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26

Elements of Pharmacy.

part i.

same, — about the 800th degree of Fahrenheit; and by raisingthe temperature the illumination increases, until a perfectlywhite light is produced, which is the highest point of ignition.Ignition is supposed to arise from the extrication of the light,which is regarded as a constituent of the ignited body, by therepulsive agency of the additional caloric: but this explanationof the phenomenon is liable to some objections, and the realcause remains still undetermined.

As a pharmaceutical agent, caloric is of the first importance:in the majority of cases it produces decomposition; but in someit favours combination. The decomposition most easily effectedby it, is the separation of the more volatile from the more fixedingredients of compounds. Thus, in the process of distillation(see Operations), when weak spirits are heated, the alcoholseparates from the water, owing to its superior volatility, and,by condensation in a different vessel, is obtained as a distinctsubstance. Almost all compounds into which oxygen hasentered without having occasioned combustion, as nitric acid,hyperoxymuriatic acid, and some metallic oxides, suffer like-wise decomposition by caloric. All compound bodies contain-ing combustibles are also decomposable by it; as are alsocompounds consisting of two or more combustible ingredients,in combination with oxygen, as almost all animal and vegetablematters. On the contrary, the compounds which are little ornot at all affected by caloric, as far as regards their composi-tion, are those which have been formed by combustion; suchas water, phosphoric acid, and carbonic acid. 1 The properapplication of caloric for the purpose of obtaining new combin-ations by lessening the force of aggregation, and thus favour-ing the attraction of affinity; or for producing decompositionsby weakening or destroying altogether the force of theseattractions, so as to obtain the principles of bodies in a distinctstate, constitutes the most important feature of operative phar-macy. (See Operations.)

2. Light.Light is a substance consisting of very subtle particles, whichare constantly emanating in straight lines from luminous bodies.Some philosophers, however, following Hooke and Huygens,believe that light is merely the state of undulation of a highlyrare, elastic substance, which it is supposed fills the whole ofthe universe: but this hypothesis is inconsistent with the fact,that a ray of light which has suffered ordinary refraction inpassing through a crystal in one direction, should suffer ordi-nary refraction in passing through it in another direction ;whilst this fact perfectly accords with the idea that light consists

1 Tftomscm's Chemistry, 4th edit. i. 546.