LIFE AS COMPARED WITH THE PHYSICAL PORCES. 37 



we compare a mammal with a bird on the one hand and with 

 a reptile on the other. Both these two live longer than 

 mammalia. But Avhilst the bird breathes more rapidly and 

 abundantly than the beast, whilst its blood circulates more 

 rapidly and has a higher temperature than that of the beast, 

 the reptile deviates in the opposite direction. The bird, it 

 may be said, differs from a lump of stone more widely than 

 does the beast, but the reptile differs less. What then is the 

 standard according to which life could be measured ? 



1 2. A further difference between life and the physical forces, 

 or modes of energy, is that we can convert the lotter into 

 each other. But we can effect no such transformations with 

 life. Let us take an instance; a lump of coal contains, 

 stored up, a quantity in proportion to its weight and its 

 quality. If we burn it, this chemical energy is converted 

 into heat. If we allow this heat to act upon the boiler of 

 a steam engine the heat is converted into mechanical motion. 

 These transformations are not arbitrary in their quantity but 

 quite definite. 



13. We may go further : if we use the mechanical poAver to 

 make a coil of wire revolve between the two poles of a mag- 

 net the mechanical power is thus transformed into electricity, 

 which again by certain arrangements can be made to appear 

 as light (arc-light). Similar transformations backwards and 

 forwards can be effected in different directions. We can 

 convert heat directly into light. We can transform motion 

 into heat as in the well-known process which ensues if the 

 axle of a wheel in quick rotation is not supplied with any 

 lubricant to reduce the friction. We convert mechanical 

 power into electricity by rubbing a piece of glass or amber 

 with rough dry silk — the manner m which electricity was 

 jBrst observed. 



14. But can we transform any one of these forces or agencies 

 — call them what we please — into life ? No ; life as we know 

 it, can certainly exist only between certain limits of tempera- 

 ture, probably at the outside between — 100° to + 300° 

 Fahrenheit. If we reduce the temperature still lower all 

 forms of life die out as do most long before reaching this 

 lower limit. If we heat liviDg beings too strongly they also 

 cease to exist. Some of the spores of microscopic organisms 

 can resist a temperature exceeding that of boiling water. 

 But very few of the larger and more highly developed 

 animals and plants can bear a heat exceeding 120"" or below 

 10° Fahrenheit. 



