194 Dr. Play fair on Transformations 



atoms of water which are feebly attached. In fact we know 

 that the addition of alcohol equally aids the precipitation, 

 the action here being a chemical separation of the water, 

 as in the other it is mechanical. The diminished solubility 

 of the salt, after it has been influenced by cohesion and sepa- 

 rated from water, has its counterpart in many similar in- 

 stances ; for example, in the small solubility of anhydrous 

 sulphate of iron. The effect of agitation on a solution of 

 sulphate of soda, saturated while hot and allowed to cool, I 

 ascribe to the same cause. The supposed effect of cohesion 

 or elasticity in these cases is nothing more than that con- 

 stantly observed in ordinary phagnomena, when the gravity 

 of a substance is different from that of the medium in which 

 it exists. The vesicles of water in the atmosphere may be so 

 small that they float in it and produce fogs ; but when ag- 

 gregated together by the motion of the air, they form drops, 

 which precipitate to the ground with a rapidity proportionate 

 to their size : the converse of this is also true. Thus, the 

 particles or aggregated atoms of carbonic acid in water may 

 be so very small, that, with the slight affinity of the latter 

 added, they may be enabled, when in a state of rest, to re- 

 main without resuming their elastic form; but agitation 

 causes a larger system of aggregated atoms, and the gas now 

 escapes in small bubbles. 



The first instance of cohesion applies in the precipitation 

 of tartar. At the moment of formation the particles may be 

 so widely apart, that, aided by their slight affinity for water, 

 they remain without aggregating to any considerable extent. 

 Brisk agitation, and the presentation of an extended surface, 

 effect their aggregation and cause a speedy precipitation. It 

 may be that these are really instances of combination favoured 

 by motion ; but presuming that they are, the general argu- 

 ment is not aflfected, that other decompositions perfectly ana- 

 logous are produced where the exciting body is in a state of 

 rest. 



The third theory of these decompositions is, that catalytic 

 bodies act by exerting a feeble chemical affinity on one of the 

 constituents of the body decomposed. This view was intro- 

 duced by Mercer *, and supported by several very ingenious 

 experiments communicated to the British Association at its 

 meeting in Manchester. One of these was, that protoxide of 

 manganese had the singular power of hastening the oxidation 

 of starch in nitric acid f. The metallic protoxide, from its 



* Reports of British Association, vol. xi. 2d Part, p. 32. 

 f The experiment is easily made by dissolving 1 ounce oxalic acid in 

 J- a pint of water at 180° F., and adding to this 1 oz. colourless nitric acid of 



