538 
Proceedings of the Royal Irish Academy. 
the presence of the metal, especially as, from the comparatively large 
amount of the substance taken — four or five grains'^' — there was little 
possibility of error. Where, however, the result seemed to be at all 
doubtful, it was confirmed by one or more analyses in the wet way. 
The zinc appeared to be always most plentiful where there was the 
largest quantity of magnesium ; but, as I am at present only concerned 
in proving the existence of the former metal in the above minerals, I 
am not just now in a position to positively assert this, deferring any 
quantitative estimation until I have continued the investigation so far 
as to enable me to select the best typical examples. In the meantime 
these notes on the subject may not be without some interest, bearing as 
they do on a matter of much importance, from a mineralogical as well 
as a chemical standpoint. 
It may be thought that the zinc might as well be considered to re- 
place the other members of the isomorphous group, which are known 
to occur in traces, and occasionally in quantity, in some of these mine- 
rals. Eut this would be a replacement of a replacing element, and I 
believe it is invariably considered that the accessory mineral substitutes 
itself for part of the essential metal. On this ground alone, all the 
minerals here referred to being strictly magnesian, the zinc must be re- 
garded as replacing magnesium. At the same time I do not doubt that 
in cases where there is no magnesium, it may be equally found to re- 
place the other metals. f Eut when we remember the affinities of the 
two metals, it will hardly be thought a far-fetched notion to suppose 
that, were a preference possible, the zinc would associate itself with the 
magnesium, in the same way as some other metals are found, in nature; 
notably those of the platinum group, which, possessing a very peculiar 
relation to one another independently of their isomorphism — which is 
not thorough — are nearly always found together. 
It will be seen that the majority of the minerals examined are spe- 
cies that are most often found in nature forming component parts of 
igneous rocks, while two rocks of that class are themselves included. 
* This was rendered possible, in some cases, by the use of paraffin oil for the blow- 
pipe lamp. By this means a very large and hot flame was procured, capable of decom- 
posing a much larger quantity of rock. For reductions, and when great heat was re- 
quired in blowpipe analyses, it proved very useful, where gas was unattainable, 
t Since this paper was read I have found it replacing Iron in Iron-pyrites. \_Added 
in Press.'] 
