VOL. LVI.] PHILOSOPHICAL TRANSACTIONS. 315 



first pressed close together, so as to drive out as much of the included air as 

 possible, was filled with fixed air, by means of the bent tube d; one end of 

 which is fixed into the wooden ring, and the other fastened into the mouth of 

 the bottle e, containing marble and spirit of salt. By this means the fixed air 

 thrown into the bladder mixed with the air in the bottle, and came in contact 

 with the fixed alkali. The fixed air was by degrees absorbed, and crystals were 

 formed on the surface of the fixed alkali, which were thrown to the bottom by 

 shaking the bottle. When the alkali had absorbed as much fixed air as it would 

 readily do, the crystals were taken out and dried on filtered paper, and the 

 remaining solution evaporated; by which means some more crystals were procured. 

 N. B, It seemed as if not all the air discharged from the marble was of a 

 nature proper to be absorbed by the alkali, but only part of it; for when the 

 alkali had absorbed somewhat more than -f of the air first thrown into the 

 bladder, it would not absorb any more: but on pressing the remaining air out 

 of the bladder, and supplying its place with fresh fixed air, a good deal of this 

 new air was absorbed. I cannot however speak positively as to this point; as 

 I am not certain whether the apparatus was perfectly air-tight.* 



These crystals do not in the least attract the moisture of the air; as I have 

 kept some, during a whole winter, exposed to the air in a room without a fire, 

 without their becoming at all moist or increasing in weight. Being held over 

 the fire in a glass vessel, they did not melt as many salts do, but rather became 

 white, and calcined. They dissolve in about 4 times their weight of water when 

 the weather is temperate, and dissolve in greater quantity in hot water than cold. 

 It was found, by the same method that was made use of for the volatile 

 sal ammoniac, that these crystals contain -,*oVo of their weight of fixed air, and 

 that 2035 grains of them saturate as much acid as 1000 grains of marble. 

 Therefore these crystals contain more air, in proportion to the quantity of acid 

 they saturate, than marble does, in the ratio of 21 1 to 100. 



Exper. l6. — As these crystals contain about as much fixed air, in proportion 

 to the quantity of acid that they can saturate, as volatile sal ammoniac does, it 

 was natural to expect that they should produce the same effects with a solution 

 of Epsom salt, or a solution of chalk in spirit of salt; as those effects seemed 

 owing only to the great quantity of fixed air, contained in volatile sal ammoniac. 

 This was found to be the real case; for a solution of these crystals in 5 times 



• Pearl-ashes deprived of their fixed air, i. e. soap lees, will absorb the whple of the air discharged 

 from marble ; as I know by experience. But yet it is not improbable but that the same alkali, wiien 

 nearly saturated with fixed air, may be able to absorb onlyiSome particular part of it. For as it has 

 been already shown, that part oT the air discharged from marble is more soluble in water than the 

 rest; so it is not unlikely but that part of it may have a greater affinity to fixed alkali, and be ab- 

 sorbed by it in greater quantity than the rest. — Orig. 



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