PHYSICAL AND LITERARY. 33 



EXPER. XXIL 



Thirty grains, of the marie of Ex- 

 periment 3. were, by burning, reduced 

 to twenty. Ihefe, juil taken fi'om the 

 VoL,in. E fire, 



in their natural flate, as each particle is fuppofed to be 

 furrounded by-a coat cf clay. As to this particular, it 

 muft be confidered, that acids contain a confiJerable 

 quantity of water, which a.9s upon the clay, at the 

 fame time that the acid is adling upon the calcarious 

 earth. Hence it is, that the addition of water to the 

 purer acids, facilitates the refolution of the marks to 

 which they are applied. Befides this, in all pieces of 

 marie, fooie particles of calcarious earth muft be es^^o- 

 fed. The acid, in difTjlving thefe, fets at liberty their 

 entangled air. This now rellorsd to an elaftic fta:e, 

 pulhes every way with great force, breaks the Ihells of 

 clay contiguous to it, and, by that means, expofes tnovs 

 of the calcarious particles to the contad of the acid. 

 Thus the folution is carried on. 



Some of the foregoing reafoning may be applle^i fn 



favour of the following fuppofition. It is probable, that 



•in a compound confifting of calcarious earth and fand, 



I the particles of the former furround and cement thofe of 



f.the latter. From this arrangement it is, that wl-en fach 



I a compofition is calcined, and put into water, the whole 



.relents into powder, tho' only the calcarious earth is 



aflfefired, 



