Copy 1

The London dispensatory / By Anthony Todd Thomson.

  • Anthony Todd Thomson
Date:
1811
Catalogue details

Licence: Public Domain Mark

Credit: The London dispensatory / By Anthony Todd Thomson. Source: Wellcome Collection.

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    independent of the presence of air, and is a simple effect of caloric. Aériform substances are not susceptible of ignition. The degree of temperature at which al! bodies capable of ig- nition begin to be ignited, or become red hot, is ne-rly the same,—about the 800dth degree of Fabrenheit; and by raising the temperature the illumination increases, until a perfectly white light 1s produced, which is the highest point of ignition. It 1s excited by percussion and friction, as well as by the direct communication of caloric to substances; as in the case of flint and steel, &c. Ignition is supposed to arise from the extrica- tion of the light, which is regarded as a constituent of the ig- nited body, by the repulsive agency of the additional caloric ; but this explanation of the phenomenon is liable to some ob- jections, and the real cause remains still undetermined. As a pharmaceutical agent, caloric is of the first import- ance; im some cases producing decomposition, in others favouring combination. The decomposition most easily ef- fected by it is the separation of the more volatile from the more fixed ingredients of compounds. Thus in the pro- cess of distillation (see Operations), when weak spirits are heated, the alcohol separates from the water, owing to its superior volatility, and by condensation is obtained as a distinct substance. Almost all compounds into which oxygen has entered without. having occasioned combustion, as nitric acid, byperoxymuriatic acid, and some metallic oxides, suffer likewise decomposition by caloric. All compound bodies cons, taining combustibles are also decomposable by it; as are also compounds consisting of two or more combustible ingredients, in combination with oxygen, as almost all animal and vegeta- ble matters. On the contrary, the compounds which are little or not at all affected by caloric, are those which have been formed by combustion; such as water, phosphoric acid, and carbonic acid’, The proper application of caloric for the purpose of obtaining new combinations by favouring the attraction of af- finity, and lessening the force of aggregation ; or for producing decompositions by weakening or destroying altogether the force of these attractions, so as to obtain the principles of bor dies in a distinct state, constitutes the most important feature of operative pharmacy. (See Operations.) g. Licur. Light isa substance consisting of very subtle particles which are constantly emanating in’ straight lines from luminous bo- dies, The size of these is too minute to be appreciated; but rr Nt ee ae RS ‘ 3 Thomson’s Chemistry, @th edite i. 54G. dg
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