378 POISONS, CONTAGIONS, MIASMS. 



are frequently evolved from the earth in cellars, 

 mines, wells, sewers, and other places, are amongst 

 the most pernicious miasms. The former may he 

 removed from the air by alkalies ; the latter, by 

 burning sulphur (sulphurous acid), or by the 

 evaporation of nitric acid. 



The characters of many organic compounds are 

 well worthy of the attention and study both of phy- 

 siologists and pathologists, more especially in rela- 

 tion to the mode of action of medicines and poisons. 



Several of such compounds are known, which to 

 all appearance are quite indifferent substances, and 

 yet cannot be brought into contact with one 

 another in water without suffering a complete 

 transformation. All substances which thus suffer 

 a mutual decomposition, possess complex atoms ; 

 they belong to the highest order of chemical com- 

 pounds. For example, amygdalin, a constituent of 

 bitter almonds, is a perfectly neutral body, of a 

 slightly bitter taste, and very easily soluble in 

 water. But when it is introduced into a watery 

 solution of synaptas, (a constituent of sweet 

 almonds,) it disappears completely without the 

 disengagement of any gas, and the water is found to 

 contain free hydrocyanic acid, hydruret of benzule 

 (oil of bitter almonds), a peculiar acid and sugar, 

 all substances of which merely the elements existed 

 in the amygdalin. The same decomposition is ef- 

 fected when bitter almonds, which contain the 

 same white matter as the sweet, are rubbed into a 



