44 Experiments and Ohfervations 



deprived of its acid, is rendered cauftic, and will therefore 

 greedily imbibe the acid from the air, after which it will be 

 brought back to its original flate, and will form new combina- 

 tions with alkaline vapours, and the fame procefs will be 

 continually repeated, till it is itfelf diffolved in water, borne 

 away by the air, or abforbed by plants, to all ■which cafualties it 

 isfubje£l. That acids exift in the air is proved, i ft. From their 

 being capable not only of evaporation, but of being rendered 

 permanently elaltic, and even in this ftate capable of combining 

 with alkalies, which can in like manner be rendered perma- 

 jiently elaftic. From their union, a white cloud is produced, 

 which contains a neutral fait. 2d, If quick-lime is kept dry, 

 and expofed a long time to the air, it will become effete, and 

 lofe its caufticity by its re-union with an acid. 3d, If a cloth 

 moiftened by a ftrong folution of pot-afbes is expofed to the 

 air, it will be covered with faline chr)'ftals of vitriolated tartar. 

 4th, If the earth from which falt-petre has been made is expofed 

 to the air, it will recover the nitrous acid it had loft. In 

 fome parts of the Eaft-Indies falt-petre is made by fetting 

 fire to a long grafs which grows on the declivities of hills ; that 

 afties fexves as the alkaline bafe of nitre (which is a neutral fait 

 compofed of an acid and an alkali) the acid of which ftoats in 

 the air and combines itfelf with the alkali, forming the fait which 

 is waflied down by the rains and received in refervoirs at the 

 foot of tlie hills^ where the water is evaporated and the fait 

 chryftalized. 



