1815.] Smithson Tennant, Esq. 85 
communicated to the Royal Society in the year 1799; shewing that 
carbonate of magnesia is an ingredient in the latter species of lime- 
stone, which he describes as an extensive stratum in the midland 
counties, and as being found also in many other situations, particu- 
larly among the primitive marbles, under the name of Dolomite. He 
gives the proportions in which lime and magnesia exist in many 
specimens of this lime-stone, and infers from its slow solution in 
acids, and from its crystalline structure, that it is rather the result 
of chemical combination than of a casual mixture of the two 
earths. This conjecture has since been verified by the goniometrical - 
researches of Dr. Wollaston, and by the near agreement of Mr. 
Tennant’s analysis with the constitution of the mineral as inferred 
from the law of definite proportions. 
Mr. Tennant had found that grain will scarcely germinate, and. 
soon perishes, in moistened and perfectly mild carbonate of mag- 
nesia; and that the injurious effects of the magnesia in agriculture do 
not depend on its property of long remaining caustic, but probably 
on some inherent quality of the earth itself. He also made many 
experiments on the germination aad growth of various seeds and 
plants in different mixtures of the simple earths; and transporting 
‘portions of soil, either from his own estates, or from different parts 
of England, to the neighbourhood of London, he tried on a small 
scale the effects of various manures in promoting the growth of 
different vegetables, and endeavoured in this way to obtain hints for 
improved modes of cultivation. 
In the year 1802 he communicated to the Royal Society his che- 
mical examination of Emery, which had hitherto been considered as 
an ore of iron, He showed that it consists principally of alumina, 
and that it nearly agrees with the Corundum of China, which had 
been analyzed a short time before by Klaproth. 
In the month of July during the same year, in endeavouring to 
make an alloy of lead with the powder which remains after treating 
crude platina with aqua regia, he observed remarkable properties in 
the powder, and found that it contained a new metal. But while 
he was engaged in pursuing this investigation, the attention of two 
French chemists was accidentally directed to the same object, In 
the autumn of 1803 M. Descotils had discovered that the powder 
contains a metal which gives a red colour to the ammoniacal preci- 
pitate of platina ; and M. Vauquelin having treated the powder with 
alkali, obtained from it a volatile oxide, which he considered as be- 
longing to the same metal. 
In the spring of the year 1804, Mr. Tennant having completed 
the course of his experiments, communicated the results to the 
Royal Society. He shewed that the powder consisted of two new 
metals, to which he gave the names of Iridium and Osmium, and 
that these might be separated from one another by the alternate 
action of heated alkali and of acid menstrua. By crystallization 
from the acid solution, he obtained a pure salt of iridium, from 
