VOL. t,XV.] PHILOSOPHICAL TRANSACTIONS. 58Q 



The Process, — As chalk and oil of vitriol are capable of producing the desired 

 effervescence, and are the most eligible on account of their cheapness, he has, 

 in describing the process, mentioned only these 2 ingredients. Various other 

 substances may however be employed for the same purpose; but none perhaps 

 are so unexceptionable as those named. In the other acids a proper degree of 

 fixity is wanting, during the effervescence ; the nitrous and marine have so 

 much volatility, that there is always a risk of some of the acid fumes passing 

 the valve, and thus rendering the water acid, which it was intended to impreg- 

 nate only with fixed air. To begin the process, it is necessary to fill the vessel 

 A up to the dotted lines, with diluted oil of vitriol. By confining the height of 

 the surface of the effervescing mixture to the dotted lines in the glass a, none of 

 the acid will be driven through the valve, during the intumescence that attends 

 the escape of the fixed air. The glass b is to be totally filled with water, and 

 the vessel c is to be put on it. Some powdered chalk is then to be thrown into 

 the glass a, and the vessels are to be immediately placed as in fig. 5, except that 

 the stopper belonging to c is to be left out. When the acid in the lowermost 

 vessel acts on the chalk, the extricated air passes the valve in the middle glass ; 

 and as the construction of this valve allows the fixed air from the effervescing 

 substances to pass, but denies a passage to the water in a contrary direction, the 

 separated air ascends to the upper part of the middle glass, and at the same time 

 a portion of water, equal in bulk to the intruding air, passes up the bent tube 

 into the uppermost vessel. As the effervescence goes on, the fixed air con- 

 tinues to accumulate in the middle vessel, and the uppermost one to be filled 

 with the water that has given place to the air. The quantity of chalk to be 

 thrown into the acid at one time, must be determined by the capacity of the 

 uppermost vessel. Should more air be extricated than is sufficient, in the con- 

 duce of the process, to fill that vessel, the water will run over the top of it, and 

 will continue to run as long as any air ascends in the middle vessel, or till the 

 surface of the water is below the extremity of the bent tube. Both these acci- 

 dents are to be carefully avoided ; as in one case the whole would be wet and 

 disagreeable ; and in the other, a quantity of fixed air would be unnecessarily 

 lost. Half a drachm of chalk will, in general, produce air enough to fill the 

 uppermost vessel with water ; and it must be remembered, that the chalk em- 

 ployed to produce the effervescence, should be finely powdered, as a selenetic 

 crust will otherwise form around it, and thus prevent the action of the acid on 

 the interior part. To keep the neck of the glass clean, through which the 

 chalk is put, it will be necessary to include the chalk loosely in paper ; and this 

 circumstance is by no means to be neglected, as the accurate junction of the 

 glasses depends on it, and consequently the whole of the process. When the 

 uppermost vessel is filled with water, and there is therefore a considerable quan- 



