The elements of experimental chemistry (Volume 1).

  • Henry, William, 1774-1836.
Date:
Nov. 1814
Catalogue details

Licence: Public Domain Mark

Credit: The elements of experimental chemistry (Volume 1). Source: Wellcome Collection.

Provider: This material has been provided by the National Library of Medicine (U.S.), through the Medical Heritage Library. The original may be consulted at the National Library of Medicine (U.S.)

    62/422
    An increased temperature promotes chemical union by dimin- ishing or overcoming cohesion. Thus metals unite by fusion, and salts are rendered more soluble in water. Whenever heat is an obstacle to combination, it produces its effect by increasing elas- ticity. Hence water absorbs a less proportion of gas at a high than at a low temperature. A reduction of the temperature of elastic bodies, by lessening their elasticity, facilitates their union with other substances. In certain cases, an increased tempera- ture has the combined effects of diminishing cohesion and increas- ing elasticity. When sulphur is exposed to oxygen gas, no com- bination ensues, until the sulphur is heated ; and though the elas- ticity of the gas is thus increased, yet the diminution of cohesion of the solid is more than proportionate, and chemical union ensues between the two bodies. Such are the most important circumstances, that modify the exertion of chemical affinity. Of their influence, sufficient illus- trations have been given to prove, that in every case of combina- tion and decomposition, we are not to consider the force of affinity abstractedly ; but are to take into account the agency of other powers, as cohesion, quantity, insolubility, elasticity, efflorescence, and temperature. By the action of these extraneous powers, Berthollet has endeavoured to explain certain facts which are not easily understood on any other principle. Of these the most im- portant are, lstly, the establishment of proportions in chemical compounds ; and 2dly, the modification produced in the affinities of bodies by chemical union. 1. Independently of these extraneous forces, Berthollet imag- ines that there are no limits to combination, or that two bodies, which are now susceptible of union only in one or in few propor- tions, might, if these forces were annihilated, be united in every proportion. The causes which he has assigned, as chiefly regula- ting proportion, are cohesion and elasticity. To take one of the simplest cases, the proportion, in which a salt can be combined with water, depends on the balance between the chemical affinity of the bodies for each other, and the cohesive attraction of the salt. In this case, then, cohesion is the limiting power. As an example of the influence of this force when more energetic affinities are ex- erted, if we add to diluted sulphuric acid a solution of barytes, a compound is formed, consisting of sulphuric acid and barytes, h, in consequence of its great insolubility or cohesion, is in-