Another reagent is constituted by iron ammonium sulphate; the extent of the penetration of Neradol D, which gives an intense blue coloration with iron salts, into the leather may be determined by washing the pelt treated with Neradol D, making a cut, again washing and treating the cut with a few drops of a weak solution of iron ammonium sulphate. Those parts of the pelt which have been converted into leather then appear deep blue; on the other hand, those which have been in contact with Neradol D, but have not yet been converted into leather, are light blue. Those parts which have not yet been in contact with Neradol D appear pure white; the results of this reaction are therefore opposite to those obtained by the use of indigotine. 8. Combination Tannages with Neradol D Whereas mixtures of Neradol D and vegetable tannins impart properties to the leather consistent with the proportions in which these materials are present, it is not possible to combine Neradol D with mineral tanning agents or fats (_e.g._, fish oils, etc.) in such a way that a leather characterised by the properties of either material is obtained. Experiments were carried out using (1) chrome salts plus Neradol D; (2) aluminium salts plus Neradol D; and (3) oils plus Neradol D, and the following conclusions were arrived at:-- 1. CHROME-NERADOL D liquors, containing comparatively larger amounts of Neradol D, act too rapidly on the pelt and draw the grain; smaller amounts of Neradol D seem without influence on the finished leather, which possesses pronounced characteristics of chrome leather. Another disagreeable factor is the following: the chrome salts must possess a certain degree of basicity in order to produce good leather; the Neradol D must, on the other hand, possess a certain acidity to produce the optimum results, and it is hence impossible to balance practically the basicity of the chrome salts and the acidity of the Neradol in order to justify the presence of both. If one of the two is used separately before the other, a leather always results possessing the characteristics of the material first employed, provided the time of action has been sufficiently extended. If insufficient time has been allowed, the characteristics imparted by the main tanning agent are not altered. 2. ALUMINIUM SALTS AND NERADOL require practically the same basicity and acidity respectively, and when combined always yield a leather possessing mainly the properties of one of the components. In addition to this fact, leathers tanned with aluminium salts possess great softness and stretch, those tanned with Neradol D greater firmness and less stretch, and these opposing qualities completely compensate one another and render _nil_ the value of such mixtures. In addition to this, the presence of aluminium salts produces no better fixation on the leather fibre of basic coal-tar dyes, so that in this respect also a combination of aluminium salts and Neradol D is of no value.