Among the methods for indirect haemoglobin estimation, that of calculation from the amount of iron in the blood appears to be quite exact, since haemoglobin possesses a constant quantity of iron of 0.42 per cent. This calculation may be allowed in all cases for normal blood, for here there is a really exact proportion between the amounts of haemoglobin and of iron. Recently A. Jolles has described an apparatus for quantitative estimation of the iron of the blood, called a "ferrometer;" which renders possible an accurate valuation of the iron in small amounts of blood. However for pathological cases this method of haemoglobin estimation from the iron present is not to be recommended. For if one tests the blood of an anaemic patient under the microscope for iron one finds the iron reaction in numerous red blood corpuscles. This means the presence of iron which is not a normal constituent of haemoglobin. Other iron may be contained in the morphological elements (including the white corpuscles) as a combination of proteid with iron, which is not directly recognisable. It is further known that in anaemias the amount of iron of all organs is greatly raised (Quincke), apparently often the result of a raised destruction of haemoglobin ("waste iron," "spodogenous iron"). In many cases too, it should be borne in mind that the administration of iron increases the amount of iron in the blood and organs. From these considerations we see how unreliable in pathological cases is the calculation of the amount of haemoglobin from the amount of iron. We have been particularly led to these observations by the work of Biernacki, since the procedure of inferring the amount of haemoglobin from the amount of iron has led to really remarkable conclusions. For example, amongst other things, he found the iron in two cases of mild, and one of severe chlorosis quite normal. He concludes that chlorosis, and other anaemias, shew no diminution, but even a relative increase of haemoglobin: but that other proteids of the blood on the contrary are reduced. These difficult iron estimations stand out very sharply from the results of other authors and could only be accepted after the most careful confirmation. But the above analysis shews, that in any case the far-reaching conclusions which Biernacki has attached to his results are insecure. For these questions especially, complete estimations with the aid of the ferrometer of A. Jolles are to be desired.