic anhydride, cyanogen, and several others, have been liquefied. But in other cases, a combination of the pressure obtained by means of a condensing syringe, with the application of a very low temperature, has been requisite. A convenient form of apparatus for this purpose has been devised by Andrews, who has succeeded in reducing the non-condensible gases in fine glass tubes, by pressure alone, to less than of the volume which they occupy under the ordinary conditions of the atmosphere, and in exposing them, still under this pressure, to a cold of from -77° to — 101° C.

Carbonic anhydride may be manufactured in large quantities, and stored up in the liquid form, in strong wrought-iron vessels. The apparatus used for this pur

410

LIQUEFACTION OF CARBONIC ANHYDRIDE.

[196. pose was devised by Thilorier. A modification of it is represented in fig. 151. It consists of two very strong hollow cylinders of wrought-iron, one of which, a, is

FIG. 151.

D

employed as a retort for generating the gas; the other, B, is a receiver, in which it is accumulated. The generator,

A, is charged with a mixture of 125 kilos. (2 lb.) of finely powdered hydric sodic carbonate (the common bicarbonate of soda) (NaHCO), and 2.83 litres (61b.) of warm water: a copper tube, C, containing o'68 kilo. (1 lb.) of oil of vitriol, is cautiously introduced, and the head of the apparatus, furnished with a stop-cock of peculiar construction, is screwed

down and rendered tight by a leaden washer. The generator is then reversed, so as to mix the materials, which, by their reaction, liberate carbonic anhydride; this gas accumulates in the upper part of the vessel, where it is liquefied by its own pressure; a strong tube, E, is screwed on laterally to both vessels, A and B, in order to connect them together. The receiver, B, is kept cool by being immersed in melting ice. As soon as the stop-cocks are opened, the liquefied gas distils over; the stop-cocks are then closed, the vessels A and B are separated, and a fresh charge is introduced into the generator.* The same operations are then repeated, until a sufficient quantity of the liquefied gas has been obtained. Communicating with the stop-cock of the receiver is the tube, b, which passes down nearly to the bottom of the vessel, and terminates in an open end, so that as soon as the stop-cock is opened, a jet of the liquid anhydride is, by the pressure of its own vapour, forced up the vertical tube, b, and it escapes from the vessel through a fine nozzle, e, which is screwed to the stop-cock. The issuing liquid immediately begins to evaporate with great rapidity; by this means so large a quantity of latent heat is carried off in the escaping gas, that a portion of the liquid is converted into the solid form. If the jet of liquid be made to play into a cylindrical box, D, furnished with lateral apertures for the free passage of the gas, the solidified anhydride may be collected in the form of a flocculent deposit, of snowy whiteness, which gradually evaporates in the air, without undergoing previous liquefaction. This may be seen by placing a few flakes of the solid in a retort, the mouth of which is immersed in water: the gas, as it rises in bubbles, can thus be collected.

If means be taken to cut off the supply of heat from external objects by placing the solidified anhydride in a glass vessel,

*By attaching a condensing syringe to the bottom of the generator, by means of which water may be pumped into the vessel, Mr. Addams displaced the condensed gas, and thus saved a considerable portion which would otherwise have been lost.

196.]

SOLIDIFICATION OF CARBONIC ANHYDRIDE.

411

-

covered externally with flannel, enclosing this in a second glass, and covering the whole with a card, and thus making, in fact, an extemporaneous ice-pail, the solidified anhydride may be kept in open air for some hours. As will readily be supposed, the temperature of this solid is extremely low. According to the experiments of Faraday, it is as much as 106° (-77° C.). It may, notwithstanding, be handled with impunity, and may be put into water, without causing the water to freeze. These paradoxical effects are, however, easily explained. The cold solid never really touches either the water or the hand, because owing to the rapidity with which it evaporates, it is constantly surrounded by a badly conducting atmosphere of its own vapour; but if it be really brought into contact with any solid or liquid, which may be done by moistening the solid anhydride with ether, which has a strong adhesion to carbonic anhydride, its low temperature is at once manifested, and this low temperature is maintained by its continual evaporation, which constantly carries off a large quantity of heat in the latent state. By placing some mercury in a basin, pouring on it a small quantity of ether, and adding a little solidified carbonic anhydride, the mercury will, in a few seconds, be converted into a malleable solid, although before the metal will freeze it is necessary that the temperature be reduced as low as 37°9 (-38-8° C.). If the frozen mercury be transferred to a vessel containing a small quantity of water, the metal will be quickly thawed, but spicule of ice will be formed, showing that the process of liquefaction in the case of mercury, as in all other instances, is attended with a disappearance of heat.

By accelerating the evaporation of the bath of carbonic anhydride and ether, Faraday was enabled to command a still greater reduction of temperature, and in the vacuum of the air-pump he obtained by this means a degree of cold which he estimated at -166° (-110° C.). In such a cold bath, many of the liquefied gases were frozen, and were obtained in the shape of solids, clear and transparent as ice. Among the number which assumed this form was carbonic anhydride itself. (Phil. Trans. 1845, 155.) Even without the aid of pressure, but simply by employing a bath of carbonic anhydride and ether in the air, the following gases— viz., chlorine, cyanogen, ammonia, sulphuretted hydrogen, arseniuretted hydrogen, hydriodic acid, hydrobromic acid, and carbonic anhydride—were obtained in the liquid form, and were sealed up in tubes. The tubes used were of green bottle-glass, bent as represented in fig. 152; to these tubes brass caps and stop-cocks were, when necessary, securely attached, by means of a resinous