The sea-weeds tried by the author amount to six speeies.
| Fucus cartilagineus, | Fucus filamentosus, |
| membranaceus, | Ulva pavonia, |
| rubens, | linza. |
But he could discover no traces of it in any of these, nor in certain corallines and sponges which he also tried.
The intention of the present paper is to state the result of a series of observations on the connection of production of nitre with the state of the atmosphere; and the account begins with a description of the situation of the laboratory of the Ashmolean Museum, where these observations were principally made; the pavement being nine feet below the level of the street in which the museum stands, and seventeen below the highest part of its ceiling, which is arched, and, as well as the side walls, consists of a calcareous freestone.
The saline efflorescence takes place principally on three sides that are surrounded by high ground, and but little on the fourth side, where the ground without is on a level with the pavement.
It is observed, that even in the midst of those parts that abound most in nitre, there are certain places which always produce much less than others; and even insulated patches, which are always and entirely free from any appearance of efflorescence, showing that these gradations depend on some difference in the texture or composition of the stone. It is also remarked, that such differences are often not by gradual transitions, but occur abruptly at the passage of a line, on one side of which there appears an abundant crop, and on the other never the slightest efflorescence; but this does not depend on the joints of the masonry, but takes place indifferently on the surface of the stones composing the wall, and of the mortar by which they are cemented.
With regard to the influence of different states of the atmosphere on the production of nitre, Dr. Kidd observes, that it is most abundant in clear frosty weather, and that in a moist state of the atmosphere the formation either does not take place, or goes on very slowly. Sometimes also, that which has already formed disappears, as if the moisture occasioned it to be re-absorbed into the substance of the wall: but the author was not able to detect it in a portion of the stone taken from near the surface, and lixiviated for that purpose; and he also observes, that it occasionally disappears in dry frosty weather, when no absorption could be supposed to take place.
Wishing to ascertain whether the presence of atmospherical air was necessary to the production, the author coated a productive part
