CHEMICAL TOXICOLOGY 297 



GROUP D. 



It is not possible to give a single comprehensive scheme 

 for the detection of the various acids, alkalis, and soluble 

 salts comprised under this heading. An excellent method 

 of separation for soluble compounds of this class is afforded 

 by dialysis, taking advantage of the fact that the soluble 

 crystalloids pass through a membrane into surrounding 

 pure water or dilute solution, whilst the colloid protein 

 substances do not so diffuse, or if so, only extremely slowly. 

 For the practice of dialysis the most convenient membrane 

 is a diffusion shell or thimble of parchment pa,per. Failing 

 this, an intact sausage-skin membrane may be used. The 

 material under examination is made into a thin paste with 

 water, and placed in the shell, which is then immersed in 

 a narrow cylinder of distilled water. The dialysate is 

 removed from time to time, fresh water being added. 



Acids, bases and soluble salts are thus obtained in a fairly 

 pure watery solution. Sometimes, however, it is sufficient 

 to considerably dilute the material and filter ; but the 

 filtration is often very slow, and dissolved proteins are not 

 removed. 



The subsequent handling of the solution dependp on the 

 nature of the substances under research, and as a rule no 

 difficulty exists in separating and identifying by means of 

 evaporation to crystallising point, neutralisation, precipita- 

 tion, and the like operations. 



As regards alkalis, these are recognised by means of 

 indicators, which are dye-stuffs having different colours 

 according as to whether the solution is acid (contains 

 hydrogen ions), or alkaline (containing OH ions). In this 

 group may be found the strong alkalis — potash, soda, lime, 

 strontia, and baryta — and weak alkalis, such as ammonia, 

 ammonium carbonate, carbonates and bicarbonates of sodium 

 and potassium, and borates and silicates of the alkalis. It 

 ought to be carefully remembered that the rumen and intes- 

 tinal contents are normally alkaline. Allowance must be 



