of a solid mass of metal is to be ascertained, it may be heated throughout to a certain degree, and then surrounded by water of 32°, observing the increase of temperature which is gained by the water, and calculating the specific heat in the same manner as before. This was the method of Wilcke; but Lavoisier and Laplace substituted ice for water, placing, by means of an apparatus called the Calorimeter, the heated body in the centre of a quantity of ice, and determining the caloric evolved, by the quantity of ice melted in each in

stance.

When this comparison is extended to a great variety of bodies, they will be found to differ very considerably in their capacities for caloric. The results of numerous experiments of this kind are comprised in a table of specific caloric.*

The capacities of bodies for caloric influence, considerably, the rate at which they are heated and cooled. In general,· those bodies are most slowly heated, and cool most slowly, which have the greatest capacities for heat. Thus, if water and quicksilver be set, in similar quantities, and at equal distances before the fire, the quicksilver will be much more rapidly heated than the water; and, on removal from the fire, it will cool with proportionally greater quickness than the water. By ascertaining then the comparative rates of cooling, we may determine, with tolerable exactness, the specific caloric of bodies.

It has been doubted whether the specific heats of bodies are permanent so long as they retain their form; in other words, whether a quantity of heat, which raises a body through a certain number of degrees at any one temperature, will raise it through an equal number of degrees at other temperatures. This subject, to which Mr. Dalton had formerly turned his attention, has been lately investigated more completely by Petit and Dulong.§ They heated the body to be tried to the required temperatures, and ascertained what number of degrees of heat it communicated to a certain quantity of water. Repeating these trials at various points of the thermometric scale, they found that the specific heats of bodies

See the Appendix.
New System, voi. i. p. 53.

See Martine, on Heat, page 74. § Annals of Philosophy, xiii.

are greater at high than at low temperatures. specific heat of iron was found to be as follows:

Thus the

Specific beat.

0.1098

0.1150

0.1218

0.1255

The same law was found to extend to various other bodies, as is shown by the following Table:

Another important law, deduced by Petit and Dulong, from their researches on heat is, that the atoms of all simple bodies have precisely the same specific heat. This will appear from the following Table, the third column of which expresses differences so small, that they may reasonably be imputed to unavoidable inaccuracies in the method of determining the true weights of the atoms.

The determination of the specific heat of gases is a difficult and important problem, which has successively employed the labour and ingenuity of Crawford, Lavoisier and De la Place, Leslie, Gay Lussac, Dalton, Delaroche and Berard, and Clement and Desormes. The details of the experiments of Delaroche and Berard are given in the 85th volume of the Annales de Chimie, preceded by an historical review of the labours of their predecessors. The following Table contains the general results :

TABLE OF THE SPECIFIC HEATS OF SOME GASES.

Taking water as unity, the specific heats of the gases are as follow:

Table of the Specfiic Heats of the Gases, water being taken

After having thus determined the specific heats of the gases, MM. de la Roche and Berard ascertained that the specific heat of any one gas, considered with respect to its volume

augments with its density, but in a proportion less than the increase of density. On this subject, M. M. Clement and Desormes have given the following results:

153

CHAPTER IV.

OF LIGHT.

THE laws of light, so far as they relate to the phenomena of its movement, and to the sense of vision, constitute the science of OPTICS; and are the objects, therefore, not of Chemistry, but of Natural Philosophy. It may be proper, however, by a brief statement of its physical properties, to recal them to the memory of the reader.

1. The light of the sun moves with the velocity of 200,000 miles in a second of time, in consequence of which it passes through the whole distance between the sun and the earth in about eight minutes.

2dly. While it continues to move through a transparent medium of uniform density, its motion is in perfectly straight. lines, but in passing obliquely out of one medium into another, it undergoes a change of direction. If the new medium be denser than the old, the ray of light is bent or refracted nearer to the perpendicular; but in passing out of a denser into a rarer medium it is refracted from the perpendicular; and there is a constant proportion between the sine of the angle of incidence and that of refraction. Transparent media, also, not only cause a change of the direction of a ray, but decompose it into its constituent parts, an effect which has been called dispersion.

3. In general the amount of refraction is proportional to the density of a body, but inflammable substances cause a greater refraction than might have been inferred from their densities, and the refractive power of the same inflammable substance bears a proportion to its perfection, insomuch that this property may be used as a test of its purity. Thus Dr. Wollaston found that genuine oil of cloves has a refractive power of 1.535, while that of an inferior quality did not ex