332 



NA rURE 



{Feb. 24, 1876 



Against these advantages the process of analysis advo- 

 cated by the speaker involves more trouble and more 

 careful manipulation than are usually bestowed upon what 

 are called " commercial" analyses, and although these 

 drawbacks ought not to be paramount considerations, 

 where such important issues are involved, yet if any other 

 more simple method existed from which trustworthy 

 quantitative information about the organic matter in water 

 could be obtained, the more troublesome process would 

 cease to have a raison d'etre. 



Such a simple alternative method of determining organic 

 nitrogen, but not organic carbon, is now very extensively 

 used by chemists. It is known as the "albuminoid 

 ammonia" method, and depends upon the fact that, by 

 boiling with an alkaline solution of potassic perman- 

 ganate, most nitrogenous organic bodies are decomposed 

 with evolution of ammonia. From the amount of am- 

 monia so evolved, the proportion of organic nitrogen is 

 calculated. A critical examination of the results obtained 

 by this method conclusively demonstrates that it is in- 

 capable of converting into ammonia either the whole, or 

 any definite proportion, of the organic nitrogen of potable 

 waters. Indeed, this is shown not only by the following 

 instances, but also by numerous others in which known 

 quantities of nitrogenous organic matters of known com- 

 position were submitted to the process. 



Results of analysis expressed in parts per ioo,oco : — 



Artificial Watei's containing Peaty Matter. 



Organic nitrogen ^tyl'lbuminoT 

 by combustion, ammonia process. 



Sample No. i '068 part ... '016 part. 



,, No. 2 '042 „ ... "016 „ 



,, No. 3 -076 „ ... 022 „ 



„ No. 4 1-015 „ ... -308 „ 



„ No. 5 1175 ,. ••• -422 „ 



„ No. 6 -029 „ ... on „ 



on 

 •012 

 •024 

 •oc6 

 •030 

 •003 

 •004 

 •006 

 •018 



It is almost superfluous to say that any opinion as to 

 the quality of a sample of water, based upon the albu- 

 minoid ammonia obtained, must be entirely untrust- 

 worthy. 



Dr. Frankland sun med up with the following con- 

 clusions, to which he had been led by the experiments of 

 himself and others — 



1. That the "albuminoid ammonia" process of ana- 

 lysing water affords no evidence whatever of the absolute 

 quantity, either of organic matter, or of organic nitrogen 

 present in potable water. 



2. That it does not indicate, even approximately, the 

 relative quantities either of organic matter or of organic 

 nitrogen in different samples of such water. 



3. That it affords no indication, either of the presence 

 or of the proportion, of albuminoid as distinguished from 

 other nitrogenous organic compounds. 



4. That the " combustion " process, though more 

 troublesome, is the only method at present known which 

 affords any trustworthy information respecting the orga- 

 nic matters present in potable waters. 



5. That it is the only method which even professes to 

 determine organic carbon in such waters. 



6. That the determinations by it of organic carbon and 

 nitrogen are fairly accurate, notwithstanding the very 

 minute quantities of matter dealt with, and that the errors 

 even of a comparatively inexperienced analyst fall far short 



of the limits which would affect a verdict upon the quality 

 of the water submitted to investigation. 



7. That it is the only process which discloses the pro- 

 portion of nitrogen to carbon in the organic matter of 

 waters, such information being often of prime import- 

 ance in reference to the origin of the organic matter. 



8. That since the improvements which have been made 

 in the mode of evaporating the water to be analysed, the 

 process can now be conducted in any laboratory and with 

 a moderate expenditure of time and labour. 



RELATION BETWEEN THE LIMIT OF THE 

 POWERS OF THE MICROSCOPE AND THE 

 ULTIMATE MOLECULES OF MATTERS 



'T^HE subject which I have selected for my address is 

 -*■ the relation between the limit of the powers of the 

 microscope, and the ultimate molecules of organic and 

 inorganic matter. I think I may at all events claim for 

 this question sufficient novelty. Until the last few years 

 the subject could scarcely have been attempted, and even 

 now so many necessary facts are imperfectly known, that 

 nothing more can be done than to fill the gaps with plausible 

 assumptions. This necessarily imparts more or less of a 

 speculative character to some of my remarks ; but it 

 appears to me that in his annual address the president of 

 a society cannot do better than endeavour to point out the 

 general bearings of what is already known on some great 

 question, even if for no other object than to prove the need 

 of further inquiry. 



Though fully impressed with the imperfect state of our 

 knowledge, yet, even now, the facts are sufficiently definite 

 to indicate, if not to prove, the existence of as wide a 

 world of structure beyond the limit of the power of the 

 microscope, as what has been revealed to us by it is 

 beyond the powers of the unassisted eye. 



I propose to divide my subject into three heads— 



1. The limits of the power of the microscope. 



2. The size of the ultimate molecules of organic and 

 inorganic matter. 



3. Conclusions to be drawn from the general facts. 



In considering the limits of the power of the micro- 

 scope, I shall assume that the instrument itself is perfect, 

 and that the invisibility of the objects examined is in no 

 way dependent on the want of the necessary characters. 

 The point to which I particularly wish to direct attention 

 is the limit of visibility depending on the constitution of 

 light, beyond which light itself is of too coarse a nature to 

 allow of our seeing objects distinctly defined. This 

 question has been treated of in an admirable manner by 

 Helmholtz in the jubilee volume oi Poggendorff''s Annalen 

 (1874, p. 573). The conclusion to which he arrives is that 

 the limit depends on the confusion in the image due to 

 the bright interference fringes overlapping the dark out- 

 lines of the object. This limit vanes directly as the 

 wave-length of the light, and inversely as the sine of half 

 the angle of the aperture of the object-glass when illumi- 

 nated by means of a condenser of equal aperture. Accor- 

 ding to this principle the limit for dry object glasses of 60° 

 aperture is, roughly speaking, about equal to the wave- 

 length of the light, and for the largest possible aperture 

 equal to \ the wave-length. In the case of immersion 

 object glasses of the same aperture, the limit is about | of 

 that for dry. 



On comparing this theory with the results of observa- 

 tion, the agreement is very striking. It indicates exactly 

 the same law for the increased defining powers of lenses of 

 large aperture, as has been determined by experiment, and 

 gives for the theoretical limit of distinct visibility sjjoacfth 

 part of an inch, which is exactly the same as the mean 

 of the experimental determination of a number of the 

 most skilful microscopists. It also shows why in the case 



I Anniversary Address of the President of the Royal Microscopical Society, 

 H. C. Sorby, F.R.S., &c. Abstract by the Author. 



