94 CLASSICS OF MODERN SCIENCE 

 their senses notwithstanding the utmost care to retain it; and they 

 have always found, upon further inquiry, that subtle part to be air, 

 which having been imprisoned in the body, under a solid form, was set 

 free, and rendered fluid and elastic by the fire. We may therefore 

 safely conclude, that the volatile matter lost in the calcination of 

 magnesia, is mostly air; and hence the calcined magnesia does not 

 emit air, or make an effervescence when mixed with acids. 



The water, from its properties, seems to contain a small portion of 

 volatile alkali, which was probably formed from the earth, air and 

 water, from some of these combined together ; and perhaps also from 

 a small quantity of inflammable matter, which adhered accidently to 

 the magnesia. Whenever chemists meet with this salt, they are in- 

 clined to ascribe its origin to some animal or putrid vegetable sub- 

 stance ; and this they have always done, when they obtained it from 

 the calcareous earths, all of which afford a small quantity of it. 

 There is, however, no doubt, that it can sometimes be produced in- 

 dependently of any such mixture, since many fresh vegetables, and 

 tartar, afford a considerable quantity of it. And how can it, in the 

 present instance, be supposed, that any animal or vegetable matter 

 adhered to the magnesia, while it was dissolved by an acid, separated 

 from this by an alkali, and washed with so much water? 



Two drams of magnesia were calcined in a crucible, in the manner 

 described above, and thus reduced to two scruples and twelve grains. 

 This calcined magnesia was dissolved in a sufiicient quantity of spirit 

 of vitriol, and then again separated from the acid by the addition of an 

 alkali, of which a large quantity is necessary for this purpose. The 

 magnesia being very well washed and dried, weighed one dram and 

 fifty grains. It effervesced violently, or emitted a large quantity of 

 air, when thrown into acids ; formed a red powder, when mixed with 

 a solution of sublimate ; separated the calcareous earths from an acid, 

 and sweetened lime-water ; and had thus recovered all those properties 

 which it had but just now lost by calcination. Nor had it only re- 

 covered its original properties, but acquired besides an addition of 

 weight, nearly equal to what had been lost in the fire; and as it is 

 found to effervesce with acids, part of the addition must certainly 

 be air. 



This air seems to have been furnished by the alkali, from which it 

 was separated by the acid ; for Dr. Hales has clearly proved, that 



