PREFACE. v 



now given, that views have been thus accidentally imputed to the author which he does not enter- 

 tain. 



It did not enter into my plan to reprint any of the valuable memoirs which have been published 

 by other chemists on these topics, more especially on those relating to the chemical agencies of 

 light I may, however, direct the attention of the readers to a description by Sir J. HERSCHEL, of 

 the spectrum alluded to in the Appendix (645). It is inserted in the Philosophical Magazine for 

 February, 1843. In this paper will be found what probably will prove to be the true theory of the 

 action of light on Daguerreotype plates, and the reader of this volume will be the more interested 

 in it, inasmuch as it contains criticisms on several of the leading doctrines here set forth. 



The different chapters of the Appendix are, for the most part, reprinted from sources mentioned 

 under the title of each. In a few instances slight changes have been made in them ; changes which 

 have been rendered necessary by the mode in which they were originally published. Thus some 

 of these papers were printed in various American, and others in European journals, and it some- 

 times happens that experiments are in consequence described twice over. This repetition I have 

 avoided by removing the paragraphs involved. In this manner I have entirely omitted to republish 

 in this work a paper on the constitution of the atmosphere, given in the Philosophical Magazine for 

 October, 1838, because nearly all its experimental matter had been previously published in the 

 American Journal of Medical Sciences, and is given in Chapter VI. 



The time has now arrived when both Vegetable and Animal Physiology are to have their founda- 

 tions laid on Chemistry and Natural Philosophy, the only basis which can elevate them from their 

 present deplorable position to that of true sciences. It is with this impression that the explanations 

 which I have given in this book of the mode by which light acts in determining organization, and 

 of the mechanical causes by which such organized matter is transmitted from point to point of 

 living systems (for these are the leading facts which this work is designed to illustrate), are offered 

 to the attention of chemical philosophers. 



JOHK WILLIAM DKAPBR. 

 Umxrnty of Scw-York, October 1, 1844. 



N.B. The references to the Appendix are made by the number of the paragraph, thus (Ap.. 100, 

 101). When the place referred to is in the body of the work, the letters AP. are omitted, thus 

 (100, 101). 



