Elements of Pharmacy.

part 1.

ance it affords in overcoming the cohesive attraction, as hasbeen already noticed.

2." In liquid bodies this force also operates, but in a lessdegree than in solids, their particles being at greater relativedistances, and moveable with regard to each other by a verysmall force; but as their mobility does not change their relativedistances, they remain within the sphere of this attraction, andare kept together. The exertion of this power varies in dif-ferent liquids : it is greater in mercury than in water, and inthis than in alcohol. It offers, however, scarcely any resist-ance to the combination of fluids with other bodies: andthence, the mutual affinity of two bodies is always favoured,when one of them is in the liquid state.

3. This attraction is not exerted over aeriform substances ;for while these remain at the temperature necessary for thepreservation of their aerial state, their particles mutually repeleach other, and would recede to an indefinite distance, werethey not prevented by the pressure of the surrounding bodies.Thus, a portion of air which can be contained in a vessel of1 cubical inch of capacity, will fill a vessel of 100 cubicalinches of capacity, if the pressure which confines it within thesmaller vessel be removed.

One of the most important results of this variety of conti-guous attraction, in a pharmaceutical point of view, is theformation of crystals, or the regular and determinate figuresassumed by many bodies in passing from the fluid to the solidstate, when nothing opposes the union of their particles ac-cording to the laws of aggregation.

The process of crystallization requires that the particles ofthe substance to be crystallized become moveable; and, conse-quently, in order to obtain any body in a crystalline state, itmust first be rendered fluid, either by solution in a liquid, orby fusion.

The crystallization of salts is usually effected in the. firstmethod. When a salt is much more soluble in hot water thanin cold, as is the case, for example, with sulphate of soda,nothing more is required for its crystallization, than to saturateboiling water with the salt, and set the solution aside to cool.As the caloric is dissipated, the saline particles graduallyapproach each other, and uniting, form solids of that regularshape which characterizes the crystals of this peculiar salt.But when the salt is almost equally soluble in hot and in coldwater, as muriate of soda, for instance, its crystallization canbe effected only by evaporating a part of the fluid; and themore slowly this takes place, the mutual attraction of the par-ticles is more regularly effected, and the more regular is the

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