ADAPTATION OF FISH TO TEMPERATURE 557 
2. It was found that dextrose solutions were not able to afford 
any protection against the effects of a sudden rise in temperature. 
From these and similar experiments with CaCl, solutions it fol- 
lows that the protective action of sea-water or a Ringer solution 
against high temperature is not an osmotic but a specific effect 
of the salts of the sea-water. 
3. It was ascertained how long it takes to immunize the fish 
against the harmful effects of a sudden transfer to a temperature 
of 35°C. It was found that by keeping the fish for thirty hours or 
more at a temperature of 27° they were immunized against a tem- 
perature of 35°. Often a noticeable immunizing effect was pro- 
duced already by an exposure of sixteen hours or even a little less 
to a temperature of 27°. Fish kept for two days at 27° were able 
to survive if suddenly transferred to distilled water of 35°C. 
4. The immunity against a temperature of 35° acquired by keep- 
ing the fish for two days at 27° is not lost or weakened if the fish 
_are afterwards kept as long as thirty-three days at a temperature 
of from 10° to 14°. Our experiments have not been extended 
beyond this period of time: 
5. The immunity against a temperature of 35°C. is also main- 
tained if the fish are kept after the two days’ exposure to 27° for 
two weeks at a temperature of 0°.4 C. 
6. Fish immunized against a temperature of 39° and then kept 
at a temperature of from 10° to 14° for eleven days did not lose 
their immunity. 
7. A longer exposure of fish to a femperature of 0°.4 may finally 
lower their resistance to high temperature. 
8. In order to immunize fish to a temperature of 39° it is not 
necessary to expose them continuously to a higher temperature. 
An intermittent exposure to a higher temperature during a number 
of hours each day will bring about the same effect. 
9. Various suggestions for a possible theory of these phenomena 
are made. 
