22 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



sulphate of potash identical with that prepared from oil of 

 vitriol and " tartar" or potash. 



These actions were studied carefully by John Mayow, a 

 London Physician (1645-1697), who pointed out in 1674 

 that " although [acids] and [alkalis] pass into a neutral sub- 

 stance when they meet, yet they do not, as is generally 

 supposed, entirely destroy each other," since when the 

 conditions are suitable both the acid and the alkali may 

 be recovered from the salt. Thus: 



" If oil of vitriol is poured upon nitre, which consists of 

 an [alkali] and of a volatile [acid] (as was shown above), 

 the fixed salt [i.e., the base] of the nitre will soon leave its 

 own acid and will enter into union with the acid of the 

 vitriol, which is more concordant with it .... That the 

 case is so, is clear, for if nitre mixed with oil of vitriol be 

 distilled, the spirit, or [acid] of the nitre will pass under 

 a mild heat into the receiving vessel, while yet in other 

 circumstances that spirit will not be carried up except by a 

 very vehement fire .... It is a corroboration of this view 

 that the mass left in the retort after a distillation of this 

 kind, closely resembles vitriolated tartar, and can be properly 

 substituted for it." (" Of the combination of contrary salts," 

 Medico-physical Works, A.C.R. XVII. 161162.) 



As oil of vitriol was found to liberate nitric acid from any 

 nitrate and muriatic acid from any muriate, it was regarded 

 as stronger than either of these acids, whilst vinegar was 

 found by similar tests to be weaker than the three mineral 

 acids. It is recognised now that this rough and ready 

 method is not the best test of the strength of an acid, as it 

 depends too largely on the readiness with which the various 

 acids can be driven off in the form of vapour. But 

 observations such as these were of great importance 

 because they showed clearly that all salts possessed a 

 dual structure ; they thus prepared the way for the system 

 of classifying salts which is described in the preceding 

 paragraphs. 



