264 Prulpltation cf Magnejta. — Air-Vtfds of Fif.; 



this article, that it be made to pofliifs thcfe qualities in as liigh a degree as is pofTihle. It had 

 long fincc occurred to the writer of this, that the effcils which arc attributed, by all who treat 

 ddi the fubjefl, to a very fmall quantity of otlier earths in the alkali which is ufed in the pre- 

 cipitation, were fomewhat difpvoportionate to the afligncd caufc, and that a part of them 

 were probably rather owing to a deficiency of carbonic acid *. An accidental piece of in- 

 formation which he received lately from a pra£lical man, that magncfia was always to be 

 obtained " beautifully light" by the addition of a fmall proportion of fal fodx to the vege- 

 table alkali employed, and a very loofc experiment which he has fincc made with a view to 

 this objc£l, appear to corroborate fuch an idea. The magncfia contained in four ounces of 

 Epfom-falt was precipitated with a filtered folution of common pearl-alh, wafhed, dried, 

 and a portion of it then re-diflblved by vitriolic acid, and again precipitated with the fame 

 alkali, with the addition of one-fourth of carbonate of foda. The powder was certainly 

 more light and impalpable after the fccond precipitation. An addition of carbonic acW to 

 the alkaline folution employed, will probably operate in two ways: it will not only render 

 the magnefia lighter, but in fome degree actually purer, by precipitating the aluminous and 

 filiceous earths before held in folution by the pot-afli in a more cauftic ftate. In this refpedl, 

 and in this only, perhaps, if a fulRcicntly fmall quantity of water be ufed, the aqua kali of the 

 prefent Pharmacopoeia is inferior to the oil of tartar per deliquium of the old ones. There is 

 poflibly a limit to the proportion of this ingredient, which can be admitted into the procefs 

 with a due regard to economy ; perfedly neutralifed carbonate of magnefia being by no 

 means infoluble. If an alkali, in an highly effervefcent ftate, be added to a weak folution of 

 any magnefian fait, it is well known that no precipitation whatever will take place. What 

 remained in the fupernatant liquor might, however, if thought of fufllcient value, be after- 

 wards precipitated with a cauftic alkali, and referved for calcination ; or indeed would of 

 itfelf fubfidc during the fubfequent evaporation for obtaining the vitriolated tartar. The 

 bed procefs in all refpeds may be cafily afcertained by experiment, and the matter appears 

 to deferve it. 



D, 



V. 



On the ElaJIic Fluid contained in the Air-VeJJeh of Fijh. 



'R. Francis Rigby Brodbelt, of Jamaica, in a letter to Dr. Duncan f, gives the fol- 

 lowing account of fome obfervations and experiments which he has made on the gas con- 

 tained in the air-bladder of the fword-fi(h : 



" I will relate to you a few experiments which I made during my paflage to this ifland. 

 I had often wiflied to determine what is the nature of the gas which is contained in the air- 



• I find, on examination of the common mngncfia with a deep magnifier, that its levity proceeds from an 

 a£tu3l radiated cryllalhzation, like that of fnow : its form may be advantageoiifly (ten when juft rcpaniing inio 

 Hocks in a gtafs tube. Hence perliapi it would be improved by the addition of the alkali in very fmall por- 

 tions, at intervals. Merc wafhin^, by deftroying the ramifications of the cryftals, confiderably augments the 

 fpccific gravity of magncfia : an cffc<ft which would pcrhajis bi- befl prevented by performing this operation 

 carefully with dillilled water, prcvioufly boiled on a portion of it more than fufficient for it] faturation. F. 



\ Annals of Medicine, by Drs. Duncan, for 1796, p. 393- 



bladder 



