PREFACE. 



DURING the past forty years I have devoted much time to the experi- 

 mental investigation of scientific topics, and have published the results 

 in various journals, pamphlets, and the transactions of learned societies. 

 They have been largely disseminated in European languages, and many 

 of the conclusions they have presented have been admitted into the 

 accepted body of scientific knowledge. 



It has therefore become desirable for me to collect these scattered 

 memoirs and essays together, and, since they are too voluminous to be 

 published in full, to offer an abridgment or condensation of those that 

 are of less interest. I propose in this book to include only such as are 

 connected with the effects of Radiations or of Radiant energy, these 

 having been distinguished by the American Academy of Science, as mani- 

 fested by its award to me of the Rumford medal for discoveries in light 

 and heat. A statement of the action of the Academy is annexed. 



Besides these, I have several other memoirs on chemical, electrical, 

 and physiological topics, some of them hitherto unpublished. These, for 

 the present, I must reserve. 



Among many other subjects treated of in these pages, the reader will 

 find an investigation of the temperature at which bodies become red-hot, 

 the nature of the light they emit at different degrees, the connection be- 

 tween their condition as to vibration and their heat. It is shown that 

 ignited solids yield a spectrum that is continuous, not interrupted. This 

 has become one of the fundamental facts in astronomical spectroscopy. 

 At the time of the publication of this Memoir, no one in America had 

 given attention to the spectroscope, and, except Fraunhofer, few in Eu- 

 rope. I showed that the fixed lines might be photographed, doubled 

 their number, and found other new ones at the red end of the spectrum. 

 The facts thus discovered I applied in an investigation of the nature of 

 flame and the condition of the sun's surface. I showed that" under cer- 

 tain circumstances rays antagonize each other in their chemical effect, 

 and that the diffraction spectrum has great advantages over the prismat- 

 ic, which is necessarily distorted. I attempted to ascertain the distribu- 

 tion of heat in the diffraction spectrum, and pointed out that great ad- 



