ALKALIES. 229 



and reasoning : this remark is perhaps equally true of the principal 

 acids, and both these important classes of bodies should be placed as 

 early as possible in the hands of the student. It has been already 

 stated, in the plan of the work, that in teaching, I have found the 

 most convenience in introducing the alkalies before the acids ; al- 

 though my preference is not so decided that I should have any seri- 

 ous objection to the opposite course. But, I am not willing to post- 

 pone the history of the alkalies and earths until we come to that of 

 the metals, and to treat of them merely as appendages of those bodies ; 

 and I should be still more reluctant, for the sake of avoiding this diffi- 

 culty, to bring in the metals first, or in connexion with the simple 

 combustibles, as some authors have done ; nor is it a sufficient reason, 

 that the alkalies* and earths then fall in naturally as metallic oxides. 

 It is true that modern discovery has increased the difficulty of giving 

 a strictly logical definition of an alkali ; but the bodies that have usu- 

 ally been called by this name are, in some of their forms, familiarly 

 known ; they have also a sufficient number of properties in common, 

 to distinguish them from other classes of bodies,f and this is the most 

 important point to be attained in our arrangements. It is true also 

 that their properties graduate into those of some of the earths; but 

 it is sufficient to designate the latter as alkaline earths, and to leave 

 the remainder of them to be called earths proper. 



Explanatory Statement. 



The alkalies, when they are to be prepared pure for chemical pur- 

 poses, are generally extracted from their saline combinations, and it 

 is therefore necessary to premise, that a salt is composed of an acid 

 and a base : the alkaline salts have, of course, an alkaline base, and 

 the object of our processes is to separate the acid, and leave the base 

 isolated, and free also from accidental bodies, commonly called im- 

 purities. 



In giving the history of potassa, soda and ammonia, only two acids 

 need be mentioned : potassa and soda, as they occur in commerce, 

 are usually found combined with the carbonic acid ; and ammonia 

 both with that and with the muriatic acid. The carbonic acid, com- 

 posed of carbon and oxygen, is a gaseous body, which when com- 

 bined with the alkalies, blunts their properties, but it is easily remov- 

 ed from these combinations, partially by heat and completely by the 

 superior affinity of lime. It is also entirely expelled by stronger 

 acids, but a new salt is, in that case, formed ; and in general the form- 

 ing of such a compound, would rather retard than advance our pro- 



* Ammonia excepted, which no one arranges under the metals. 

 i It is scarcely necessary to add, that I do not include the new alkaline vegetable 

 proximate principles, morphia, delphia, quinia, strychnia, &c. 



