CHEMICAL AFFINITY IS UNIVERSALLY DIFFUSED. 19 



as ten or twelve times its volume of that gas. Metallic platinum exhibits the same 

 property in a far higher degree ; for from the atmospheric air, * gaseous mixture 

 in which oxygen forms only the fifth part, that metal (in the form of a black 

 powder) condenses on its surface, at the ordinary temperature, an enormous quantity 

 of oxygen gas (without any nitrogen,) and acquires thereby properties, which it 

 does not otherwise possess.^) And when oxide of chromium, fragments of porce- 

 lain, or asbestus, at high temperatures, effect the combination of two gases, 

 oxygen and hydrogen, or oxygen and sulphurous acid, which gases do not combine 

 at the same temperature, unless when in cqntact with these solid bodies, it is to the 

 chemical attraction or affinity of these solid bodies that we must ascribe this 

 effect. 



The solution of a salt in water is an effect of affinity, and yet no one property, 

 either of the salt or of the solvent, is thereby altered, except only the cohesion of 

 the saline particles. 



Sea salt, the crystals of which are usually anhydrous, takes up, at very low 

 temperatures, 38 per cent, of water of crystallization ;* not because any new 

 cause acts which increases its affinity for the particles of water (for cold is no 

 cause, but the absence of a cause,) but because the higher temperature acted as an 

 obstacle, opposing their chemical combination. The force of affinity is all the 

 time present and undiminished. 



When we add alcohol to the solution of a salt in water, we observe, that now 

 the salt separates from the liquid in the form of crystals, doubtless only because, 

 by the addition of another chemical force, the amount of attraction between the 

 particles of the salt and those of the water has been altered. 



The aqueous particles, which were combined with the saline particles, manifest 

 an attraction for the particles of alcohol ; and as the latter have no affinity, or only 

 a very feeble affinity, for those of the salt, the attraction of the saline particles for 

 each other is strengthened. This attraction was present in equal force before the 

 addition of the alcohol, but the resistance which opposed their union (the chemical 

 attraction for them of the aqueous particles) was more powerful.! The alcohol was 

 not the cause of the separation. The cause of the separation of the salt from the 

 liquid, its crystallization, is at all times the force of cohesion ; and by the alcohol 

 the cause which opposed its manifestation was removed. 



The affinity of potash for sulphuric acid is known, and sulphate of potash 

 readily dissolves in water. If we add, to a saturated solution of that salt, an 

 equal volume of aqua potassae of sp. g. 1.4, there is immediately formed a crystal- 

 line precipitate of sulphate of potash, and the sulphuric acid is separated in this 

 form from the water. 



In these cases the chemical effect (the separation) depends on the presence of a 

 certain quantity of the liquid which is added (such as aqua potassse, alcohol, &c.,) 

 but in many other cases there is required only a slight alteration in the quality of 

 the solvent to effect separations of this kind. 



When hydrochloric acid is added to a solution of ferrocyanide of potassium, 

 ferrocyanic acid is set free, and remains dissolved in the liquid. If now the vapor 

 of boiling ether be passed rhrough the mixture, there occurs, after a few moments, 

 a complete separation. The whole of the ferrocyanic acid is deposited from the 

 liquid in the form of white or bluish-white crystalline scales, which generally 

 appear in such quantity as to render the whole mass semisolid. In proportion as 

 the vapor of ether is dissolved by the water, the latter fluid loses entirely its solvent 

 power (its affinity) for the ferrocyanic acid. The coagulation of albumen by- 

 ether depends on a similar cause. 



The capacity of solids to become moistened by liquids, and, in short, all 



( ! ) According to Dcebereiner, platinum black condenses 252 times its volume of 

 Dxygen. Its effect in oxidizing alcohol, pyroxilic spirit, &c., is familiar to every 

 chemist. W. G. 



* Crystallization of sea salt. 



i Precipitation of salt from its solution by alcohol ; of sulphate of potash by caustic potash; of 

 ferocyanic acid by ether ; of suspended mud by alum. 



