590 PHILOSOPHICAL TRANSACTIONS. [aNNO 1775. 



tity of fixed air in the middle one, these two vessels are to be separated from the 

 lowermost, and the air and water are to be agitated together, to promote their 

 union. If, during the agitation, a stopper be put into the uppermost glass, the 

 descent of the water in it will not show the absorption of the fixed air by the 

 water, as the external atmospherical air will enter below, at the valve, to fill the 

 space which the absorbed fixed air would otherwise leave void. But, on the 

 contrary, if the uppermost vessel be open, during the agitation, the pressure of 

 the atmosphere on the surface of the water in that vessel, will force the water 

 down into the middle one, as fast as the absorption of the fixed air below will 

 allow it room. This latter method may be pursued, when a person wishes to 

 know the quantity of fixed air that the water can absorb; but in common use, 

 it will be better to stop the uppermost vessel, as the air and water may be then 

 more forcibly agitated without inconvenience, and of course the impregnation 

 more expeditiously efl?ected. During the effervescence, the uppermost glass is 

 to remain open, and it is only to be stopped when the agitation is performed. 

 It is not to be expected, that the impregnation will be considerable at first ; it 

 will indeed be necessary to repeat the process, with the same water, 4 or 5 

 times, before it will be highly impregnated. After an agitation therefore, when 

 a stronger impregnation is wished for, the uppermost vessel is to be opened, 

 and raised from the middle one, to allow the water to descend, that was before 

 driven up. When the middle glass is again full, a fresh quantity of chalk is to 

 be put in the lowermost vessel, and the agitation to be repeated, as soon as the 

 effervescence ceases. It is seldom necessary to repeat the process more than 4 

 times, to produce a very strong impregnation ; but should it be thought proper 

 to have the water as highly saturated with fixed air as it admits of, nothing more 

 than a repetition of the same process is requisite. In this account of the appa- 

 ratus, he had purposely confined himself to the method of uniting fixed air 

 with water ; but it is to be observed, that many curious experiments may be made 

 with it, both in chemistry and pharmacy. By its assistance. Dr. N. had been 

 enabled to imitate very perfectly, the common mineral waters, and to make 

 aqueous solutions of substances that were before deemed insoluble in water. 

 These circumstances however he had reserved for a future paper, which he 

 should have the honour to present to the society, as he had not then been able 

 to arrange the several facts, which this apparatus had made him acquainted with, 

 in the manner he could wish. 



p. s. Since the foregoing paper was read. Dr. N. had contrived a glass valve, 

 which seems preferable in some respects to the ivory one. The following is a 

 description of it. It consists of 3 pieces, as in fig. 7- The superior and 

 inferior pieces are perforated, but the middle one is without perforation, having 

 only its upper part convex, and its under part plane. In fig. 8 is a [xirpendicular 



