PREFACE. x j 



duced ; but this lias always been formally indicated under the title of 

 NOTES, or included in parentheses. An instance of the former occurs on 

 page 45, of the latter on page 30. Wood-cuts and their explanation, sel- 

 dom occurring in the original publications, have been introduced. They 

 have for the most part been obtained from some popular articles pub- 

 lished by me in Harper's Magazine. 



Except in a few instances, I have adhered, in these memoirs, to the chem- 

 ical nomenclature in use at the time they were written. Though often 

 very weighty reasons may be given that the designations under which 

 substances pass might be made more in accordance with their constitu- 

 tion or properties, and therefore more correct, yet such are the con- 

 fusions, the inconveniences, the difficulties attending an introduction of 

 new names, that sweeping changes of nomenclature should never be in- 

 troduced until they have become absolutely indispensable. In Memoir 

 X., which treats of the action of the leaves of plants under the influence 

 of yellow light, I have preferred to retain the term carbonic acid instead 

 of any of its more recent synonyms. 



Here and there the reader will detect statements that seem to be con- 

 tradictory. On examination, however, he will find that this arises from 

 changes which the general progress of science had made necessary. As 

 an illustration of what I mean, I may refer to what is given as regards 

 wave-lengths (from Mosotti) on page 112. These numbers do not agree 

 with the more exact ones of Angstrom, page 120. It is better in such 

 cases to let the original statements stand. 



The pages offered in this volume, though not very numerous, repre- 

 sent a very large amount of work, the occupation of many years. Ex- 

 perimental investigation, to borrow a phrase employed by Kepler respect- 

 ing the testing of hypotheses, is " a very great thief of time." Some- 

 times it costs many days to determine a fact that can be stated in a line. 

 The things related in these memoirs have consumed much more than 

 forty years. 



Such a publication, therefore, assumes the character of an autobiog- 

 raphy, since it is essentially a daily narrative of the occupations of its 

 author. To a reader imbued with the true spirit of philosophy, even 

 the shortcomings easily detectable in it are not without a charm. From 

 the better horizon he has gained he watches his author, who, like a 

 pioneer, is doubtfully finding his way, here travelling on a track that 

 leads to nothing, then retracing his footsteps, and again, undeterred, 

 making attempts until success crowns his exertions. To explore the 

 path to truth implies many wanderings, many inquiries, many mis- 

 takes. 



Perhaps, then, since this book is a sort of autobiography, its reader 



