416 Dr. Lamont on Phenomena observed during 
between such a mixture and a true chemical compound. <Ad- 
mitting, then, the possibility of a variation of composition in a 
mineral species, independent of the phenomena of isomorphism, 
it becomes of importance to distinguish this new class of phzeno- 
mena by a separate term; and I would propose for this purpose 
the word Allomerism. By this word I would designate a variation 
in the proportions of the constituents of a crystallized compoundwith- 
out any essential change in the crystalline form. If, then, we also 
use the word ¢ypical to indicate the condition of definite composi- 
tion, we may speak of those specimens ofa mineral species which 
contain an excess of one or the other constituent, as allomeric 
variations from the typical composition. The degree of allomerism 
would then be measured by the excess of the allomeric con- 
stituent above the typical composition.. Thus the crystals of 
Sb Zn® containing 4:2°3 per cent. of zinc would be said to have 
the typical composition ; while those containing 55 per cent. of 
zine would be distinguished as an allomeric variety, the degree 
of allomerism in this instance amounting to 12 per cent., and 
zine being the allomeric constituent. In the case of the 
mineral Discrasite, it is probable that no specimen having the 
typical composition has yet been analysed. Those specimens 
whose analyses are given in Dana’s ‘ System of Mineralogy,’ are 
all probably adlomeric varieties of the mineral, silver bemg the 
allomeric constituent, and thé degree of allomerism varying 
from 4 to 7 per cent. It is unnecessary, however, to multiply 
examples, as the above are sufficient to illustrate the use of the 
term. 
LV. On Phenomena observed during Total Eclipses of the Sun. 
By Dr. Lamont, Astronomer Royal at Munich*. 
[With a Plate.] 
| my yearly report for 1854, I have endeavoured to give a 
novel explanation, and one that differed from all the views 
hitherto entertained, of the violet prominences that are noticed 
in total solar eclipses. According to the explanation there set 
forth only in general terms, the phenomenon is produced by 
little masses of clouds which become condensed in our atmo- 
sphere in the cone of the moon’s shadow, owing to the depres- 
sion of the temperature which takes place there. From the cir- 
cumstance of there occurring in 1860 in Spain a total eclipse of 
the sun which may be observed under very favourable conditions, 
and the opportunity there will then be afforded for arriving at data 
for confirming or refuting my hypothesis, I am induced to return 
* Fyrom Dr. Lamont’s Yearly Report on the Bogenhausen Observatory for 
1858 (Munich, 1859); translated and communicated by W.G. Lettsom, Esq. 
