416 Rev. J. B. Emmett ou the Specific Gravities 



fresh-water plants, of several orders, existed in abundance, 

 prior to the formation of the new-red-sandstone strata, in which 

 the beds of salt occur. 



On reviewing the chemical history of these subjects, it ap- 

 pears, in conclusion, that a series of new and exact researches 

 on the contents of sea-water and of the rock-salt and brine of 

 all countries, is now required by science. Subordinate to this 

 would be required an investigation of the interesting points 

 involved in the following: viz. the limits of compatibility with 

 each other of the various salts concerned, their mutual action 

 during the evaporation of solutions containing them, and the 

 state of combination in which the elements of the salts ob- 

 tained from sea-water actually exist in that fluid. The last 

 labours of Dr. Murray (Trans. Roy. Soc. Edin, vol. viii.) and 

 Dr. Marcet, in particular, especially so far as relates to the 

 second of these subjects, require to be examined and recon- 

 ciled. 



16, St. James's-street, Clerkenwell : May 5, 1829. 



LXI. On the Specific Gravities of Compound Bodies. By 

 the Rev. J. B. E.^imett. 

 To the Editors of the Philosophical Magazine and Annals. 

 Gentlemen, 

 TJ AVING had occasion to institute some inquiries into the 

 -*--■■ specific gravity of certain compounds, I have found that 

 some highly erroneous tables comjiuted by Hassenfratz are 

 generally adopted. The errors arise from two sources. Where 

 substances which are soluble in water are concerned, a known 

 quantity was accurately weighed in air, and afterwards in a 

 bottle filled with mercury. It is well known that on account of 

 the air contained in porous bodies, such as lime, hydrate of lime, 

 magnesia, calcined alum or borax, and small crystals, as well 

 as on account of capillary action, mercury does not enter into 

 the pores ; and consequently the resulting specific gravity is 

 not that of the solid matter, but that of the volume made up 

 of the mass of solid matter and of the pores or interstices to- 

 wether. The error is often very great: for examj^le, Hassen- 

 fratz assigns the specific gravity 0*4229 to calcined alum, al- 

 though the powder sinks rapidly in water : also, the specific 

 gravity of the powder consisting of lime 2 and water 1, he gives 

 0*8983 ; yet this powder sinks in water. The specific gravity of 

 lime must exceed his estimate 1*4558 ; for the specific gravity 

 of its carbonate is 2*7; it consists of 26*5 lime and 20*7 car- 

 bonic acid ; this acid escapes during calcination, yet the volume 

 is but litde altered; hence in this porous state its giavity will 

 be about 1*5: in this state it mechanically absorbs a large 



(juantity 



