SJftfe PHILOSOPHICAL TRANSACTIONS. [aNNO 1774. 



absorbent earths do alsoM-emain suspended in it; but, so soon as any part of this 

 air is expelled by heat, those earths begin to separate from the water, which then 

 becomes white and turbid; and when, by continuance of the heat, more of this 

 air is expelled, more of the earthy particles also separate from the water, in the 

 same proportion as its air is separated from it; and while only a small portion of 

 the air remains, some portion of the martial earth also remains dissolved in the 

 water, as appears from its giving a slight tinge of the purple, when mixed with 

 galls: but none of these earths are any longer detained in the water, than while 

 it continues impregnated with some mephitic air: when this air is entirely sepa- 

 rated from the water, it is wholly decompounded, having lost its distinguishing 

 brisk, and pungent taste, and its power of striking a purple colour with galls ; its 

 more volatile and elastic principles being exhaled, its metalline and absorbent 

 earths then subside in a white flocculent sediment, and no other substance 

 remains dissolved in the water, save only the small portion of alkaline and neutral 

 salts, which enter its composition. 



From this short recapitulation of the abovementioned experiments, it there- 

 fore appears, that the Pouhon water undergoes a decomposition when its air is 

 expelled from it by means of heat. The opposite extreme of cold is also found 

 to produce the same effect of decompounding the Pouhon water, when this its 

 aerial principle is expelled from it by means of congelation. For -having poured 

 some of this water into open tin vessels, that were placed in the common freez- 

 ing mixture of sea salt and snow, as soon as the water began to shoot into ice, 

 at the bottom and sides of the vessels, very minute bubbles of air incessantly 

 arose in it, and were discharged from its surface with such force, as to carry with 

 them small particles of the water to a considerable height; and continued thus to 

 fly off, till all the water was congealed. The ice was very white, from the 

 minute bubbles ef air, which were every where interspersed through it, and by 

 which the frozen water considerably increased in bulk, so as to rise at its surface 

 into a very convex form. The water, when thawed, was white and turbid, and 

 soon let fall its metallic and absorbent earths in a white sediment: it then had 

 almost lost all its taste; and, being mixed with tincture of galls, only gave a 

 slight purple tinge. By a '2d congelation, it seemed almost entirely deprived of 

 its air, and, with it, of the remaining part of its white earths; and, when 

 decanted from its sediment, no longer struck a colour with galls. From these 

 experiments it therefore appears, that as soon as this water is deprived of its air, 

 whether it be by heat or by cold, it is no longer capable of keeping those earths 

 dissolved, which, while it is impregnated with this air, continue suspended in it. 



In these decompositions of the Pouhon water, by heat and by cold, no vola- 

 tile spirit, either acid or sulphureous, nor any other subtile matter, has been 

 found to fly from it, save only its mephitic air: while this air is present in the 



