But it may happen, that the above salt is so mixed with alimentary matter, as to defy the action of the tests; in this case we must endeavour to obtain from it the pure earth, by precipitating the suspected fluids by the sub-carbonate of ammonia; when a _carbonate of baryta_ will fall down, which must be dried on a filter, and calcined with charcoal. CANTHARIDES. _Spanish Flies_—_Blistering Flies_. (_Cantharis Vesicatoria_, Sp. 1, of Latreille.)[337] Cantharides are imported into this country in their entire state, and are so kept in the shops; their form and general appearance are too well known to require description, and they will rarely become the objects of inquiry; in powder, however, they may be presented to us for investigation, and it is therefore essential that the forensic physician should be acquainted with the appearances which they assume in the state of disintegration. This powder has a greenish colour, tinged with grey, and abounding with shining points of a very beautiful green colour, and which may be recognised in whatever state of division the powder may exist, even after it has passed through a silken sieve. Its odour is acrid and nauseous; when thrown on burning coals it emits that peculiar smell, which generally attends the destruction of animal matter by heat. The chemical history of _cantharides_ is still involved in some obscurity; according to _Robiquet_, who has furnished us with the most satisfactory analysis, they contain various fatty principles; the phosphates of lime, and magnesia; and the acetic and uric acids; together with a peculiar crystalline principle, in which the vesicatory properties wholly reside, and to which the name of _cantharidin_ has been given by _Dr. Thomson_.[338] It may be obtained in plates, having a micaceous lustre; when perfectly pure it is insoluble in water, but it is rendered soluble in that fluid, by the presence of a yellow matter which exists in native combination with it; it is very soluble in oils. _Symptoms of poisoning by Cantharides._