110 



combustible substances are produced by plants ; and its mechanical 

 value is to be estimated by determining the heat evolved by burning 

 them, and multiplying by the mechanical equivalent of the thermal 

 unit. Taking, from Liebig's Agricultural Chemistry, the estimate 

 2600 pounds of dry fir wood for the annual produce of one Hessian 

 acre, or 26,910 square feet, of forest land, (which in mechanical 

 value appears not to differ much from estimates given in the same 

 treatise for produce of various kinds obtained from cultivated land,) 

 and asuming, as a very rough estimate, 4000 thermal Units centigrade 

 as the heat of combustion of unity of mass of dry fir wood ; the 

 author finds 550,000 foot-pounds (or the work of a horse-power, 

 for 1000 seconds), as the mechanical value of the mean annual pro- 

 duce of a square foot of the land. Taking 50° 34' (that of Giessen,) 

 as the latitude of the locality, the author estimates the mechanical 

 value of the solar heat which, were none of it absorbed by the atmo- 

 sphere, would fall annually on each square foot of the land, at 

 530,000,000 foot-pounds ; and infers that probably a good deal 

 more, y oV o ^^ the solar heat, which actually falls on growing plants, is 

 converted into mechanical effect. 



When the vibrations of light thus act during the growth of plants, 

 to separate, against forces of chemical affinity, combustible materials 

 from oxygen, they must lose vis viva to an extent equivalent 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 or- 

 ganic products, and may be reproduced 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 mechanical 

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

 as subject to Carnot's principle ; and a possible connection 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-established theory 



