denoting the whole heat, sensible and latent together, of the substance at the respective temperatures B, C, D.

103. If now it be assumed that hard ice is essentially colder than ice-cold water, we can easily see why two wet pieces of ice will have the water between them frozen when they come into contact. For the ice on both sides of the layer of water will now be colder than it, and hence a new distribution of heat will take place, the consequence of which will be that the water will be frozen, becoming as it were the centre of the block.

104. It might be said that the laws of conduction are against this hypothesis, and that we cannot conceive a piece of ice entirely surrounded for a considerable length of time by water at 32°, or a little over it, to have in its interior a temperature lower than 32°, however small we may imagine this difference to be; but we think this objection must vanish if it be assumed that the supposed intermediate states between ice and water correspond to intermediate quantities of latent heat. For in this case the heat which is conducted from the outside into the body of a block of ice is not altogether influential in adding temperature, since in each small addition of temperature a certain quantity of heat becomes latent. Let us consider, for instance, what would take place if a large mass of sealing-wax were to be gradually melted by agitation in a pan of liquid sealing-wax over the fire. As the heat was conveyed to the lump of wax, envelope after envelope would become liquid and drop off, mixing with the liquid mass until a very small solid nucleus was left: but as long as there was left a solid nucleus, however small, we should surely be entitled to assume that the temperature of the centre of this nucleus was lower than that of the melted wax.

We imagine that there is no impossibility in conceiving that something of the same kind takes place in ice. Heat will no doubt be conveyed into the interior of a block of ice

that is left for a long time in water a little above 32°, but this heat (as remarked by Professor Forbes) will exhibit its action rather in diminishing the size of the block of ice than in completely equalizing its temperature throughout.

105. If we imagine this objection to be obviated by these remarks, there are three questions started by the hypothesis which can only be decided by experiment.

1. Is the interior of a block of ice in fact colder than

the exterior?

2. Is the interior of such a block harder than the exterior? 3. Does soft ice possess more latent heat than hard ice? With regard to the first of these points certain experiments made by Professor Forbes would seem to indicate that the interior of a block of ice is slightly colder than the exterior. For, in the first place, he found that a thermometer buried in the heart of a block of ice fell decidedly below 32° Fahr., and he also found that rapidly-pounded ice was colder than melting ice. This last experiment has been tried by the author of this work with the same result. With regard to the second point, Professor Forbes has remarked that the surface of a block of ice is much softer than hard cold ice. With respect to the third point, Person's deductions from Regnault's experiments are in favour of the view that soft ice possesses more latent heat than hard ice. On the whole, we think the gradual liquefaction of ice is a view which appears not only to be supported by analogy, but to be the best explanation of observed facts: nevertheless it would be desirable that this view should be confirmed by further experiments.

106. Substances which change their composition in passing from the liquid to the solid state. When a solid is dissolved in a liquid until it refuses to dissolve any further, we have what is termed a saturated solution. But what is a saturated solution at one temperature will

not be so at another. In general, a hot liquid dissolves more than a cold liquid. The consequence is that, if the temperature of a saturated solution be diminished, we have a deposition of solid matter in the shape of crystals, and the liquid which is left behind is saturated for the reduced temperature. If the solution contain two salts of unequal solubility, of different crystalline forms, and having no chemical action upon each other, a greater or less separation of these two salts may be produced by crystallization; by this means nitre is purified from common salt.

107. Solutions are subject to the same anomalies as water and the like liquids. Thus if we have a solution of Glauber's salt at a high temperature, and if it be allowed to cool gradually and at rest without the admission of air, it will retain the salt in solution, even though the temperature be much reduced. But if it be agitated, or if air be admitted, or, better still, if a crystal of Glauber's salt be dropped into it, crystallization will immediately commence, attended, as in the case of water, with a rise of temperature.

108. If instead of a saturated solution we have a weak solution of certain salts, such as sea water, this, when lowered in temperature, will change its state in a different way. At a temperature which is always lower than the freezing point of water such a solution will freeze, producing nearly pure ice and leaving the salt behind. Mr. Walker, who accompanied Sir L. McClintock in the "Fox," made numerous experiments on sea water: he used cold as a means of separating the salt from the water, but was by this means unable to obtain water of less density than 1.002.

Rudorff has made many experiments on this subject, and finds that in saline solutions generally the freezing point is below 32° Fahr., but the extent to which it is lowered depends upon the nature of the salt.

CHAPTER VII.

Change of State.-Production of Vapour and its Condensation.

109. When sufficient heat is applied to a body it generally assumes the gaseous state; unless it be of such a nature that it will under ordinary circumstances be decomposed before assuming this state. By means of a certain application of electricity, it is probable that the most refractory substances, such as carbon, can be made to appear as gases, although only in very small quantity.

Generally when a solid passes into a gas it first assumes the intermediate state of a liquid, but sometimes its passage into a gas is completed without the intermediate form of liquidity being assumed. This is called sublimation; while the passage of a liquid to the gaseous state goes under the general name of vaporization. In whatever way the gaseous condition is produced it always requires a considerable amount of latent heat. Thus a pound of water at 212° Fahr. will absorb a great quantity of heat before it is entirely converted into steam, although the steam does not possess a higher temperature than 212°. In the same manner as before we may apply the following formula, and say—

Steam at 212° = water at 212° + latent heat of steam.

The latent heat of gases is greater than that of liquids, and we will afterwards shew how it may be measured. This latent heat has to be disposed of in some sensible form, when the gas which possesses it is reconverted into a liquid, and thus the latent heat of gases is of great service in retarding the change from the liquid to the gaseous or from the gaseous to the liquid state, which, but

for the great latent heat of gases, would be inconveniently sudden.

A

vapour

Elastic fluids have been divided into gases and vapours, but the distinction between these is merely conventional. denotes a substance in the gaseous form which at ordinary temperatures appears as a liquid or solid, while a gas denotes a substance which under ordinary circumstances appears in the gaseous form, and which can only be reduced to the solid or liquid form by intense pressure or intense cold. Our subject may be divided into the following parts.

1. Vaporization, or the conversion of a liquid into a gas; and sublimation, or the conversion of a solid into a gas.

2. Liquefaction and solidification of vapours and gases. 3. Elasticity and density of vapours and gases, with a few remarks upon hygrometry.

VAPORIZATION AND SUBLIMATION.

110. Vaporization is the general name for a process of which there are three varieties, namely

1. Evaporation, where a liquid is converted into a gas quietly, and without the formation of bubbles.

2. Ebullition, where bubbles of gas are formed in the mass of the liquid itself.

3. Vaporization in the spheroidal condition, where a liquid evaporates slowly, although in apparent contact with a very hot substance.

111. Vapours are formed in vacuo more readily than in air. The presence of air or of any foreign gas retards the formation of vapours, but in vacuo a liquid is very quickly converted into vapour. If a small quantity of water, alcohol, or ether be introduced up through a barometer tube into the Torricellian vacuum at the top, as soon as it reaches this it is converted into vapour, which shews itself