﻿462 Dr. E. J. Mills on Statical and 



tained in the idea of motion, such motion being understood of itself 

 — that is (in ordinary language), without reference to anything 

 moved. This criterion will sufficiently enable us to decide upon 

 the relative value of most of the current theories, of chemistry 

 especially. I purpose in this paper to apply it to the contro- 

 versy about acid, alkali, base, and salt; but the following histo- 

 rical digression is a necessary preface to the argument*. 



2. The word acid was probably used at first only in a concrete 

 sense, signifying vinegar. The solvent properties of what we 

 now call acids were noticed by Geber. Tachenius (an iatro- 

 chemist) defines them as forming salts with alkalies, to which 

 they have a certain oppositeness or antagonism. Boyle and 

 others noticed that they reverse the colorific effects of alkalies. 

 Freind testifies to the general acceptance of the Tachenian doc- 

 trine during the first third of last century, and strongly protests 

 against it ; the corrosive, colorific, and fermentative properties 

 are, he states, often shared by acids and alkalies alike, " and 

 what in respect to one body is named alkali, is, if compared 

 with some other, by the very same writers calPd an acid. So 

 that in vain we endeavour to fix the boundaries which separate 

 each kind." 



Various opinions have been proffered as to the source of the 

 acid properties. Becher and Stahl ascribed them to the pre- 

 sence of the universal acid, Sylvius to a fiery matter, Meyer 

 to an "acidum pingue " — the last two being supposed to be 

 common also to alkalies. Lemery assured himself of the sharp- 

 ness of the constituent particles of acids ; Mayow, Scheele, and 

 Lavoisier ascribed acidity to oxygen. Davy, on the other hand, 

 first considered hydrogen' as the acidifying principle; but he 

 afterwards discarded the notion of a principle, and held, if I may 

 so illustrate his view, that the acidity of any substance is a kind 

 of resultant whose direction is hydrogen. The doctrine that " an 

 acid is a salt of hydrogen" may be assigned to Dulong and 

 Gerhardt (the former especially). 



A question of nomenclature has arisen recently with reference 

 to this subject. Professor Williamson maintains t that the words 

 acid and base "were introduced to describe bodies of oppo- 

 site pi'operties which are more or less completely lost in the salt 

 or compound of acid and base" (p. 423), or "belong to the 

 idea of compounds of fundamentally opposite properties which 

 unite to form one or more molecules of a comparatively neutral 



* The following historical details have been purposely compressed as 

 much as possible. Authority for many of them will be found in Kopp's 

 Geschichte der Chemie, or in Watts's ' Dictionary of Chemistry ; ' but they 

 are partly supported by original research. 



t Cheim Soc. Journ. vol. xvii. 



