394 PHILOSOPHICAL TRANSACTIONS. [aNNO I767. 



tion, which being collected and dried, weighed 1^ 1 grs. It proved to be en- 

 tirely a calcarious earth, except a small part, which was magnesia. This Mr. C. 

 found in the following manner. A little of this earth, being mixed with spirit 

 of salt, dissolved entirely ; which shojvs it to consist solely of an absorbent earth, 

 but does not show whether it is a calcarious earth or magnesia. The remainder 

 was saturated with oil of vitriol: a great deal of matter remained undissolved, 

 which, as the earth was shown to be entirely of the absorbent kind, must have 

 been selenite, or a calcarious earth saturated with the oil of vitriol. The clear 

 liquor strained from off the selenite, yielded on evaporation only 1 8 grs. of solid 

 matter, which proved to be Epsom salt ; so that all the earth, except that con- 

 tained in the 18 grs. of Epsom salt, must have been of the calcarious kind. That 

 contained in the Epsom salt is well known to be magnesia. 



The water remaining after distillation, and from which the earth was sepa- 

 rated, was evaporated, first in a silver pan, and afterwards in a glass cup, till it 

 was reduced to about 3 oz. Not the least earth was precipitated during the eva- 

 poration, till it was reduced to a small quantity ; there then fell SQ grs., which 

 were entirely selenite : so that all the unneutralized earth in the water was sepa- 

 rated during the distillation. The liquor thus evaporated was of a reddish colour, 

 like an infusion of soot. 



Many waters contain a good deal of neutral salt, composed of the nitrous acid 

 united to a calcarious earth; the most convenient way of ascertaining the quan- 

 tity of which, is to drop a solution of fixed alkali into the evaporated water, till 

 all the earth is precipitated ; by which this salt is changed into true nitre, and is 

 capable of being crystallized. For this reason some fixed alkali was dropped into 

 the evaporated water till it made no further precipitation. The earth thus pre- 

 cipitated weighed 36 grains, and was entirely magnesia. The liquor was then 

 further evaporated, but no nitre could be made to shoot: being then evaporated 

 to dryness, it weighed 256 grs. It gave not the least signs of containing any 

 nitrous salt, either by putting some of it on lighted charcoal, or by making a 

 match with a solution of it, but appeared to be a mixture of sea salt and vitrio- 

 lated tartar, or some other salt composed of tl;ie vitriolic acid. As Mr. C. had 

 heard of no other London water, that had been examined with this view, but 

 what had been found to contain a considerable proportion of nitrous salt, it 

 seemed very remarkable that this should be entirely destitute of it. He now 

 proceeded to the experiments made on the distilled water. 



The distilled water, especially that part of it which came over first, became 

 opaque, and let fall a precipitate, on dropping into it a solution of sugar of lead. 

 It also became opaque by the addition of corrosive sublimate, much in the same 

 manner that the plain water did before distillation. It was found, by drop- 

 ping into it a little acid of vitriol and committing it to evaporation, to contain a 



