1815.] Smithson Tennant, Esq. 93 
vestigation to which he applied himself was an endeavour to ascer- 
tain from whence the fodine found in several marine plants is de- 
rived. On this he had laboured assiduously during the spring and 
summer of 1814; and early in September in that year, the evening 
before he left London, he mentioned toa friend that he had de- 
- tected iodine in sea water. A tarnishing which he had observed in 
silver leaf, * Jed him to promise himself a successful termination of 
these researches; but it is not known what were the decisive expe- 
riments by which he had succeeded in making this discovery. 
Mr. Tennant had always lamented his omission to visit the Con- 
tinent of Europe-during the short peace of 1802. He therefore 
took an early opportunity, after the general pacification of 1814, of 
passing over to France for the purpose of observing those changes 
which the eventful period of the last twenty years has produced, 
and of renewing his personal communications with men of science 
at Paris, from which he had been so long debarred. His own ex- 
perience had taught him how much may be known, which has not 
been communicated in books. In this respect he was not disap- 
pointed ; for in one of his letters, written a short time before he 
left Paris, he mentioned with much satisfaction how many interest- 
ing facts he had collected which would enliven his Cambridge 
lectures. 
He went to France early in September, 1814; and the following 
passage of a letter, in which he relates his first sensations, may be 
worth transcribing, both because it affords somewhat of a specimen 
of his general manner, and may perhaps recall to the recollection of 
his friends several of his favourite topics and opinions. ‘“ After a 
short and favourable passage of three hours and a half, we got into 
the harbour of Calais, with its immense pier. The difference of 
every thing struck ime prodigiously. 1 felt quite intoxicated with 
* The same test for iodine in iodic salts is proposed by Sir Humphry Davy 
in the Philosophical Transactions for 1514, 
+ Among the different series of experiments alluded to in this memoir, upon 
which Mr, Tennant had been engaged at various periods of his life, but which he 
had not brought te a completion, the following may deserve to be mentioned :— 
Researches on the pigments used by the ancients, 
Experiments made with a view to improve the glass employed in the construc- 
tion of achromatic lenses, 
Experiments on the refractive powers of compound bodies compared with the 
refractive powers of their constituents, 
Mr. Tennant had at one time very nearly obtained an insight into the wonderful 
class of phenomena belonging to vollaic electricity ; as appears from the following 
extract from an old note-book, in which there is no date; but Mr. T, always 
spoke of the experiment as having been made many years since, 
*€ If a piece of silver or gold is immersed in a solution of vitriol of copper, 
and the silver or gold is douched with iron or zinc, the copper is diffused upon 
them around the point of contact; upon platina, not so easily; the iron, though 
very near, occasions no precipitate upon the silyer or gold; but if iron touches 
tilver, and silver gold, the latter gets the copper.” 
