ON THE CONDHCTR^ POWERS OF VARIOUS SUBSTANCES. 
817 
atmosphere. It is more compact than ordinary chalk. The specimen used in the expe- 
riment was very moist. It gave 
-=•3; 
c ’ 
and when thoroughly saturated with moistru’e, it gave again 
^=• 3 . 
C 
The same block, when thoroughly dried, gave 
-=•19. 
c 
In the first case, the block weighed 
6808‘5 grains 
immediately after the experiment. In the second case, when perfectly saturated, the 
weight was 7602 grains before the experiment, and 7503 grains after it, while, when 
thoroughly dried, it was 
6 1 87 ’5 grains. 
Hence in the first case the block may be considered to have contained 621 grs. of water, 
nearly ^th of the weight of the dry chalk; and in the second, 1315-5 grs., while the 
conductive power in the two cases was the same. It should be observed, however, that 
the temperature of the lower surface of the block in the first experiment was 2I1°'5, 
and that of the upper surface about 163°; while in the latter the two temperatures 
were only 123°’5 and 101°'4 respectively. It is probable that the higher temperature 
might increase the conductivity by the more rapid conversion of the water into vapour. 
These results were verified by almost identical results obtained from another similar 
block of chalk. 
The specimen of cluncli on which I experimented did not, as I conceived, afibrd any 
certain test of the quantity of moisture ordinarily contained in the Chalk as it exists in 
the general mass of the chalk formation. To obtain a better test, I afterwards procured 
a specimen from the same place near Cambridge as that from which the above-mentioned 
specimens were obtained. It was taken from a spot which had not been immediately 
exposed to the atmosphere, but it contained a quantity of moisture sufficient to render 
its colour much darker than the white of dry chalk. Soon after it was taken from the 
mass it weighed 
4966 ’4 grains, 
and after being thoroughly dried by a fire for several days, it weighed 
4154-1 grains. 
It therefore lost 812-3 grs., or nearly one-fifth of its weight when dry. This proportion 
is about the same as in the case above mentioned, in which the chalk was saturated with 
k 
water. The preceding value (-3) of - may therefore be considered as applicable to the 
general mass of the Lower Chalk near Cambridge. This, I have no doubt, is near the 
