"Note. That in every sheath a small aperture is to be made, in order that it may be ignited by the match when applied, which match is made slender at the extremities, but in the middle large and filled with the aforesaid powder." Another receipt of Marcus for Greek fire is as follows:-- "Greek fire is made in the following manner. Take pure sulphur, tartar, sarcocole, (a kind of resin,) pitch, fused saltpetre, and oil of petroleum. Boil them well together. Dip tow in the mixture, and set fire to it. This fire cannot be extinguished but with vinegar or sand." The close analogy, or rather the identity, of these compositions with gunpowder as at present made, requires no comment. The more important question is the date at which this work was written. This is a matter of great doubt. Messrs Reinaud and Favé, from the fact that the receipt for the preparation of saltpetre to be found in this same book of Marcus is much more imperfect than that in the Arabian MSS., place the date of his book earlier than the thirteenth century. Again, Geber, an oriental writer, the date of whose life is doubtful, but whom our authors fix at the eighth century, has described the preparation of a salt which has been translated _nitre_, but which our authors consider to have been a sesqui-carbonate of soda, _natron_, not _nitrum_. They thence conclude that nitre was unknown to Geber, and thus, because it was known to Marcus, that he lived subsequently; and for this reason they place Marcus between the ninth and twelfth century. We have seldom seen an instance of more loose deduction than this. It is required to find the date of Marcus. Geber, whose date is unknown, is set down, upon rather weak data, as of the eighth century. Geber's translator is corrected to prove that Geber did not know saltpetre. Hassan Alrammah, an Arabian, is considered as more recent than Marcus, a Greek, because his process for saltpetre is somewhat more perfect; and from the cumulative effect of these data, each of which is very insufficiently established, and which, if established, only go to prove differences in the degrees of perfection of their respective receipts, the date of Marcus is fixed: this certainly is pushing _incertum per incertius_ very far. We fear that if no more accurate information be brought to bear on it, the epoch of Marcus Græcus will be a subject of as much controversy as ever. The paragraph in the treatises _De Mirabilibus_ of Albert the Great is so identical with that of Marcus Græcus, that there can be no doubt of its being copied from it, or derived from the same source, and is a strong additional instance of the general progress of inventions. A received publication calls attention to a fact already disclosed but forgotten, the knowledge acquired by the world since is brought to bear on the old fact; and a consequent improvement results.