438 PRINCIPLES OF CHEMISTRY 



4NaCl + 2MgS0 4 = 2NaCl + MgS0 4 + Na 2 S0 4 +MgCl 2 = 2Na 2 S0 4 -f 

 2MgCl 2 . A no less clear limit expressed itself in another of Malaguti's 

 researches when he investigated the above-mentioned reversible reactions 

 of the insoluble salts of barium. When, for example, barium carbonate 

 and sodium sulphate. (BaC0 3 + Na 2 SO 4 ) were taken, then about 72 per 

 cent* of the salts were decomposed, that is, were converted into barium 

 sulphate and sodium carbonate. But when the two latter salts wer,e 

 taken, then about 19 per cent, of them passed into barium carbonate 

 and sodium sulphate. Probably the end of the reaction was not 

 reached in either case, because this would require a considerable time 

 and a uniformity of conditions attainable with difficulty. 



Gladstone (1855) took advantage of the colour of solutions of 

 different ferric salts for determining the measure of exchange between 

 metals. Thus a solution of ferric thiocyanate has -a most intense 

 red colour, and by making a comparison between the colour of the 

 resulting solutions and the colour of solutions of known strength 

 it was possible to judge to a certain degree the quantity of the 

 thiocyanate formed. This colorimetric method of determination has 

 an important significance as being the first in which a method was ap- 

 plied for determining the composition of a solution without the removal 

 of any of its component parts. When Gladstone took equivalent quanti- 

 ties of ferric nitrate and potassium thiocyanate Fe(N0 3 ) 3 + 3KCNS 

 only 13 per cent, of the salts underwent decomposition. On 

 increasing the mass of the latter salt the quantity of ferric thio- 

 cyanale formed increased, but even when more than 300 equivalents of 

 potassium thiocyanate were taken a portion of the iron still remained 

 as nitrate. It is evident that the affinity acting between Fe and NO 3 

 and between K and CNS on the .one hand, is greater than the affinity 

 acting between Fe and CNS, together with the affinity of K for NO 3 , 

 on the other hand. The investigation of the variation of the fluor- 

 escence of quinine sulphate, as well as the variation of the rotation of 

 the plane of polarisation of nicotine, gave in the hands of Gladstone 

 many proofs of the entire applicability of Berthollet's doctrine, and in 

 particular demonstrated the influence of mass which forms the chief 

 distinctive feature of the teaching of Berthollet, teaching little appre- 

 ciated in his own time. 



At the beginning of the year 1860, the doctrine of the limit of 

 reaction and of the influence of mass on the process of chemical trans- 

 formations received a very important support in the researches of 

 Berthelot and P. de Saint-Gilles on the formation of the ethereal salts 

 EX from the alcohols ROH and acids HX, when water is also formed. 

 This conversion is essentially very similar to the formation of salts, but 



