55 



Correctionernes Bestemmelse udfortes ved Sjælp af 

 Vandprøverne I og VII, hvis Egenvægter .tidligere ere be- 



17.°5 



stemte til 1.02691 oa 



02669 ved '_ . For det paa 



1 1:.) 



første Togt anvendte Aneometer erholdtes saaledes gjennem 

 5 Aflæsninger i I Oorrectionen 0.00023 og gjennem 12 

 Aflæsninger i VII ligeledes — 0.00023. Paa samme Maade 

 bestemtes det andet Aræometers Correctioner ved 5 Aflæs- 

 ninger i I til —0.00037 og ved 8 Aflæsninger i VII til 

 — '0.00038. Under disse Athesninger var Yadsken altid 

 bragt til 17.°5 eller en meget nærliggende Temperatur, 

 hvorfra Aflæsningeme eiter de forhen gjengivne Correctio- 

 ner reducéredes til Normaltemperaturen. Gjennem flere 

 Rækker Aflæsninger i Vandprøven I ved forskjellige Tem- 

 peraturer har jeg tillige forvisset mig om, at den ved Be- 

 regning af Correctionstabellen benyttede Udvidelsescoefficient 

 for Aræometrene er passende valgt. 



Hermed er givet de fornødne Data til Reduction af 

 de paa den norske Nordhavsexpedition aflæste Egenvægter, 

 dl jeg gaar dernæst over til Bestemmelsen af Relationerne 

 mellem Saltgehalten, Chlormængden og Egenvægten. 



Til Bestemmelse af Saltmængden har. saavidt jeg ved. 

 tidligere kun været benyttet den simpleste Methode, bestaa- 

 ende i Afdampning af Våndet og Residuets Tørring ved en 

 passende Temperatur, som af de forskjellige Ghemikere er 

 bleven valgt noget forskjelligt fra 150° — 180". Denne 

 Methode har jeg imidlertid af tiere Grunde fundet lidet 

 tilfredsstillende, hvad man ogsaa paa Forhaand maatte vente. 

 Etter Graham 1 og andre taber nemlig den svovlsure Mag- 

 nesia, om hvis Tilstedeværelse i Søvandet der vel ikke kan 

 reises Tvi vi, forst ved en Temperatur af over 200° sit 

 sidste Molekyl Vand. medens man paa den anden . Side 

 allerede ved en Temperatur af betydeligt under 200" maa 

 befrygte en delvis Decomposition af den i Saltene tilstede- 

 værende Chlormagnesium. Etter de Forsøg, som jeg an- 

 stillede. viste det sig. at Saltene selv etterat være tørrede 

 ca. 20 Timer i Luftbad ved en Temperatur fra 170° — 

 ISO" endnu indeholdt ikke ubetydelige Mængder Vand (om- 

 kring 15 Mgr. pr. Gr. Salt), medens de tørrede ved lidt 

 lavere Temperatur indeholdt noget mere. Samtidig under- 

 søgtes ogsaa Saltene paa fri Magnesia, hvorved jeg i Strid 

 med ældre Angivelser fandt. at de bestandig selv ved Tør- 

 ring ved 160° til 170" indeholdt uventet store Qvantiteter, 

 saaat der for hvert Gr. tørret Salt fandtes en Magnesia- 

 mængde tilstrækkelig til at neutralisere over 20 Mgr. HC1 (ved 

 Tørring ved 180" fandt jeg endog en enkelt Gang 40 Mgr.). 

 Bestemmelsen af den frie Magnesia foretoges ved Saltenes 

 < tplosning i en afmaalt Mængde titreret Svovlsyre og der- 

 paa følgende Retitration med fortyndet Natronlud af be- 

 kjendt Styrke. Ved Anvendelse af Rosolsyre som Index 

 erholdtes her en meget skarp Endereaction. 



Phil. Mag. J. (i — 422. 



The determination of the corrections was performed 

 with the water of samples I and VII, whose specific grav- 



ity at \'. had been found to be respectively 1.02691 and 



1.02669. For the areometer used on the first voyage. .*> 

 readings with the water of sample 1 gave the correction 

 — 0.00023, and 12 readings with the water of sample VII 

 likewise — 0.00023; in the same manner, the corrections 

 for the other areometer were determined. by 5 readings 

 with the water of sample I, to be — 0.00037, and. by 8 

 readings with the water of sample VII. to be — 0.00038. For 

 these readings. the fluid was always brought to 17.°5, or 

 as near that temperature as possible, the readings ha ving 

 in the latter case to be reduced. by means of the correc- 

 tions given above, to the normal temperature. Several 

 series of readings with the water of sample I, at different 

 temperatures, convinced me that the coefficient of expan- 

 sion for the areometer which I had computed for prepar- 

 ing the Table of Corrections was practically correct. 



Having now specified the data necessary for reducing 

 the specific gravities read on the Norwegian North-Atlantic 

 Ex4>edition, I shall proceed to determine the relation be- 

 tween the specific gravity of sea-water and the amount 

 of salt and chlorine it contains. 



For determining the amount of salt, the only method 

 formerly resorted to was, so far as I am aware, the sim- 

 plest, viz. that of evaporating the water and then drying 

 the rcsidue at a proper temperature, which has been vari- 

 ously tixed by different chemists at from 150" to 180°. 

 This method, however. has proved in several respects de- 

 fective, as was indeed to be expected. According to Gra- 

 ham 1 and others, sulphate of magnesia, the presence of 

 which in sea-water can hardly admit of doubt, does not 

 part with its last molecule of water till exposed to a temp- 

 erature of more than 200° whereas, on the other hand. it 

 is highly probable that partial decomposition of the chloride 

 of magnesium contained in the salt takes place consider- 

 ahly below 200°. Even after the salts had been dried for 

 about 20 hours in an air-bath at a temperature of 170° — 

 180°, they were still found to contain, according to my 

 experiments, a considerahle quantity of water (about 15'""'' 

 salt per gramme); dried at a lower temperature, the 

 amount was somewhat greater. I also tested the salts for 

 free magnesia, and found, in direct opposition to earlier 

 statements. that. even when dried at 160" — 170°, they in- 

 variably contained a very large amount, the quantity of 

 magnesia to every gramme of dried salt being sufticient to 

 neutralize more than 2t»'"''HCl (oncé, when' dried at 180°, 

 even -!(>'"■'). For determining the free magnesia, the salts 

 were dissolved in a given quantity of titrated sulphuric 

 acid. and the fluid then retitrated w T ith dilute soda-lye of 

 known strength. With rosolic acid as the index, the final 

 reaction was verv decided. 



1 Phil. Mag. J. ii. p. 122. 



