258 Prof. Thomson on the Power of 



When the vibrations of light thus act during the growth of 

 plants, to separate, against forces of chemical affinity, combustible 

 matei'ials from oxygen, they must lose vis viva to an extent equi- 

 valent to the statical mechanical effect thus produced; and 

 therefore quantities of solar heat are actually put out of existence 

 by the growth of plants, but an equivalent of statical mechanical 

 effect is stored up in the organic products, and may be repro- 

 duced as heat, by burning them. All the heat of fires, obtained 

 by burning wood grown from year to year, is in fact solar heat 

 reproduced. 



The actual convertibility of radiant heat into statical mecha- 

 nical effect, by inanimate material agency, is considered in this 

 paper as subject to Carnot's principle ; and a possible connexion 

 of this principle with the circumstances regarding the quality of 

 the radiant heat (or the colour of the light), required to produce 

 the growth of plants, is suggested. 



On the Power of Animated Creatures over Matter. 

 The question, " Can animated creatures set matter in motion 

 in virtue of an inherent power of producing mechanical effect ?" 

 must be answered in the negative, according to the well-esta- 

 blished theory of animal heat and motion, which ascribes them 

 to the chemical action (principally oxidation, or a combustion at 

 low temperatui-es) experienced by the food. A principal object 

 of the present communication is to point out the relation of this 

 theory to the dynamical theory of heat. It is remarked, in the 

 first place, that both animal heat and weights raised or resistance 

 overcome, are mechanical effects of the chemical forces which act 

 during the combination of food with oxygen. The former is a 

 dynamical mechanical effect, being thermal motions excited ; the 

 latter is a mechanical effect of the statical kind. The whole me- 

 chanical value of these effects, which are produced by means of 

 the animal mechanism in any time, must be equal to the mecha- 

 nical value of the work done by the chemical forces. Hence, 

 when an animal is going up-hill or working against resisting 

 force, there is less heat generated than the amount due to the 

 oxidation of the food, by the thermal equivalent of the mecha- 

 nical effect produced. From an estimate made by Mr. Joule, it 

 appears that from | to g- of the mechanical equivalent of the 

 complete oxidation of all the food consumed by a horse may be 

 produced, from day to day, as weights raised. The oxidation of 

 the whole food consumed being, in reality, far from complete, it 

 follows that a less proportion than |-, perhaps even less than f, 

 of the heat due to the whole chemical action that actually goes 

 on in the body of the animal, is given out as heat. An estimate, 

 according to the same principle, upon very imperfect data, however, 

 is made by the author, regai-ding the relation between the thermal 

 and the non-thermal mechanical effects produced by a man at 



