509 



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. 



Observations respecting the natural production of Saltpetre on the walls 

 of subterraneous and other Buildings. By John Kidd, M.D. Pro- 

 fessor of Chemistry at Oxford. Communicated by William Hyde 

 Wollaston, M.D. Sec. R.S. Read June 16, 1814. [Phil. Trans. 

 1814,^.508.] 



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 seven- 

 teen 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 abun- 

 dant in clear frosty weather, and that in a moist state of the atmo- 

 sphere 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 pur- 

 pose ; 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 



