350 BACTERIA. 



tooth containing iron, even in minute quantities, will, after an exposure from 

 one to sixty minutes, assume a blue colour — Prussian blue being formed. 

 One source of error is introduced in the necessary use of an iron instrument 

 in extracting the teeth, but this will only affect those points on the external 

 surface of the tooth with which the forceps come in contact, and therefore 

 may be easily eliminated." He found by these experiments that there were 

 minute traces of iron in Nasmyth's membrane, in the dental pulp (though 

 not constantly), in carious dentine and in enamel, and he considers that it 

 is quite possible that the sulphide of iron that may be formed during putre- 

 faction of the pulp — a process that is set up by micro-organisms — may have 

 something- to do with the discoloration, though it is quite possible that 

 much of the discoloration is due to the iron that is taken into the mouth 

 along with the food, the putrefactive processes set up in the mouth liberating 

 the sulphuretted hydrogen, which, combining with the iron brought from 

 outside, gives the discoloration already referred to. 



In addition to this coloration, the result of the formation 

 of inorganic salts, we have those violet, magenta, green and 

 yellow colours that appear to be distinctly organic in 

 character, and to be the result of the metamorphoses of 

 albuminoid substances. As above stated, these may be 

 related to the aniline colours, though this has certainly not 

 yet been proved to be the case. It may be well to bear in 

 mind, however, in this connection what takes place in the 

 process of colour formation set up by the Bacillus iluorescens 

 putidus, in which we have not only a colour resembling an 

 aniline colour, but we have a distinct odour of trimethyla- 

 mine, a substance nearly allied to the cyanogen compounds 

 from which, as we know, the most beautiful red, blue, and 

 yellow products are readily obtained when combining with 

 iron in certain definite ways. Of course this is only given 

 as an example of what might take place, and not as repre- 

 senting any accurate work that has been done, for it appears 

 that up to the present very little definite kncfwledge has been 

 obtained as to the nature of the pigments contained within the 

 protoplasm of micro-organisms or of those diffused from it 

 into the surrounding tissues. What we do know is, that a 

 large number of the saprophytic decomposition-producing 

 bacteria give rise, when grown under certain conditions, to 

 most exquisite colour products, that by altering the con- 

 ditions, as in the case of the Micrococcus prodigiosus — subject- 

 ing it to a higher temperature, for example — the power of form- 

 ing these colours may remain in abeyance, the energy of the 

 protoplasm being diverted into the formation of some other 

 substance — in this instance, lactic acid. It has, however, 



