46 PHARMACY 



formed without any apparent change in the appearance of the liquid 

 with possible disastrous results. 



In order to avoid this form of incompatibility some knowledge 

 of the chemistry of the agents must be understood. A good working 

 basis is that substances are incompatible if used as tests for each 

 other, or if they are antidotes. 



Physical or Pharmaceutic Incompatibility results in the 

 production of mixtures of unsightly appearance due to physical 

 changes. This is largely a question of solubility and often occurs 

 when solids or liquids are added to solutions, thereby changing their 

 densities. It occurs when there is a combination of such substances 

 as are physically incapable of mixing. The most common physical 

 incompatibilities result from mixing alcoholic solutions of resinous 

 substances with water (fluidextracts, tinctures, spirits, etc., ginger, 

 cannabis, camphor), but may not in any way effect the action of 

 the drugs. 



Physiologic or Therapeutic Incompatibility is where two or 



more drugs are prescribed which are antagonistic or contra-acting 

 to each other, in which case they may almost exactly neutralize each 

 other or one may weaken the action of the other. Arecoline and 

 atropine are good examples, yet no two drugs exactly oppose each 

 other throughout their entire range of action and some latitude is 

 always permitted. 



Incompatibility is a subject very much overdrawn and unneces- 

 sary stress is placed upon it. Although it is possible to find a large 

 number of incompatibilities for any active chemical, but few of these 

 are ever likely to be encountered in prescription writing ; and accord- 

 ing to Bastedo, of these few, the result not infrequently makes no 

 practical change in the medicinal value or is deliberately desired. 

 According to the same author, the following are those most likely 

 to be encountered in the practical use of drugs : 



I. Incompatibility Depending on Change of Solvent. 



A. Precipitate when added to Aqueous Liquids. Substances in 

 alcoholic solution and insoluble in water : as in spirits, fluidextracts, 

 and tinctures, especially resinous ones, like tincture of cannabis, 

 benzoin, myrrh. 



B. Precipitation when added to Alcoholic Liquids. Substances 

 in aqueous solution and insoluble in alcohol: as solutions of many 

 salts (sodium sulphate, ammonium chloride) and mucilage of acacia. 

 Mere insolubility as of oils or bismuth subnitrate in water, makes 

 these really incompatible with the solvent. 



II. Chemical Incompatibilities. Eule 1. Acids and salts 

 of acid reaction are incompatible with alkalies and salts of alkaline 

 reaction and the halogen salts. 



Eule 2.^ Highly oxidized substances, like chromium trioxide 

 (chromic acid), potassium . permanganate, and potassium chlorate 



