IV 



PREFACE. 



beam acts, and hovr, of the rays of which that beam is composed, the yellow ray of light is the 

 operative principle. The mode by which the resulting decomposition is effected leads to an in- 

 vestigation of the absorption of light. 



As will be seen from the Appendix, in these investigations I have not restricted myself exclusively 

 to the subjects before me, but have, in most instances, followed them out into their physical details. 

 The chapters of the Appendix have all been published in various American and foreign works, each 

 chapter being a memoir, more or less complete, on the topic on which it treats. In that Appendix, 

 therefore, there will be found a large amount of experimental matter, of which an immediate use is 

 not made. But in philosophy no new facts are superfluous. At first sight, there might seem to be 

 no connexion between the fixed lines of the solar spectrum and the production of chlorophyl, yet, as 

 will be hereafter understood, very great use may be made of the former in investigations on the 

 nature of the latter. 



The remark which has just been made respecting the vague ideas which are entertained in rela- 

 tion to the mode of action of light, might also have been made in relation to capillary attraction. 

 This force, with exosmose and endosmose, are very favourite resources of the botanist in his diffi- 

 culties. In the chapters in which these topics are considered, I have endeavoured to show the 

 true relation which exists between endosmosis and capillary attraction, and how the latter itself 

 springs from electric excitement. I have shown how, in strict conformity with this principle, the 

 flow of the sap and the circulation of the blood may be explained ; the systemic circulation, for ex- 

 ample, arising as a necessary consequence of the deoxydation of arterial blood. 



The theory of these circulatory movements taking place in organized structures I have not here- 

 tofore published, though they have been taught in my lectures in this University. It will be seen 

 that the peculiarity is in tracing the action to chemical changes ; that the systemic circulation arises, 

 as has just been said, from the deoxydation of arterial blood ; the pulmonary circulation from the 

 oxydation of venous blood ; and the flow of sap in flowering plants from the carbonization of water 

 by the sunlight in their leaves. 



It might be supposed that Chapter XVIII., AP., is intended as a reply to the criticisms on some 

 parts of Chapter XIII., which have been published by M. EDMOND BECQUEREL, in the Annales de 

 Chimie. When that chapter was written, I had not, however, had the opportunity of seeing his 

 valuable memoir. The experiments there given were not made with the iodide, but the bromide 

 of silver, a circumstance which relieves me from much of the weight of the criticisms of that chem- 

 ist. It is not only from my own experiments, but also from those of Sir J. HERSCHEL, that I am led 

 to differ from M. BECCIUEREL, in regarding the iodide and bromide of silver as exhibiting very dif- 

 ferent changes in the spectrum. M. BECCIUEREL'S idea that there is but little difference between them, 

 does not seem to be consonant to experiment. 



Of the Appendix, several chapters have already been translated into different European lan- 

 guages, and a variety of criticisms upon them have been published. It is this circumstance which 

 has caused me to hasten the printing of the whole of these papers in this collected form now offered 

 to the public. Whatever care be taken in the preparation of such translations, it will necessarily 

 happen, that on many points mistakes will be made as to the meaning and views of an author. 

 This has happened in the present instance, both in Germany and France : for example, in Chapter 

 XV., which has been republished in Paris in the Annales de Chimie, errors of the kind occur. 

 Those who are familiar with these matters will find, when they look over th&e papers in the form 





