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atures ; but, from the very marked differences in the properties 
of this substance when in the solid state, it seems probable 
that solid sulphur, like gaseous sulphur, is capable of passing 
from one molecular grouping into another. It may be remarked, 
in passing, that sulphur yields a line spectrum only at very high 
temperatures. Phosphorus and arsenic, again, present us with 
instances of probable changes in molecular weights correlated 
with decided changes in general chemical and physical proper- 
ties. Indeed, the whole series of facts grouped under the term 
Allotropy finds its only rational explanation on the assumption 
that the molecules of the elementary bodies may exist in various 
more or less complex groupings. 
When we turn to compounds we find numerous instances of 
the same kind. Are not the so-called polymeric compounds 
of organic chemistry instances of the great change which is 
brought about in the properties of a compound by doubling or 
tripling the weight of its molecule ? 
An oxide of nitrogen represented by the formula N 2 0 4 
exists ; when this compound is heated its physical properties 
are altered. From a deep-red-coloured gas it is converted into a 
colourless gas ; but the elementary composition of each gas is 
the same, only the molecular weight of the colourless compound 
is represented by the formula N0 2 . In a sense these two 
bodies are the same ; in another and a truer sense they are 
different. Spectroscopically considered, there is a marked dif- 
ference between these compounds. 
If the facts which Mr. Lockyer has amassed concerning 
changes in the spectra of the elements be considered from a 
chemical standpoint, it seems to me that they are in keeping 
with other phenomena of a more purely chemical nature, and 
that both series of phenomena point to the compound nature 
of the elements, in the sense that the molecules of these bodies 
are composed of particles (atoms) exhibiting properties different 
from those of the elements themselves, but at the same time 
that the atoms of any one element all exhibit the same pro- 
perties. 
Whether these elementary atoms are or are not themselves 
capable of undergoing decomposition must remain an open 
question. 
I do not enter into the question of the meaning of those 
short lines which Mr. Lockyer states are to be found occurring 
in the spectra of different elements after eliminating all im- 
purities. Until the methods adopted for eliminating impurities 
are published in detail, and until we are possessed of exact 
information with regard to the effect of very small quantities 
of impurities in modifying the elementary spectra, it seems to 
me preferable to abstain from framing hypotheses which may 
eventually be found not to rest on any sure foundation of fact.. 
