99 
I his diy slaking prodaces heat ; and the dilatation is so considerable as 
to bnist casks, 01 other wooden vessels in which lime may be contained. 
These phenomena, says Fourcroy, are occasioned chiefly by the water 
contained in the atmosphere, and the force with which the lime tends to 
unite with it. For by heating lime which has been slaked with air in a retort, 
till it becomes red hot, water is obtained, and the lime returns to its original 
state. 
Water acts very powerfully on quicklime. Pour a small quantity of 
that fluid on a quantity of lime, the water is instantly absorbed, and the 
mass of lime appears as dry as before; but it soon bursts and breaks into 
pieces. 
The heat excited in it by this operation is so strong, as to produce a re¬ 
markable hissing noise: the water is reduced to vapour of a peculiar smell, 
which communicates a green colour to a paper tinged with syrup of violets. 
The lime soon falls down to a powder; and the heat, motion, and smoke 
gradually disappear. If the process be performed at night, in a place per¬ 
fectly dark, a great many luminous points are observable all over the surface 
of the lime. 
This remarkable phenomenon did not escape the active mind of Mayow, 
who, in Chapter XIV. of his elaborate work, treating on the heat from quick¬ 
lime and the union of different salts, remarks, that this substance is a com¬ 
pound of two contrary principles, an acid and an alkali, and that the nitro- 
aerial-particles (oxygen) is so inherent in the acid, and in so dry a state, that 
they are inert, and as it were fixed; but when water pervades these two 
bodies, the laws of affinity then act, and these two opposite natures combine, 
and give out the heat which was before in a dormant state. 
May we not rather attribute this process to the air in the interstices of 
» 
of the water, and in part to a decomposition of the water; or does our 
present knowledge of the nature of lime give a sufficient solution of this 
question ? 
Seeing then that there are great chemical actions going on in the bowels 
of the earth, and observing the slow circulation of air, the heat conse¬ 
quently liberated from these natural processes becomes stagnant, and accounts 
readily for that uniformity observed at certain depths. 
