214 THE COXDITIOXS (3F LIFE IX THE SEA [PART III 



caution, since the presence of marine bacteria soon alters the 

 composition of the nitrogen compounds present. Natterer found 

 that the salts of nitrous and nitric acids were very scarce in the 

 samples of Avater examined by him — so much so that the quantities 

 just lay on the limits of quantitative expression, and are quoted 

 as " high," " relatively high," " relatively low," " low," and so on. 

 But ammonia salts were more abundant, and were present in 

 measurable quantities. They were most abundant in the water 

 samples taken from the bottom layers, or in the water filtered 

 from samples of mud collected from the bottom deposits. They 

 were least abundant in the upper well-lighted layers of the sea, 

 where plant life was more abundant than at lower depths. If 

 we exclude the results of analyses of bottom samples of water 

 we find that Natterer made 112 estimations of the amount of 

 ammonia salts in the water of the Mediterranean area, with the 

 following results : 



87 samples contained 0*0077 to 0'06 milligramme of NHg per litre 



19 » „ 0061 „ 0-1 



5 „ „ 0012 „ 015 



1 sample „ 0-32 „ „ „ 



Natterer's results are apparently unexceptionable and we may 

 take them as representing the nitrogen salts present in the waters 

 of a warm sea area. Raben's results are therefore of particular 

 interest since they refer to the waters of the colder temperate 

 seas. In 1904 water samples were collected, with all possible 

 precautions, by the German International Fishery Investigation 

 steamer, Poseidon, in the North Sea and Baltic. The samples 

 were collected in glass tubes previously evacuated of air, and 

 containing mercuric chloride for the sterilisation of the contents. 

 After filtration, so as to remove organisms, Raben estimated the 

 nitrites, nitrates, and ammonia, separately, having previously 

 examined the methods of determining these substances in minute 

 quantity. Nitrous acid was distilled off from the sample, after 

 the addition of acetic acid, and was estimated by means of the 

 metaphenylene-diamine sulphuric acid reaction. Ammonia was 

 distilled off from the sample after addition of magnesium oxide 

 and was estimated, with elaborate precautions, by Nessler's re- 



