14

by which the quantity projected will be knownlet the balance be then restored; the differenceof weight between the quantity projected andthat requisite to restore the balance will discoverthe weight of air lost during effervescence; ifthe loss amounts to 13 per cwt. of the quantityof marl projected, or from 13 to 32 per cwt.the marl essayed is calcareous marl. This ex-periment is decisive, when we are assured bythe external characters above mentioned, thatthe substance employed is marl of any kind;otherwise some sorts of the sparry iron-ore maybe mistaken for marl. The experiments to dis-cover the argillaceous ingredient (being toodifficult for farmers) I omit. The residue leftafter solution, being W'ell washed, will, whenduly heated, generally harden into a brick.

Argillaceous Marl contains from 68 to 80per cent, of clay, and consequently from 32 to20 per cent, of aerated calx. Its colour is greyor brown, or reddish brown, or yellowish orbluish grey. It feels more unctuous than theformer, and adheres to the tongue: its hard-ness generally much greater. In water it fallsto pieces more slowly, and often into square