was said on page 285, that the quantity of heat expended in the formation of vapour, is the same whether the vaporization takes place in vacuo or in a space filled with air, and at whatever temperature the vapour may be formed.

When a volatile liquid, like ether, is to be kept boiling for a considerable time in contact with another substance without allowing any of it to escape, a glass tube open at both ends is attached to the flask and kept cool with water by means of Weigel's condensing apparatus, or merely surrounded with moistened paper, so that the vapour which rises into the tube may be condensed and run back into the flask. Mohr's ap paratus (Ann. Pharm. 18, 232); also Carriol and Berthemot's apparatus (J. Pharm. 18, 112).

b. Liquefaction or Solidification by the Affinity of Ponderable Bodies for the Ponderable Base of the Gas.

If any ponderable body, whether it be in the solid, liquid, or gaseous state, has a stronger affinity at the given temperature for the ponderable substance contained in a gas, than heat has for the same substance, and if this stronger affinity is able to exert itself (pp. 36....38), the two ponderable substances combine, and the latent heat of the gas (or of both gases, as the case may be) is rendered sensible, provided the new compound is not itself gaseous.

To this case belongs the condensation of oxygen gas by the combination of oxygen with hydrogen, boron, phosphorus, selenium, and metals; that of chlorine by combination with phosphorus, selenium, iodine, and metals; of acid gases by ammoniacal gas and many other salifiable bases; of all gases by water, alcohol, and other liquids; of aqueous vapour by acids, salifiable bases, salts and other bodies; of alcohol and ether vapours by water, oil of vitriol, fat oil, or camphor, &c.

When aqueous vapour at 100° is brought into contact with pulverized salts, citric acid, hydrate of potash, or sugar, these substances absorb part of the water and form a solution, the temperature of which is several degrees above 100°, and approaches very closely to the temperature at which the aqueous solutions of these substances boil (p. 269). Thus also alcohol vapour of 83-3° raises the temperature of chloride of calcium to 99°. (Faraday, Ann. Chim. Phys. 20, 320.) Aqueous solutions of acids and salts enclosed together with water in a vessel full of air at the ordinary temperature, absorb so much the more vapour of water, as their boiling points are higher: solutions of different substances whose boiling points are the same often absorb equal quantities of aqueous vapour. (Graham, Edinb. J. of Sc. 8, 326.)

Bonsdorff's Evaporating Receiver. (Pogg. 15, 604.) A basin filled with oil of vitriol is placed on a large glass plate, a number of cup-shaped glasses arranged in the basin, and on the top of these above the oil of vitriol are placed dishes containing the liquid to be evaporated. A glass bell jar is inverted over the whole, and kept close to the plate by means of grease. The evaporation goes on gradually at the ordinary pressure and temperature.

The hygrometer of De la Rive (Bibl. univ. 28, 285) is founded on the different degrees of temperature indicated by a thermometer exposed to the air after immersion in oil of vitriol-the rise of temperature being produced by the condensation of the aqueous vapour in the air, and varying with the degree of humidity. It has however been shown by Gay

VOL. I.

U

Lussac (Ann. Chim Phys. 30, 89) that the density of the air likewise influences the rise of temperature.

Woulfe's Apparatus. To cause gases to combine with liquids, they are conducted by means of tubes twice bent at right angles, from the flask or retort in which they are generated, through a number of bottles, Woulfe's Bottles, which are furnished with two or three openings. The liquid contained in these bottles absorbs the gas as it enters in single bubbles. To prevent the liquid from passing back, the third opening is fitted, either with a straight tube open at both ends and dipping into the liquid, or else with a Welter's Safety-tube, i. e., a tube twice bent, and having in the middle a bulb containing water.

Degrees of Incandescence, according to Pouillet.

525°: incipient redness;-700°: dull red; 800° commencing cherryred; 900° brighter cherry-red; 1000° full cherry-red; 1100° dark yellowish red; 1200 bright ignition; 1300° white heat; 1400° strong white heat; 1500... 1600 dazzling white.

A. By alteration of the state of Aggregation.

It has already been shown that the passage of a solid body to the liquid state, and that of a liquid or solid to the gaseous state, is always accompanied by absorption of heat; and that when these substances are brought back to their former condition, the heat is again set free.

B. By the combination of Ponderable Bodies one with the other, and decomposition of the resulting Compounds.

The combination of ponderable substances, with or without the destruction of existing compounds, is always accompanied by disengagement or absorption of heat.

The most striking instances of the Development of Heat are those which accompany the production of light, as noticed on page 181, those namely which take place in the combination of oxygen and chlorine with metals and other combustible bodies,-of bromine, iodine, sulphur, and phosphorus with several metals; likewise in the combination of strong acids with strong salifiable bases, and in that of strong acids and strong bases with water,-in all cases, that is to say, in which combination is brought about by strong affinity, and in which the combining bodies are decidedly opposite in chemical nature.

To determine the quantity of heat set free in combustion, the process is made to go on in a space surrounded with water or ice, and the quantity of ice melted, or the rise of temperature in a given quantity of water, observed.

In the following table, Column A contains the name of the combustible substance;-B its symbol or formula;-C the formula of the compound produced during the combustion;-D shows how many parts of water are heated 1° by the combustion of 1 part of the combustible body, or the number of degrees by which the temperature of 1 part of water would be raised;-E the name of the observer;-F shows how many parts of water are heated 1°, when 1 part of oxygen combines with the combustible substance.-If we would find how many units of heat are developed by the burning of 1 atom of the combustible body, we must multiply the number in column D by the atomic weight of the substance: similarly by multiplying the numbers in column F by 8, we obtain the quantity of heat which 1 atom of oxygen develops in combining with the different substances.

Cr denotes Crawford;-Da: Dalton;-Dg: Dulong;-Dz: Despretz;—

Hs: Hess;-Lv: Lavoisier;-Rf: Rumford. When two results by the same observer are given, the first is denoted by ', the second by 2.

¶ I have likewise added the more recent determinations by Grassi (Gr), Favre & Selbermann (FS), and Andrews (A).

The quantities of heat furnished by other kinds of wood are intermediate between those yielded by oak and elm. (Rumford.) [For more on this matter, vid. Woody Fibre.] One part of oxygen produces with phosphorus, zinc, or tin 5325 units of heat, the same quantity therefore as with iron, and 14 times as much as with carbon. The quantity of heat evolved by charcoal during its combustion is the same under whatever pressure the oxygen gas may exist. Since the volume of carbonic acid gas produced is equal to that of the oxygen consumed, it follows that oxygen gas and carbonic acid gas must contain the same absolute quantity of heat. In the combustion of metals, on the other hand, in which oxygen suffers condensation, the quantity of heat must be greater as the pressure to which the gas is subjected is less. (Ann. Chim. Phys. 37, 180 and 182; also Pogg, 12, 519 and 520.) Dulong's determination of the