CANTHARIDES. 
§§ 657. 658.] 
501 
an acid anhydride, but H. Meyers’ researches have shown that it is a 
/3-lactone of a ketonic acid, the structural formula of which is :— 
It is soluble in alkaline liquids, and can be recovered from them by 
acidifying and shaking up with ether, chloroform, or benzene ; it is 
almost completely insoluble in water. 100 parts of alcohol (99 per 
cent.) dissolve at 18° *125 part ; 100 of bisulphide of carbon, at the 
same temperature, -06 part; ether, -11 part; chloroform, 1-2 part; 
and benzene, -2 part. Cantharidin can be completely sublimed, if 
placed in the subliming cell (described at p. 264), floating on mercury ; 
a scanty sublimate of crystals may be obtained at so low a temperature 
as 82-5° ; at 85°, and above, the sublimation is rapid. If the cantharidin 
is suddenly heated, it melts ; but this is not the case if the temperature 
is raised gradually. The tube melting-point is as high as 218°. Potassic 
chromate with sulphuric acid decomposes cantharidin with the produc¬ 
tion of the green oxide of chromium. An alkaline solution of permangan¬ 
ate, iodic acid, and sodium amalgam, are all without influence on an 
alcoholic solution of cantharidin. With bases, cantharidin forms crystal- 
lisable salts, and, speaking generally, if the base is soluble in water, the 
“ cantharidate ” is also soluble ; the lime and magnesic salts dissolve 
readily. From the soda or potash salt, mineral acid will precipitate 
crystals of cantharidin ; on heating with pentasulphide of phosphorus, 
0 -xylol is produced. 
§ 657. Pharmaceutical Preparations of Cantharides. — The B.P. 
preparations of cantharides are :— 
Acetum cantharides, or vinegar of cantharides, containing about -04 
per cent, of cantharidin. 
Tincture of cantharides, containing about -005 per cent, of cantharidin. 
A solution of cantharides for blistering purposes, Liquor epispasticus, 
a strong solution of the active principle in ether and acetic acid, con¬ 
taining about -16 per cent, of cantharidin. 
There are also—an ointment ; a blistering paper, Charta epispastica ; 
a blistering plaster, Emplastrum cantharides ; and a warm plaster, 
Emplastrum calefaciens. 
§ 658. Fatal Dose. —It is difficult to state the fatal dose of can¬ 
tharidin, the unassayed powder or tincture having mostly been taken. 
A young woman died from T62 grm. (25 grains) of the powder, which 
is perhaps equivalent to 6*4 mgrms. (1 grain) of cantharidin, whilst the 
smallest dose of the tincture known to have been fatal is (according to 
