x PREFACE. 



vantages arise if wave-lengths are used in the description of photographic 

 phenomena. I published steel engravings of that spectrum so arranged. 

 I made an investigation of phosphorescence, and obtained phosphores- 

 cent pictures of the moon. Up to this time it had been supposed that 

 the great natural phenomenon of the decomposition of carbonic acid 

 by plants was accomplished by the violet rays of light, but by perform- 

 ing that decomposition in the spectrum itself, I showed that it is effected 

 by the yellow. Under very favorable circumstances, I examined the ex- 

 periments said to prove that light can produce magnetism, and found 

 that they had led to an incorrect conclusion. The first photographic 

 portrait from the life was made by me : the process by which it was ob- 

 tained is herein described. I also obtained the first photograph of the 

 moon. I made many experiments on and discovered the true explana- 

 tion of the crystallization of camphor towards the light. When Da- 

 guerre's process was published, I gave it a critical examination, and 

 described the analogies existing between the phenomena of the chemical 

 radiations and those of heat. For the purpose of obtaining more accu- 

 rate results in these various inquiries, I invented the chlor - hydrogen 

 photometer, and examined the modifications that chlorine undergoes in 

 its allotropic states. Since in such researches more delicate thermom- 

 eters are re-quired than our ordinary ones, I entered on an investigation 

 of the electro-motive power of heat, and described improved forms of 

 electric thermometers. In these memoirs will be found a description of 

 the method made use of for obtaining photographs of microscopic ob- 

 jects, together with specimens of the results. In a physiological digres- 

 sion respecting interstitial movements of substances, I examined the pas- 

 sage of gases through thin films such as soap-bubbles, and the force with 

 which these movements are accomplished, applying the facts so gathered 

 to an explanation of the circulation of the sap in plants, and of the blood 

 in animals. Returning to an inquiry as to the distribution of heat and of 

 chemical force in the spectrum, I was led to conclude, in opposition to 

 the current opinion, that all the colored spaces are equally warm ; and 

 that, so far from one portion the violet being distinguished by pro- 

 ducing chemical effects, every ray can accomplish special changes. This 

 series of experiments on radiations is concluded in this volume by an 

 examination of the chemical action of burning-lenses and mirrors. 



I have endeavored to reproduce these memoirs as they were original- 

 ly published. When considerations of conciseness have obliged me to 

 be contented with an abstract, it has always been so stated, and the 

 place where the original may be found has been given. Sometimes, the 

 circumstances seeming to call for it, additional matter has been intro- 



