336 



KNOWLEDGE. 



September, 1910. 



other words a cold-blooded animal can. under certain 

 circumstances, become ^\■arm-blooded. and a warm- 

 blooded animal can become cold-blooded, so that all 

 seems confusion. 



The important )ioint is not whether tile animal 

 feels hot or cold to human touch at any f^dven 

 moment of observation, but whether the animal can 

 or can not maintain a constant temperature while 

 the temperature of its environment alters. 



In health, birds and mammals are able to kee[) 

 their biid\- temperature the same, no matter how high 

 or how low (within limits) the temperature of the 

 environment goes. The temperature of all other 

 creatures rises with a rise and falls w ith a fall of the 

 outside temperature. In this latter class we must 

 include the human infant. hil)ernating mannnals and 

 newly-hatched birds. 



The power possessed by all li\ing things, (dants 

 included, to produce heat as one of the manifestations 

 of their vitality is referred to as thermogenesis. No 

 doubt the actual amounts of heat produced by 

 different animals are very different : a fish in unit 

 time per unit of its weight is producing vastl\- less 

 heat than a man. and a man less than a bird : but 

 the\' are all producing heat. Tlie thermogenic 

 centres are chiefly muscles, including the lieart, the 

 liver and the glands : these, and not the blood, are the 

 thermogenetic foci. A frog whose blood has been 

 replaced b\' a weak solution of common salt still 

 continues to excrete carbon dioxide for some da\s. 

 showing that oxidations are still going on in it 

 although it has no blood at all. It is just as certain 

 that an animal producing carbon dioxide has been 

 and is producing heat, as that a candle which is 

 liberating carbon dioxide is producing its heat through 

 the oxidation of the carbon of its fat. 



Before we go further into the subject of animal 

 heat, it would be well to understand the difference 

 between temperature and fjuantity of heat. A 

 difference of temperature between two bodies. A and 

 B, is the expression of the fact that heat is leaving 

 A for B or vice versa. If two bodies are at the 

 same temperature, no heat w ill pass from the one 

 to the other. Temperature is of course usualh' 

 measured by the visible expansion of some substance, 

 mercury or alcohol, which can take in heat from a 

 hotter bod\-, or on the other hand allow heat to 

 leave it for a colder one. Temperature is to heat 

 what potential is to electricity. Temperature tells 

 us nothing about quantit\- of heat, but merely 

 whether heat is or is not leaving a given bod\', 

 and, in a certain sense, how fast it is lea\ing it. 

 Several things all of the same temperature ma\-, 

 however, feel very different to us on our handling 

 them ; some will feel w arm, some cold. For instance 

 the marble mantelpiece and a " Tweed " coat may 

 be at the same temperature as tested by a ther- 

 mometer, yet the former feels cold compared with 

 the latter. The chief reason is that marble is a good 

 conductor of heat compared \\ith the coat, and 

 abstracts from our skin so much more heat in a 

 given time than does the coat. Thus, getting in 



between linen sheets is so much " colder " than 

 getting into blankets, because linen is so much better 

 a conductor of heat than is wool. 



Measurement of the quantity of heat is effected by 

 an instrument known as the calorimeter. The 

 principle of this is that the heat produced by any 

 particular substance is all absorbed by a known weight 

 of water, which is. in consequence, raised a certain 

 number of degrees in temperature. A calorie, or unit 

 of heat, is defined as the quantity of heat transferred 

 to one kilogramme of water (2'2-lbs.) in order to 

 raise it one degree Centigrade in temperature. A 

 water-calorimeter is, then, essentialb' a metal box 

 immersed in a known quantity of water. To 

 ascertain how much heat an animal gives out in a 

 certain time, it is onl\' necessary to place it in the 

 inner box, which is of course properly ventilated. 

 The heat that radiates from its skin is conducted 

 through the box to the water : the heat of its expired 

 air is absorbed hv the water, the temperature of 

 which, we shall sa\", has risen n degrees. \\"e shall 

 suppose that the animal has done no external work 

 while it has been in the calorimeter. If the weight 

 ol waterisW units. then \\' X n isequal tothecaloriesof 

 heat lost h\ the animal in a gi\'en time. 



The problem might nnw he faced, from what 

 materials does the bod}- manufacture heat ? Since 

 the body onh- very exceptionall\- absorbs any heat 

 from outside sources — heat of the Tropics or Turkish 

 bath — it is quite clear that it nuist be constantly 

 taking in the wherewithal to manufacture heat so 

 continuously. Except in starving animals, the 

 source of heat is the oxidation of the food. 



Just as a steam-engine transforms the potential 

 chemical energ\- of the coal into the kinetic forms 

 of energ\- — heat and external work (movement) — 

 so the animal body transforms the potential energy 

 of the food into the acti\e forms of heat and muscular 

 movement. Both are machines for transforming 

 energy ; the one is of iron, the other of protoplasm, 

 but in both equally is the great law of the Conserva- 

 tion of Energy obej-ed. This ma\- be said to have 

 been one of the mathematico-phj'sical intuitions 

 of that great mathematico-physical physiologist, 

 Helmholtz. For a long time it was thought that the 

 animal organism was outside the pale of that great 

 generalisation, but it is now known that an animal 

 in a calorimeter will produce as many calories of 

 heat in a given time as would have been generated 

 b\- the burning of a weight of food equal to that of 

 the food digested during the period in question. It 

 is onl\- a question of difference of rate in the setting 

 free of the heat ; in burning it we get all the heat 

 out at once ; in digesting it we get out just as much 

 heat, but it is liberated much more slowly. The 

 inhabitants of cold countries are pre-eminently fat- 

 eaters ; the notion that fats give heat and sugars 

 energy is probabl}- not altogether fallacious. 



The actual amount of heat that each adult loses 

 per twent3--four hours is, in round numbers, about 

 three thousand calories, the ultimate source of which 

 is the oxidation of the various oxidisable chemical 



