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(207) Indirect Methods of Estimating Calorific Equivalents.The difficulties experienced in effecting the direct combustion of the metals in oxygen, chlorine, iodine, and sulphur, in such a manner as to ensure the perfect conversion of the metal into a given compound, unmixed with any other body of higher or lower degrees of oxidation, &c., are so considerable, that Favre and Silbermann were led to attempt the solution of this problem by indirect means, upon a principle previously suggested and applied by Dr. Woods (Phil. Mag. 1851 [4], ii. 268). An examination of one of the methods employed in the case of the oxides will furnish an idea of the general principle upon which they proceeded.

Whenever a metal is acted upon by an acid, or when one metal is employed to precipitate another metal from any of its salts, as when zinc is dissolved in sulphuric acid, or when copper is precipitated by means of zinc from a solution of its sulphate, heat is evolved. The calorific effects thus obtained are, however, complicated results: for several chemical processes concur in each operation, some of these processes being attended with the absorption, others with the evolution of heat. The calorimeter, of course, only measures the difference of these quantities.

Now, if it be assumed that the quantity of heat which is absorbed when a compound is separated into its elements is the same as that evolved in the formation of that compound, it becomes possible to calculate the value of the calorific action of any one particular chemical operation in the entire process, provided that the heat produced or absorbed in the other portions of the process be determined by other experiments. Suppose, for instance, we take the case of the solution of zinc in dilute sulphuric acid-the elevation of temperature observed will be the resultant of the following operations :

In the first place, heat is evolved by the combination of an equivalent of zinc with one of oxygen: let this amount of heat=x.

Secondly, heat is produced by the solution of the zincic oxide in sulphuric acid: let this a.

=

Thirdly, heat is absorbed by the separation of the oxygen and hydrogen during the decomposition of a quantity of water equivalent to that of the zinc dissolved: let this b.

If T be the number of heat units indicated by the rise of temperature observed in the calorimeter, supposing a and b to be known from previous experiments, it is obvious that r= T-a+b.

207.]

INDIRECT MEANS OF ESTIMATING HEAT EVOLVED.

433

Experiment shows that T, the heat evolved during the solution of 1 gramme of zinc, is equal to 5679 heat units. The solution in sulphuric acid of 1 gramme of zinc after its conversion into oxide, gave for a a quantity equal to 335'54; and b, the heat absorbed during the decomposition of a quantity of water equivalent to a gramme of zinc, was found by another experiment to be equal to 106037 units, or

34462 the calorific equivalent of hydrogen

or

=1060'37;

325 the chemical equivalent of zinc consequently, x, the heat attendant on the oxidation of zinc, is thus obtained :

Heat Units.

T= 567·90 +b=1060*37

1628.27

-a= 335'54

x=1292'73

This number agrees very closely with the direct determination by Andrews and by Dulong, both of whom burned the metal in oxygen. The experiments of Andrews would give the number 1305, and those of Dulong 1304. But although the results agree very well in the present instance, the divergences are much greater in the case of iron and of copper.

The following are the results deduced by Favre and Silbermann, by operations conducted upon this principle; the quantities of heat evolved being calculated for 1 gramme of each element, when combined with a single equivalent (0=8) of the bodies with which it is united.

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Notwithstanding the confidence with which these numbers

are put forward by their authors, it must be admitted that the data necessary for the calculations by which they were obtained

are as yet very incomplete. The latent heat of oxygen in the gaseous state is unknown, and other important numbers are wanting the results given in the foregoing table cannot, therefore, at present be received without great reserve.

:

(208) Mercurial Calorimeter of Favre and Silbermann.-Most of these experiments were performed by the aid of a mercurial calorimeter (Ann. Chim. Phys. 1852 [3], xxxvi. 33). This instrument may be regarded as a mercurial thermometer, with a very large bulb capable of receiving within it the substances which were submitted to experiment. It consists of a large iron or glass globe, A, fig. 159, of the capacity of about a litre, provided with

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three apertures, one at the top and two at the sides. Into one of the lateral apertures, b, is fixed obliquely a tube of thin iron or of platinum, closed at the bottom; and into this tube, which is called the muffle, is introduced another tube, c, of very thin glass, containing the substances which are to be submitted to experiment: this glass tube is fitted into the metallic tube by means of a cork, d; a small quantity of mercury is placed within the muffle, the object of this expedient being to transmit the heat rapidly from the glass tube to the body of the calorimeter. The second lateral aperture, e, terminates in a neck which is curved vertically upwards, and into which is cemented the bent extremity of a horizontal capillary tube, f, of uniform bore, open at both ends, and of about 500 millimetres in length: by means of this tube the changes in volume of the mercury can be measured upou the scale, g, g. Through the upper aperture of the globe passes a steel piston, h, moved by a screw, by which means the column of mercury in the capillary tube can be reduced at pleasure to the zero of the scale. The globe is itself enclosed in a wooden case, k, lined with swan's down in order to diminish the disturbing effects of external changes of temperature.

The value of the amount of expansion indicated was at once transformed into units of heat, by ascertaining the amount of ex

208 a.]

BUNSEN'S CALORIMETER.

435

pansion produced by the cooling of a given quantity of water from the boiling-point to a measured degree of temperature: by multiplying the number of grammes of water by the number of degrees Centigrade which it had lost in cooling, the number of units of heat was ascertained; since by our definition, a unit of heat is the quantity of heat required to raise 1 gramme of water 1°C. The number of millimetres by which the mercurial column had advanced in the capillary tube during the operation was next accurately measured; and by dividing this measured column by the number of heat units, the instrument was graduated so as to enable the observer to record at once the number of units of heat disengaged or absorbed during any chemical change.

This apparatus is excellent in principle, but it is open to certain objections in the mode of its construction :-the sides of the glass vessel are necessarily thick, to enable it to sustain the large weight of mercury with which it is filled; the glass, therefore, cannot rapidly and certainly adjust itself to the temperature of the hot mercury with which it is in contact. Moreover, the tubes are cemented into the three openings with mastic or marine glue. The apparatus to work well, shonld have been filled like a barometer or thermometer, since the presence of even a small bubble of air would materially affect the accuracy of the results. It is true that it is stated that the globe was filled with mercury in vacuo, but with cemented joints this precaution would soon be rendered useless. It is therefore necessary, in estimating the amount of confidence due to the results obtained by its use, to bear in mind these possible sources of inaccuracy. This is the more necessary, since it is principally in the numbers obtained by the use of this apparatus that the results of Favre and Silbermann differ from those of Andrews. At the same time it is to be remarked, that the results published by the French observers appear to be very consistent with each other.

(208 a) Bunsen's Calorimeter.-Bunsen has devised a very ingenious form of calorimeter, in which heat is measured by the amount of contraction which takes place during the liquefaction of ice (Pogg. Ann. 1870, cxli. 1). A cylindrical bulb of glass is blown, and in its axis a test tube, open at the top, is sealed. To the bottom of the cylinder a U tube is fused, and this latter communicates, by means of a cork, with a long horizontal capillary tube, graduated in millimetres. The lower part of the cylinder and the U tube contains boiled mercury, and the upper part of the cylinder is filled with water free from dissolved air. The whole apparatus except the capillary tube is surrounded with

snow or powdered ice, and a current of alcohol cooled in a freezing mixture is made to pass rapidly through the test tube (by fitting a cork with two tubes to the mouth of the tube); this causes a solidification of some of the water surrounding the test tube, and the cooling is continued until a considerable quantity of ice is formed. During the formation of the ice the increase of volume causes a depression of the mercury in the cylinder, and a corresponding movement of the mercury along the graduated tube. When the apparatus is to be used the alcohol is removed from the test tube, a plug of cotton wool attached to a platinum wire is passed to the bottom, and a small quantity of water introduced; the cylinder being kept in snow or powdered ice. When the column of mercury in the capillary tube has become stationary its position is noticed, and the body in which it is required to determine the amount of heat is dropped into the test tube on to the cotton wool, which thus prevents the fracture of the tube, and enables the operator to remove the substance more readily. The tube is then closed with a cork.

As the apparatus is not raised in temperature by the operation, there is no fear of loss of heat by radiation; hence one great source of the error in ordinary calorimeters is removed. The apparatus is also more delicate than any other hitherto described, as will be seen by the following experiment:-0'4 grm. of brass heated to 37° and plunged into 20 grammes of water at o' C., raised the temperature, as indicated by a thermometer, o'07° C. : but when placed in the calorimeter the mercury in the graduated tube was drawn back to the extent of 20m This apparatus promises to be of great service in the determination of specific heats and for many other purposes.

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(209) On the Heat evolved during Metallic Precipitations.— On the other hand, it must be stated that the varied and careful experiments of Andrews (Phil. Trans. 1848, 91) upon the heat evolved during the precipitation of several metals from their salts by the action of other metals, furnished numerical results differing from those calculated by Favre and Silbermann. In the experiments of Andrews, the corrections required are not in all cases completely under exact experimental control; in the displacement of copper by lead the correction amounts to one-eighth of the whole increment of heat, but in other instances the correction is trifling, not exceeding one-fiftieth of the amount of heat evolved; the numbers obtained are mutually consistent. An additional test of the accuracy of this method is afforded by the data furnished in two different series of experiments upon the amount of heat

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