328 FRAGMENTS OF SCIENCE. 
paper. He goes fully through the calculation of the 
mechanical equivalent of heat. He calculates the perform- 
ances of steam-engines, and finds that 100 Ibs. of coal, in a 
good working engine, produce only the same amount of 
heat as 95 Ibs. in an unworking one; the 5 missing Ibs. 
having been converted into work. He determines the use- 
ful effect of gunpowder, and finds nine per cent, of the 
force of the consumed charcoal invested on the moving 
ball. He records observations on the heat generated in 
water agitated by the pulping-engine of a paper manufac- 
tory, and calculates the equivalent of that heat in horse- 
power. He compares chemical combination with mechanical 
combination the union of atoms with the union of falling 
bodies with the earth. He calculates the velocity with 
which a body starting at an infinite distance would strike 
the earth's surface, and finds that the heat generated by 
its collision would raise an equal weight of water 17,356 
degrees C. in temperature. He then determines the 
thermal effect which would be produced by the earth itself 
falling into the sun. So that here, in 1845, we have the 
germ of that meteoric theory of the sun's heat which Mayer 
developed with such extraordinary ability three years after- 
ward. He also points to the almost exclusive efficacy of 
the sun's heat in producing mechanical motions upon the 
earth, winding up with the profound remark, that the 
heat developed by friction in the wheels of our wind and 
water mills comes from the sun in the form of vibratory 
motion; while the heat produced by mills driven by 
tidal action is generated at the expense "of the earth's axial 
rotation. 
Having thus, with firm step, passed through the powers 
of inorganic nature, his next object is to bring his 
principles to bear upon the phenomena of vegetable and 
animal life. Wood and coal can burn; whence come their 
heat, and the work producible by that heat? From the 
immeasurable reservoir of the sun. Nature has proposed 
to herself the task of storing up the light which streams 
earthward from the sun, and of casting into a permanent 
form the most fugitive of all powers. To this end she has 
overspread the earth with organisms which, while living, 
take in the solar light, and by its consumption generate 
forces of another kind. These organisms are plants. The 
vegetable world, indeed, constitutes the instrument where- 
