INTRODUCTORY NOTE. 



In these volumes the principal scientific writings of Joseph 

 Henry are collected for the first time. They include his 

 contributions to various societies and journals, together with 

 notices of a few of his earlier communications which were 

 never published in full. These productions, extending over a 

 long and busy life, are naturally grouped under two periods: 

 the first comprising the record of his researches from 1824 

 to 1846 (a period of 23 years), during his professorial career 

 at Albany and Princeton ; the second that of his scientific 

 work from 1847 to 1878 (a period of 32 years), during his 

 directorship of the Smithsonian Institution at Washington. 



It will be observed that Professor Henry's contributions 

 to science were given to the world from time to time through- 

 out more than half a century, and published in widely remote 

 places. Most of them are now scarce, many are practically 

 inaccessible, hardly any individuals and few public libraries 

 can be supposed to possess them all. It is noteworthy, and 

 indeed is characteristic of their author, that he sedulously 

 abstained from publishing any of his researches of the later 

 period or reproducing any of the earlier ones — very import- 

 ant though he knew them to be — through the inviting 

 channel of the "Smithsonian Contributions," or "Miscel- 

 laneous Collections," or in any way at the expense of the 

 Smithsonian fund. 



It has seemed to the Regents of the Smithsonian Insti- 

 tution that justice to the scientific name and memory of 

 their distinguished Secretary who made the Institution 



(iii) 



