and on a JVew Means of investigating them. 479 



bismuth nitrate and stannous chloride offer examples, as also 

 the thallic salts of all three. 



Between these limiting cases are certain intermediate ones 

 as to which our knowledge of the form which the salts take 

 in solution is not very exact. — It need scarcely be mentioned 

 that what has been said does not refer to dissociation into 

 anions and kations but into separation of free acid and basic 

 salt, an entirely different question. 



In the case of sulphates — to a consideration of which this 

 paper will be devoted — we know that the number of salts of 

 the metals which give a purely neutral reaction with litmus is 

 comparatively small ; that all the normal sulphates of the 

 heavy metals show an acid reaction even after any number of 

 recrystallizations although these may be made from solutions 

 rendered alkaline by the addition of free alkali, and that all 

 the normal sesquisulphates have an acid reaction. The ques- 

 tion is as to the meaning of this acid reaction and whether in 

 any or all cases free acid is present. 



The uncertainty that exists depends partly upon the imper- 

 fect nature of our indicators. Litmus, which is perhaps the best, 

 is reddened by the contact of any substance which will deprive it 

 of its alkali. Methyl orange, which is by some considered more 

 sensitive than litmus, has this disadvantage that it seems to be 

 decomposed by some metallic salts so that its color is rendered 

 lighter by them instead of darker although free acid may be 

 present. This tendency greatly diminishes its usefulness. 



Another method has been proposed for the detection of free 

 sulphuric acid in the presence of combined. The solution is 

 heated to 100° and evaporated to dryness in the presence of a 

 small quantity of organic matter which is blackened if free 

 sulphuric acid is present. As what we want is to determine 

 the nature of the solution at ordinary temperatures such a 

 method is quite worthless. 



But by means of a new reaction the conditions of such solu- 

 tions can be examined and free sulphuric acid can be detected 

 in the presence- of sulphates with great accuracy and sharp- 

 ness even when only a trace is present. 



The method is as follows : Taking the well known polariz- 

 ing salt discovered by Herapath, the sulphate of iodo-quinia, 

 it is possible to remove the whole of the sulphuric acid with- 

 out breaking up the molecule. This may be done with either 

 barium carbonate or barium hydroxide. The mode of opera- 

 tion is as follows. The barium compound is placed in a beaker 

 and covered with weak alcohol of about 70 per cent. Into 

 this the crystallized herapathite is dropped. In cold alcohol of 

 this strength it is but very slightly soluble but in the presence 

 of the barium compound it dissolves with facility and in large 



