The American dispensatory : containing the natural, chemical, pharmaceutical and medical history of the different substances employed in medicine, together with the operations of pharmacy / illustrated and explained, according to the principles of modern chemistry.

  • Coxe, John Redman, 1773-1864.
Date:
1827
Catalogue details

Licence: Public Domain Mark

Credit: The American dispensatory : containing the natural, chemical, pharmaceutical and medical history of the different substances employed in medicine, together with the operations of pharmacy / illustrated and explained, according to the principles of modern chemistry. Source: Wellcome Collection.

Provider: This material has been provided by the Harvey Cushing/John Hay Whitney Medical Library at Yale University, through the Medical Heritage Library. The original may be consulted at the Harvey Cushing/John Hay Whitney Medical Library at Yale University.

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    which is a triumph in the art of analysis, for whilst it obviates the necessity of ascertaining the exact proportion of alkali required in each experiment,* it possesses the valuable property of not in the least disturbing the phosphat of soda. (B.) Sulphat of Copper. Like the preceding test, this requires also for its success, that the arsenous acid should be combined with some alkali, in which case, by the operation of double elective at- traction, an arsenite of copper is thrown down of a very striking and characteristic colour, being that of the well known pigment called Scheele's green; if arsenic be not present in the liquid so assayed, and a fixed alkali has been employed, the result will be a delicate sky blue, instead of the grass green precipitate.! Mr. Hume avails himself also of the peculiar property of ammo- nia to form a triple metallic salt, and has employed it with copper: he takes the sulphat or acetat of that metal, and by the same process forms another test. In using this, however, care must be taken that it be not too highly concentrated, for in that state, it will not pro- duce precipitation. J Much controversy has taken place on the sub- ject of sulphat of copper as a test for arsenic, and it has been stated with more confidence than truth, that a decoction of onions has the property of imparting to the copper precipitate, which is produced by a fixed alkali, a colour and appearance analogous to tliat which is occasioned by arsenic. This opinion was boldly advanced and supported on a most important trial at the Lent assizes for Corn- wall, in 1817. Since this event, an opportunity has occurred which has enabled Dr. Paris to examine this alleged fact, by a fair and ap- propriate series of experiments, the result of which satisfactorily proved, that the opinion was founded on an optical fallacy, arising from the blue precipitate assuming a green colour, in consequence of having been viewed through a yellow medium. The phosphoric salts may also, under similar circumstances, be mistaken for arsenic, for the intense blue colour of the phosphat of copper, will thus necessa- rily appear green. This instance of opticw fallacy is not solitary, for corrosive sublimate has been said to possess the character of an alkali because it turns the syrup of violets green, whereas this ehange is to be attributed solely to the combination of the yellow hue of the sublimate with the blue colour of the violet. Whenever, therefore, such a source of fallacy can be suspected, the operator would do well to repeat his experiment on white paper, in the manner Dr. Paris has before proposed, and the results its weight of distilled water, to this add, guttatim, liquid ammonia, until a precipitate is formed: continue cautiously to add the ammonia, repeatedly agitating the mixture until the precipitate is nearly re-dissolved. The object of allowing a small portion to remain undissolved is to guard against an excess of ammonia. Wherever the test is used, the liquid to which it is added ought to be quite cold. * This is very important, for an excess of ammonia re-dissolves the yellow precipitate, and therefore defeats the object of the test. The fixed alkalies, in excess, have not such a property. \ Some objections have lately been raised to this test, in Silliman's Ameri- can Journal of Science, vol. iii. p. 354, which require attention. t This preparation is in fact a solution of the cuprum ammoniatum diluted.