THE PHYSICAL ENVIRONMENT OF ORGANISMS 



547 



rigidity with light weight, streamlining, and the acquisition of powerful 

 flight muscles with skeletal modifications for their efficient origin and 

 insertion. 



The medium and fluctuations in temperature. The temperature 

 relationships of water differ markedly from those of air or land. The 

 most important of these differences 

 are due to the specific heat of water 

 and its latent heats of freezing and 

 of evaporation. By the specific heat 

 of water is meant the amount of heat 

 (calories) required to raise the tem- 

 perature of 1 gram of water 1°C. We 

 have already seen that this amount 

 is 1 calorie. 1 Water, with a specific 

 heat of 1.0, may be compared with 

 some other substances: lead, 0.012; 

 mercury, 0.0334; carbon, 0.16; sand, 

 0.15; alcohol, 0.535. Only ammonia 

 exceeds water in specific heat, and 

 nearly all other substances have a 

 markedly smaller value than water. 



By latent heat is meant the amount 

 of heat required to change water 

 from its solid state to a fluid state 

 and from a fluid state to a gaseous 

 state without a change in tempera- 

 ture. To change 1 gram of ice to 

 water (with the temperature kept at 

 0°C.) requires 80 calories, and when 

 1 gram of water freezes, it gives up 80 

 calories to the surrounding medium. 

 The latent heat of evaporation is 

 even greater. To change 1 gram of 

 water to 1 gram of water vapor with- 

 out a change of temperature requires 

 from 590 to 540 calories (depending upon the temperature), and this 

 amount of heat is given up when the water vapor condenses. In this 

 respect water is hardly approached by any other substance. 



As a consequence of these properties the temperatures of bodies of 

 water tend to be very stable and to change relatively slowly. In a locality 

 where air and soil surface temperatures not infrequently change as much 

 as 30°C. in 24 hours, the ponds and streams will in the same period show 



1 One-thousandth of a great Calorie — see footnote, p. 63. 



Fig. 32.3. A naked marine snail, Dendro- 

 notus arborescens, with large branched gills. 

 This slow-moving creature has no need for 

 streamlining. (Courtesy American Museum 

 of Natural History.) 



