188 



are most often cited a> confirming the conception of definite chemical 

 compounds. Indeed, all the indications of a definite chemical combina- 

 tion are clearlv seen in the formation and properties of >alts. Thus, 

 >alts are produced with a definite proportion of oxides, heat is evoked 

 in their formation/' 3 and the character of the oxides and manv of their 

 physical properties are hidden in salts. Thus, when gaseous carbonic 

 anhydride combines with a base to form a solid >alt, the elasticity of 

 the u'as <|iiite disappears in its passage into the salt.'"' 1 



Judging tVom the above, a salt i^ a compound of basic and 



nation ot lime and sulphunc anhydride with water. When extreme oxides combine they 

 lornistable salt-, which are ditticiiltlv decompo-ed. and often show characteristic proper- 

 ties. The compounds of the intermediate oxides with each other, or even with basic and 

 acid oxides, present a very different case. However much alumina we may dissolve 

 in sulphuric arid, we cannot saturate the acid properties of the sulphuric acid, the 

 resulting solution will always have an acid reaction. So aUo. whatever quantity of 

 alumina is dissolved in an alkali, the resultiiiLT solution will always present an alkaline 

 reaetii ui. 



' In order to pve an idea of the quantity of heat evolved in the formation of salts, 

 I append a table of data tor rrry dilute arjncun* milittimix of acids and alkalis, accord- 

 ing to the determinations of Berthelot and Thonisen. The li-nres are pven in major 

 cal' -ries t hat is. in thousands of units of heat. Hence. l'. -'rams of sulphuric acid. 

 H SO., taken in a dilute aqueous solution, when mixed with such an amount of a weak 

 solution of call-tic soda, NallO. that a neutral salt i- form-d iwheii all the hvdn_reii of 

 the acid i- replaced by the sodium), evolves l.">sUi) units of heat. A star signifies the 

 formation of an insoluble salt. 



l .So 4 II NO , 



Xallo . . ir.-s 1:5-7 M-o . l.vt; i:;-s 



Kilo . . . ir.'T i:j-.s Fe() . }!:> ld'7 i?) 



NH- . . . }{:, \>i-:> '/.uO . . 11-7 i-s 



Ca<) . . . l.'.-t; ]:;;) l-'-< : '' 7 : '' ;l 



'l'he-e (i '.Hires cjiinidt l>e considered as the heat of ueiit ralisat ion. l)ecause tin- water 

 here plays an important part. Thus, for instance, sulphuric acid and caustic soda in 

 dis-olviii'_' in water, evolve very much heat, and the result a ni -odium sulphate very little ; 

 con~c(| ui-iil I \ . the he;il e\iil\ed iii an a nliyilroiis -tale \\ill lie dit't'erent from (hat in a 



1 1 vd rated state. Those acids wliich are not eiierj. f el ic in coiiiliiniiiir \\ it li the sal iiian- 



titv of alkali- as i- reqiiired for the formation of normal -alts of sulphuric or nitric 



acid- alwavs, howe\er. i:ive less heat. |-'or example, wit li caustic soda: carbonic acid 



_;.. Kr-J. liydi-ocyanic ii".i. 1 1 yd ro-eii sulphide :',".l. And as I'eelile liases (for example, 



I-'e ( )- al-o evuh'c less heal tlian lliose uhich an- more powerful, so a certain ueiieral 



correlation l.etwecn theniiochemical data and tlie conception of (lie measure of affinity 



-hous itself here. ,i, in other cases i.srr Chap. II.. Note 7 1. which does not, however, -,'ive 



i on f..r juduriii^i l the measure i.f the aftinit\ wliich l.inds the elements o) salts 



h\ the heal of | he formation ,,| salts I,, dilute solutions. This is rendered especially 



M the fact tli.it water is alile to decompose mam salts. ; <nd is -eparated in their 



