only rationale which seems applicable, that it is nothing more than the transition of caloric from one state to another, from a latent to a free state. Be this as it may, the combustion in such instances furnishes an anomaly to the already established doctrine, of the absorption of oxygen, or the base of the supporter, and the evolution of caloric from the gas, and not from the combustible; or, in other words, the change of caloric in the supporter from a combined to an uncombined state. The idea of _latent_ heat may be had from Dr. Black's own expression (_Black's Lectures_ by Robinson:) "By this discovery," says the doctor, "we now see heat susceptible of fixation--of being accumulated in bodies, and, as it were, laid by till we have occasion for it; and are as certain of getting the stored-up heat, as we are certain of getting out of our drawers the things we laid up in them." Murray's _System of Chemistry_, 2d edition, p. 398, and Watson's _Chemical Essays_, vol. iii, &c. may be consulted on this subject with advantage. See _Introduction_. We will consider, in the next place, the subject of _gunpowder proof_. The first examination of gunpowder is by rubbing it in the hands, to find whether it contains any irregular hard lumps. If it is too black, it is a sign that it is moist, or else, that it has too much charcoal in it; so, also, if rubbed upon white paper, it blackens it more than good powder does; but, if it be of a kind of azure colour, it is a good indication. If on crushing it with the fingers, the grains break easily, and turn into dust, without feeling hard, it is a criterion, that it has too much coal; or, if in pressing it under the fingers upon a smooth hard board, some grains feel harder than the rest, it is inferred that the sulphur is not well mixed with the nitre. By blasting two drachms of each sort on a copper plate, and comparing it with approved powder. In this proof it should not emit any sparks, nor leave any beads or foulness on the copper. The method of _burning_, which is commonly employed, Mr. Robins observes, is to fire a small heap on a clean board, and to attend nicely to the flame and smoke it produces, and to the marks it leaves behind on the boards. Another trial of powder is to expose it to the atmosphere. One pound of each sort, accurately weighed, is exposed to the atmosphere for 17 or 18 days; during which time, if the materials are pure, it will not increase any thing material in weight, by attracting moisture from the air. One hundred pounds of good powder should not absorb more than twelve ounces, or somewhat less than one per cent. See Mr. Coleman's account of the manufacture of powder in England, page 110.