308 PHILOSOPHICAL TRANSACTIONS. [aNNO 1 766. 



j4ir, ivhich is produced from Alkaline Substances, by Solution in Acids or by 



Calcination. 



Exper. 1. — ^The air produced, by dissolving marble in spirit of salt, was 

 caught in an inverted bottle of water, in the usual manner. In less than a day's 

 time, much the greatest part of the air was found to be absorbed. The water 

 contained in the inverted bottle was found to precipitate the earth from lime- 

 water ; a sure sign that it had absorbed fixed air.* 



Exper. 1. — I filled a Florence flask in the same way with the same kind of 

 fixed air. When full, I stopped up the mouth of the flask with my finger, 

 while under water, and removed it into a vessel of quicksilver, so that the mouth 

 of the flask was entirely immersed in it. It was kept in this situation upwards 

 of a week. The quicksilver rose and fell in the neck of the flask, according to 

 the alterations of heat and cold, and of the height of the barometer ; as it would 

 have done if it had been filled with common air. But it appeared, by comparing 

 together the heights of the quicksilver at the same temper of the atmosphere, 

 that no part of the fixed air had been absorbed or lost its elasticity. The fllask 

 was then removed, in the same manner as before, into a vessel of soap leys. 

 The fixed air by this means, coming in contact with the soap leys, was quickly 

 absorbed. I also filled another Florence flask with fixed air, and kept it with its 

 mouth immersed in a vessel of quicksilver in the same manner as the other, for 

 upwards of a year, without being able to perceive any air to be absorbed. On 

 removing it into a vessel of soap leys, the air was quickly absorbed like the 

 former. It appears from this experiment, that fixed air has no disposition to lose 

 its elasticity, unless it meets with water, or some other substance proper to ab- 

 sorb it, and that its nature is not altered by keeping. 



Exper. 3. — To find how much fixed air water would absorb, the following ex- 

 periment was made. A cylindrical glass, with divisions marked on its sides with 

 a diamond, showing the quantity of water which it required to fill it up to those 

 marks, was filled with quicksilver, and inverted into a glass filled with the same 

 fluid. Some fixed air was then forced into this cylindrical glass, in the same 

 manner that it was into the inverted bottles of water in the former experiments ; 

 except that, to prevent any common air from being forced into the glass along 

 with the fixed, I took care not to introduce the end of the bent tube within the 

 cylindrical glass, till I was well assured that no common air to signify could re- 



* Lime, as Dr. Black has shown, is no more than a calcareous earth rendered soluble in water by 

 being deprived of its fixed air. Lime water is a solution of lime in water : therefore, on mixing 

 lime water with any liquor containing fixed air, the lime absorbs the air, becomes insoluble in water, 

 and is precipitated. This property of water, of absorbing fixed air, and then making a precipitate 

 with lime water, has been taken notice of by Mr. M'Bride.— Orig. 



