546 PHILOSOPHICAL TRANSACTIONS. [aNNO 1774. 



takes place. The fixed alkali, quitting the unctuous substances, to which it 

 was joined in the soap, unites itself to the aerial spirit, or mephitic air, of those 

 waters, while this air, at the same time, deserts the earthy substances with 

 which it was before combined. The same new combinations seem to take place, 

 when soap is mixed with any of those waters which are usually called hard ; many 

 of which waters have been found to contain an earthy substance, dissolved by 

 means of this subtile aerial principle. 



The above obsei-vations and experiments show an exact agreement, in the 

 several ways by which the various neutral salts, and those saline concretes, 

 formed of mephitic air united to an earthy base, are decompounded. It ought 

 however here to be remarked, that the saline concretes, which exist in the 

 Pouhon water, in a dissolved state, though evidently of the neutral kind, 

 have not hitherto been obtained in a solid form; owing perhaps, in some 

 measure, to the great volatility of their spirituous principle; but chiefly to their 

 being subject to decomposition, from the precipitation of their earthy base, by 

 means of common air, during the evaporation of the water in which they are 

 dissolved, as will be shown hereafter. 



The mephitic air of the acidulee, though it is soluble in water, and imparts to 

 it its brisk and pungent taste, which has been usually stiled subacid; and though 

 it produces effects exactly similar to those of acid spirits (by readily uniting to 

 various earthy substances, which of themselves are not soluble in water, but, by 

 their union with this aerial fluid, are rendered soluble in it, and communicate 

 to the water peculiar savours, and form in it saline concretes of the neutral 

 kind; which concretes, so formed, are again separable into their component 

 ingredients, by all those ways by which the acid and alkaline principles of other 

 neutral salts are separable from each other) yet it differs from all acid spirits, 

 found in a liquid form, in its rare texture and in its elastic quality, and in not 

 striking a red colour with syrup of violets, and other blue tinctures of vegeta- 

 bles; which change, in the blue colour of those tinctures, is usually esteemed a 

 test of the presence of an acid. Besides the trials which have been made, by 

 mixing syrup of violets with pure water, impregnated with various kinds of 

 mephitic air, in which no change in the colour of the syrup was observed, he 

 had for several days suspended pieces of linen, that had been dyed blue with 

 fresh juice of violets, in the mephitic air of spa water, and also in that of chalk; 

 and, when the linen was taken out of the said air, did not j)erceive its blue 

 colour in any wise changed, though the same pieces of dyed linen were instantly 

 turned of a green colour, when exposed to the fumes of spirit of hartshorn. 

 Whether therefore, and under what relations, this aereo-saline spirit may merit 

 the title of an acid, he leaves to the determination of others. Such however it 

 has appeared to be to many philosophers, since this mephitic air is doubtless the 

 same with the acidum vagum fodinarum c^ Boerhaave and others; and with the 



