guided by percentages alone they may often be led astray. 

 The real value of a determination of the quantity of 

 organic impurity in a water is, that by it a very shrewd 

 notion can be obtained as to what has had access to that 

 water." 



However startling these statements may be to those 

 who judge of the wholesomeness of a water by the amount 

 of organic matter it may contain, we believe it to be none 

 the less an accurate description of facts. It is -within 

 our knowledge that some of our most wholesome supplies 

 sometimes contain an excess of organic matter, and that 

 the waters which give rise to typhoid fever and other 

 hardly less serious disorders are frequently just those 

 which contain the least, the difference of course being 

 that in the one case the organic matter is innocuous, in 

 the other deadly. 



Since, then, chemical analysis fails entirely to distin- 

 guish between these two kinds of matter, it may be 

 thought to be a work of supererogation to have recourse 

 to it at all. Not so, however, for what analysis fails to do 

 directly it can to a large extent do indirectly. Organic 

 matter in solution in water is more or less prone to 

 oxidation, the highly putrescible matter of sewage being 

 most so, and that derived from vegetation very much less 

 so. Hence it follows that one would expect to find the 

 oxidised nitrogen compounds in greater excess in the one 

 case than in the other, and as a matter of fact that is just 

 what we do find. Almost invariably, in all waters of 

 acknowledged wholesomeness, the quantity of nitrates 

 never exceeds a certain small amount, whereas in waters, 

 such as polluted well and spring waters, that have given 

 rise to illness, the oxidised nitrogen compounds, with other 

 accompaniments of sewage, are to be found in excess. 

 By means then of these oxidised nitrogen compounds we 

 get collateral evidence throwing light on the nature and 

 probable source of the contamination of which a mere 

 percentage estimation of organic matter would fail to give 

 the slightest indication. 



The mistake has been hitherto that the discussion has 

 been narrowed by looking at the question almost entirely 

 from a chemist's point of view. It is, however, to the 

 biologist that we must look chiefly for the future elucida- 

 tion of the subject, and he has a field of the widest 

 range, embracing mu:h untrodden ground, for his inves- 

 tigations. 



Putting on one side the specific poisons which through 

 the medium of water are able each to generate, after its 

 kind, diseases such as typhoid fever, it is highly probable, 

 judging from what has already been proved to take place 

 in analogous cases, that dangerous organic matter is not 

 poisonous as such, but acts by affording the pabulum for 

 organisms which are able to set up putrefactive changes 

 in the blood of the person drinking polluted water. Even 

 the conversion of organic matter into nitrates is not a 

 mere chemical process of oxidation, since we now know 

 that the oxidation only takes place by the help of a 

 distinct ferment. 



In the inquiry as to how far organic matter is destroyed 

 in rivers, it is clearly insufficient to rely upon laboratory 

 experiments in which diluted sewage is exposed only to 

 the oxidising influence of air. This is entirely to ignore 

 the agency of vegetation and of the vast army of 

 organisms, identical with or allied to bacteria, which, 

 being endowed with various functions of reorganisation, 

 convert the carbon and nitrogen of organic matter into 

 simpler inorganic compounds, these in turn to become 

 the food of the more highly organised aquatic vegetation. 



Whilst therefore duly recognising the practical help that 

 chemistry can aflbrd in the more limited scope that 

 properly belongs to it, we trust, in the interest of sanitary 

 science, that the enunciation of the views of so dis- 

 tinguished a biologist as Prof Huxley may have their due 

 weight with those to whom these questions are ordinarily 

 referred, and will tend to promote a better understanding 



NATURE [7///)' 8, iSSo 



and more solid ground for agreement than has up to the 

 present seemed possible. 



Charles Ekin 



THREE YEARS' EXPERIMENTING IN 



MENSURA TIONAL SPECTROSCOP V >■ 



Bv A New Hand thereat 



II. 



7/ie II7io/c- Sular Sptr/niin.—CowXA an observer, who 

 had once made close acquaintance with the glories of 

 svmmetry resident in great A of the solar spectrum, when 

 seen in the brightness of a southern noon-day, under a 

 dispersion of 33° and magnifying power of 10, ever remain 

 content therewith ? 



Never ! if a particle of soul belonged to him ! for he 

 would be imperiously constrained froin that moment to 

 feel that he must see the whole solar spectrum as it is 

 given forth effulgently to the denizens of the south by a 

 nearly zenith sun, before he died ; or to what purpose 

 would he have lived in a sun-illumined world ? 



Out, therefore, once more to Lisbon the experimenter 

 and his Wife went in 187S, with the important assistance 

 again of the Pacific Steam Navigation Company of Liver- 

 pool ; but now, armed with a rather different apparatus. 

 There was indeed the same heliostat and there were ail 

 the prisms belonging to the aurora spectroscope ; but in- 

 stead of each of them being looked through singly and 

 successively, they were now used all together, set out in a 

 curvilinear line several feet long on a large table, and 

 looked through all at once ; with telescope and collimator 

 each 32 inches in focal length ; with magnifying power of 

 20, and a further prismatic method supplanting the usual 

 employment of coloured glasses to prevent false glare in 

 the field of view ; and then what a new world was opened 

 up to behold and admire ! 



Lines multiplied on lines and in a perfection of finish 

 and refinement, sometimes of infinite thinness, sometimes 

 remarkable power ; and the classic fields of those more 

 refrangible portions of the spectrum where the great 

 spectroscopists of the age, Kirchhoff and Secchi, Lockyer 

 and Janssen, Huggins and Young, have chiefly gained 

 their laurels, as expounders of the constitution of the sun, 

 were surveyed with respect and all admiration ; but first, 

 foremost, and beyond everything else, were the glories of 

 the illimitable depths of solar colour ; colour, the best 

 leading index that has ever been invented yet, to 

 simplify and facilitate the description of all spectrum 

 place. 



After having got completely rid of those usual at- 

 tendant impurities in solar spectroscoping, viz., chemi- 

 cally coloured glasses used as shades, the large dis- 

 persions now employed enhanced rather than dulled the 

 solar colours ; raiied one's ideal of what colour in light 

 can be, and gave, through near fifty gradations, a definite 

 and ever-memorable colour-characterisation to as many 

 portions of the whole spectrum. 



In presence of siwh solar colours, it seemed to be a 

 wilful ignoring of one's best and plainest faculties to speak 

 of the spectrum colours as being only 3, or 5, or even 7. 

 They might indeed be rather spoken of as next to infinite 

 in number ; or rather still, as being just so many as there 

 are easily perceptible differences of spectral place ; but 

 for that law of locomotion of colour-bands within certain, 

 limits, already discovered by the experimenter in his 

 absorption spectra, and found equally applicable to the 

 solar spectrum. Confining therefore the number of 

 colours to something which should give each of them a 

 breadth, not likely to be overpassed by the locomotive 

 effects -j- and — on their boundaries, the following table 

 of fifteen spectral colours was prepared after much dis- 

 cussion and criticism of each individual'member of it: — 



^ Continued from p. 195. 



