INTKoIUVTION 37 



pressure will reach a maximum corresponding with the given tempera- 

 ture, and then evaporation will cease. Should steam be added it will 

 be condensed in the liquid ; if its quantity be diminished i.e., if the 

 pressure be lessened, the temperature being constant then evaporation 

 will go on. We shall afterwards discuss more fully these phenomena of 

 dissociation, which were first discovered by Henri St. Claire Deville. 

 We will only remark that the products of decomposition re-cornbine 

 with greater facility the nearer their temperature is to that at which 

 dissociation begins, or, in other words, that the initial temperature of 

 dissociation is near to the initial temperature of combination. 



(b) The influence of an electric current, and of electricity in general, 

 on the progress of chemical transformations is very similar to the 

 influence of heat. The majority of compounds which conduct elec- 

 tricity are decomposed by the action of a galvanic current, and there 

 being great similarity in the conditions under which decomposition and 

 combination proceed, combination often proceeds under the influence 

 of electricity. Electricity, like heat, must be regarded as a peculiar 

 form of molecular motion, and all that which refers to the influence of 

 heat also refers to the phenomena produced by the action of an electrical 

 current, only with this difference, that a substance can be separated 

 into its component parts with much greater ease by electricity, as the 

 process goes on at the ordinary temperature. The most stable com- 

 pounds may be decomposed by this means, and a most important fact 

 is then observed namely, that the component parts appear at the 

 different poles or electrodes by which the current passes through the 

 substance. Those substances which appear at the positive pole (anode) 

 -are called ' electro-negative,' and those which appear at the negative 

 pole (cathode, that in connection with the zinc of an ordinary galvanic 

 battery) are called 'electro-positive.' The majority of non-metallic 

 elements, such as chlorine, oxygen, etc., and also acids and substances 

 analogous to them, belong to the first group, whilst the metals, hydro- 

 gen, and analogous products of decomposition appear at the negative 

 pole. Chemistry is indebted to the decomposition of compounds by the 

 electric current for many most important discoveries. Many elements 

 have been discovered by this method, the most important being potas- 

 sium and sodium. Lavoisier and the chemists of his time were not 

 able to decompose the oxygen compounds of these metals, but Davy 

 showed that they might be decomposed by an electric current, the 

 metals sodium and potassium appearing at the negative pole. 



(c) Certain unstable compounds are also decomposed by the action of 

 light. Photography is based on this property in certain substances (for 

 instance, in the salts of silver). The mechanical energy of those vibra- 



