PREFACE. Vli 



No apology will be considered necessary for re- 

 producing the little piece " On the Absorption of 

 Light," the thirteenth in order of this collection. 

 Though it does not pretend to anticipate any of 

 the later experimental researches, and the reason- 

 ings grounded on them for concluding the con- 

 version of motion Into heat, electricity, and mag- 

 netism, it is, nevertheless, a step (though a small 

 one) in that direction, by showing that a state of 

 apparent rest In a material body Is not incom- 

 patible with the internal propagation ad tJifiniticm 

 within it of movement impressed on it from with- 

 out. It is veiy conceivable that the internal or 

 atomic organization of ponderable matter may be 

 such as to concentrate and localize, in Its Individual 

 molecular groups, the broken -up and dispersed 

 undulations caused by any external shock ; and so 

 preserve them from attaining that final state of 

 complete mutual counteraction which is there 

 contemplated. 



Some slight alterations in the wording, and 

 additions (not in every instance unimportant) to 

 the matter of the several Essays here reproduced, 

 have been made ; as well as, here and there, some 

 numerical corrections. In particular, the last little 



