820 
ME. W. HOPKINS’S EXPEEBIEXTAIi EESEAECHES 
The weight of the moist block before the experiment was 
6088‘6 grains. 
Its weight immediately after the experiment was accidentally not observed, but twelve 
hours afterwards it was 
5687'5 grains. 
It probably lost about 150 grains during the experiment, w'hich would make the weight 
taken as in the preceding cases equal to about 
5840 grains. 
Its weight after being thoroughly dried was 
5251 grains. 
Consequently, the absorption of a quantity of water =589 grains, which was neaily one- 
tenth of the weight of the block, increased the conductive power from ’23 to ’37. 
(4) Very hard Mocks . — A block of Millstone Grit gave the following results : — when 
diT: 
weight = 9925 grains. 
After being immersed in water about two days, 
-=•69, 
c ’ 
weight = 9956 grains. 
From a block of close Paleeozoic rock I obtained, when di’y. 
weight = 11085 grains; 
and after immersion in water the same time as the last block. 
weight = 11098 grains. 
In these cases the absorbing power was very small, and the difference of the conductive 
powers mdicated by the experiments scarcely exceeds the limits of error. 
General Summary of the preceding Mesidts. 
11. The conductive powers of calcareous, argillaceous, and siliceous substances — 
those which compose the great mass of the earth’s crust — are in the order in which I 
have now named these substances, when masses of them ai’e formed by the simple 
aggregations of each substance preidously reduced to dry powder, the fii'st being the 
worst conductor. The conductive power of Chalk, as it exists in mass, may be estimated 
