aquarium a somewhat brackish water circulates of A = 0.089 (ori- 
ginally water from the river Vecht). The percentage of salt of the 
lake of Geneva is given in Cart Voar’s Lehrbuch der Geologie 1. 
p. 538 as 0.1574°/, which would point to a freezing point of only 
0°.01. According to the figures collected by Dupors (see Verslagen 
1900 p. 12 and 30) A in Lakes Wener and Wetter is still lower. 
The osmotic pressure in atmospheres at 0° which we shall hence- 
forth denote by P,, is obtained by multiplying A by the factor 
12.08 according to Srentus') or 12.03 according to Jorissen *) and 
hence is in freshwater of the order of '/, to */, atmosphere. In 
such a medium the cold-blooded vertebrate animals, breathing mostly 
through gills, maintain in their blood an osmotic pressure of six 
atmospheres! With birds and mammals (see the table of HAMBURGER, 
Le. I. p. 456) also a pretty constant freezing point of the blood 
has been found. I have proposed to call the power of keeping ?, 
at a certain level, albeit within certain limits ‘“‘zdeotony” *), a property 
comparable with the homoiothermic power. That also the freshwater 
bony fishes possess this ideotony can hardly be doubted from the 
results communicated. The limits between which the figures of the 
same species lie, are narrow, only in the tench the differences are 
fairly considerable. The ideotony is mest conspicuous when the 
agreement between the cold-blooded freshwater veriebrates among 
each other and the great difference with P, of the surrounding medium 
are remembered. One is led to the supposition that for these animals 
which indeed are not closely related: Cyclostomes, Teleosteans, 
Amphibians and a reptile, the ZP, of about 6 atmospheres is an 
optimum. For warm-blooded animals there seems to be a tendency 
to maintain P, at 6°/, to 7'/, atmospheres; A’s of 0.570 with man 
and of 0.6 to 0.625 with mammals and birds are namely kept up 
with great constancy. 
The kidneys are the regulators. For the A of the urine of man 
varies between 012 and 3 (Hampurerr Le. I. p. 317) when the 
separately discharged portions are examined, whereas A for the whole 
quantity of 24 hours varies from about 1.3 to 2.4. For normal man 
Scnoutr *) found that A of the blood, provided digestion were eli- 
1) Srenius. Ofversigt af Finska Vetenskaps-Societetens Förhandlingar 46. No 6. 
1903—4. 
2) W. P. Jorrssen. Physisch-chemisch onderzoek van zeewater. Chem. Weekbl. 
le jaarg. No 49, p. 731. Sept. 1904. 
3) M. CG. Dexuuyzen. Ergebnisse von osmotischen Studien, namentlich bei Knochen- 
fischen, an der Biol. Stat. d. Berg. Museums. Bergens Museums Aarbog. 1904. No 8. 
4) D. Senoure. Het physisch-chemisch onderzoek van menschelijk bloed in de 
kliniek. Diss. Groningen. 1903. 
