Phys., 1895, 6, p. 296) heats three parts of the oxide with one part of magnesium powder.
In 1831, from a study of the specific heats of compounds, he formulated "Neumann's law," which expressed in modern language runs: "The molecular heat of a compound is equal to the sum of the atomic heats of its constituents."
Its ratio of specific heats has very nearly the ideal value 1 666, appropriate to a monatomic molecule.
The winds are liable to little variation; they blow from the west, often with great violence, for nine months in the year, and at other times from the north; and they moderate the summer heats, which are chiefly felt during the months of July and August, when the hot winds blow from the coast of Anatolia.
The extreme frosts and heats of the English climate are unknown, but occasional heavy snow-falls occur, and the sea in shallow inlets is covered with a thin coating of ice.
Thus the heat of formation of anhydrous zinc sulphate, ZnSO 4j which cannot be determined directly, may be arrived at by summation (in Hess's units) as follows: Heats of formation are still determined for the most part in a precisely similar manner.
He therefore abstained from determining for each case the specific heats of the solutions he employed, and contented himself with the above approximation.
The accuracy of heats of combustion determined in the closed calorimeter is in favourable cases about one-half per cent.
With knowledge then of the heats of formation of the substances involved in any chemical action, we can at once calculate the thermal effect of the action, by placing for each compound in the energy-equation its heat of formation with the sign reversed, i.e.
Thus if we wish to ascertain the thermal effect of the action Mg+CaO =MgO+Ca, we may write, knowing the heats of formation of CaO and Mg0 to be 131000 and 146000 respectively, 0-131000 = 0-146000+x x =15000 cal.
Since heats of formation afford such convenient data for calculation on the above method, they have been ascertained for as many compounds as possible.
Now we know the heats of formation of carbon dioxide (from diamond) and of liquid water to be 94300 cal.
The oxygen contained in the compound was deducted, together with the equivalent amount of hydrogen, and the heat of combustion of the compound was then taken to be equal to the heats of combustion of the elements in the residue.
It has already been stated that the heats of neutralization of acids and bases in aqueous solution are additively composed of two terms, one being constant for a given base, the other constant for a given acid.
The following table gives the heats of neutralization of the commoner strong monobasic acids with soda: - Hydrochloric acid Hydrobromic acid Hydriodic acid Nitric acid Chloric acid Bromic acid Within the error of experiment these numbers are identical.
The intricate water-ways and the stubborn Venetian defence baffled all his attempts to reach Rialto; the summer heats came on; the Lido was unhealthy.
Thomsen then investigated heats of combustion of various benzenoid hydrocarbons - benzene, naphthalene, anthracene, phenanthrene, &c. - in the crystallized state.
Specific Heat and Composition.-The nature and experimental determination of specific heats are discussed in the article Calorimetry; here will be discussed the relations existing between the heat capacities of elements and compounds.
The following table gives a comparative view of the specific heats and the ratio for molecules of variable atomic content.
The abnormal specific heats of the halogen elements may be due to a loosening of the atoms, a preliminary to the dissociation into monatomic molecules which occurs at high temperatures.
For a further discussion of the ratio of the specific heats see Molecule.
Specific Heats of Solids.-The development of the atomic theory and the subsequent determination of atomic weights in the opening decades of the 19th century inspired A.
Dulong to investigate relations (if any) existing between specific heats and the atomic weight.
This law-purely empirical in origin-was strengthened by Berzelius, who redetermined many specific heats, and applied the law to determine the true atomic weight from the equivalent weight.
At the same time he perceived that specific heats varied with temperature and also with allotropes, e.g.
The specific heats of carbon, boron and silicon subsequently formed the subject of elaborate investigations by H.
Nilson and Pettersson's observations on beryllium and germanium have shown that the atomic heats of these metals increase with rise of temperature, finally becoming constant with a value 5.6.
Trans., 1900, p. 233) investigated nickel and cobalt over a wide range of temperature (from -182.5° to loo°); his results are: It is evident that the atomic heats of these intimately associated elements approach nearer and nearer as we descend in temperature, approximating to the value 4.
Other metals were tested in order to determine if their atomic heats approximated to this value at low temperatures, but with negative results.
We now proceed to discuss molecular heats of compounds, that is, the product of the molecular weight into the specific heat.
Trans., 1904, 203 A, p. 139) for those elements whose atomic heats vary considerably with temperature.
He also showed that the difference of the specific heats at constant pressure and volume, S - s, must be the same for equal volumes of all gases at the same temperature and pressure, being represented by the expression R/TF'(t).
He remarks that ” the law according to which the motive power of heat varies at different points of the thermometric scale is intimately connected with that of the variations of the specific heats of gases at different temperatures - a law which experiment has not yet made known to us with sufficient exactness."
If he had ventured to assume the difference of the specific heats constant, it would have followed that F'(t) must vary inversely as T.
In the course of his inquiries he also noticed that different bodies in equal masses require different amounts of heat to raise them to the same temperature, and so founded the doctrine of specific heats; he also showed that equal additions or abstractions of heat produced equal variations of bulk in the liquid of his thermometers.
Thermal Properties.-The specific heats of most metals have been determined.
The general result is that, conformably with Dulong and Petit's law, the "atomic heats" all come to very nearly the same value (of about 6.4); i.e.
Distillable at red heats: cadmium, alkali metals, zinc, magnesium.
Latent Heats of Liquefaction.-Of these we know little.
The blower repeatedly heats the lower part of the mass of glass and keeps it distended by blowing while he swings it over a deep trench which is provided next to his working platform.
The blower then heats the end of the cylinder again and rapidly spins the pipe about its axis; the centrifugal effect is sufficient to spread the soft glass at the end to a radius equal to that of the rest of the cylinder.
To test the purity of the metal the tin-smelter heats the bars to a certain temperature just below the fusing point, and then strikes them with a hammer or lets them fall on a stone floor from a given height.
