THE BACTERIA OF COLOUR AND PHOSPHORESCENCE. 349 



solution it is quite evident that very minute traces of iron, say, acted upon 

 by the sulphuretted hydrogen set free by the decomposition of the sulphates, 

 can easily account for the production of certain pigments. Cladothrix 

 dichotoma, for example, growing in water, appears able to separate iron 

 from the surrounding substances ; this becomes accummulated in the form 

 of an oxide in the sheath. In the Beggiatoa the power of reducing the 

 sulphates is especially well marked, sulphur, which appears in regular 

 granules in the substance of the organisms, being stored up in the proto- 

 plasm to be utilized as it may be required. The sulphuretted hydrogen that 

 is formed, acts on the iron and gives rise to the formation of sulphide of iron. 

 In this way may be explained the presence of the pigment that is formed in 

 muds where these putrefactive organisms are present, as, for example, in 

 the mud of a tidal river. It should be remembered, however, that iron is 

 not the only metal that may be present, and that there is always a tendency 

 for the sulphuretted hydrogen to be set free from the sulphide and to give 

 way to the formation of oxides, especially in the presence of air and mois- 

 ture. 



Only by the application of some such explanation as this 

 is it possible to account for certain of the beautiful brown 

 colorations that make their appearance in gelatine ; for 

 example, that which surrounds the track of a needle inocu- 

 lation of some of those organisms, which, though colourless 

 themselves, give rise to most beautiful coloration of the 

 surrounding gelatine. So important is the presence of iron 

 in these cases, that Miller holds that to the action of organ- 

 isms on it is due most of the discoloration that occurs in 

 decaying teeth. 



He points out that the colours characteristic of decaying 

 dentine only make their appearance some time after the 

 process has commenced, the depth of the colour being in 

 direct proportion to the length of time that the decay has 

 been going on, and he considers that this coloration is 

 due to the formation of sulphide of iron in the decomposing 

 enamel, dentine, and pulp. He performed an experiment 

 which may be repeated by any individual who is unfortunate 

 enough to be compelled to have a tooth of his own drawn 

 for decay, or who is fortunate enough to obtain one from 

 some other source. He says : 



" A tooth was cracked in a porcelain mortar, so as to thoroughly expose 

 the pulp, and then placed in a mixture of dilute hydrochloric acid, to which 

 was added a small proportion of a ten per cent, solution of ferrocyanide of 

 potassium. The hydrochloric acid, as well as the water used for diluting it, 

 must be free from iron ; neither must any iron implement be brought in 

 contact with the freshly broken surfaces of the tooth. Those parts of the 



