84 Properties of Wood Ashes. 
» 7. Selecting magnesia as an article possessing physical proper= 
ties somewhat similar to those of ashes, I erected a cone of this 
material, and at the summit buried a partially ignited coal. Ina 
few minutes I was surprised to find the whole coal was alive 
with fire. Shortly afterwards the magnesia beneath the coal be- 
came ignited, and the bottom of the heap almost intolerably hot. 
8. Guided by the specific: gravity and the compressibility of 
the substances employed, I repeated the experiment with pulveri- 
zed chalk instead of magnesia. The chalk soon became red hot, 
ee the coal ; and: ne base of the heap, heated. esate eu- 
durane 
Thus anebeeiie that these sllisliee earths ieietiiad the same 
heat-preserving properties as ashes, and that the same downward, 
centralizing tendency of caloric was shown in all, I was led to 
the conclusion that the heat eliminated and diffused in-the sifted 
ashes was the result of the combustion of the single coal buried 
in them; and considering their low ,conducting and. radiating 
power, it appeared probable that the amount of heat apparent was 
not very far from the absolute quantity generated -duting the 
combustion. In every instance, while the central parts of the 
cones were red hot, the exterior of the ashes except at the-apex 
was. cold setiaphoss the experiment. The caloric is evolved 
faster than it is diffused, and of course it accumulates within.a 
small sphere near the coal to an igniting temperature ; combusti- 
ble matter lying at the cireumference of this sphere, would ignite 
and generate another ball of fire, and this produce another, and 
so on indefinitely, or while the last ignited spheres reached new 
combustible matter. In this manner l-conceive the calorie trav- 
elled in the fifth and sixth experiments, and I see no reason why 
it should not under similar circumstances circulate are a - 
of ashes spread over the whole earth. -. 
Satisfying myself in this manner sar the p of pulveru- 
lent charcoal was not essential to sa imasgonta in —" T 
ee: powders to similar trials. efi 
Senin the coal refused to be buried alive in such sepolchres 
as these ; ee an 
