PREPARATION OF TRANSPARENT SECTIONS OF ROCKS, ETC. 139 

I have not dwelt on the methods employed by others. If I 
had to begin again I should not adopt the plan I have done. I 
should economise my time by getting a deal of work done by 
others. But you must bear in mind that when I commenced this 
subject there were no people that could make these sections; such 
a thing was unknown. But I am very pleased to see that the 
method I had the pleasure to propose and first carry out has now 
become so universally adopted, not only in England but on the 
Continent, that there are many men who make a trade of preparing 
these sections. 
Of course there are great advantages if you have the time and 
opportunity to work these things for yourself, because you can 
learn a great deal in preparing them that you otherwise could not 
learn. You give a specimen to the lapidary and he would not 
know the desired thickness ; he would probably rub down every- 
thing to a uniform thickness, and you do not know yourself what 
thickness it should be ; but 1f prepared in the way I have described 
you can learn something about it. You can learn that there are 
certain characters that it is very important to study more fully, and 
you perhaps find that the section has been rubbed down quite thin 
enough to show some important facts; but perhaps another operator 
who did not examine beforehand might think it was not anything 
like thin enough, and would rub it down and lose some of the 
most important characters. 
There are two points to consider : first the structure of some of 
the constituent materials, and secondly the structure of the material 
existing between them. In the case of certain igneous rocks it is 
more important to be able to study the minute structure of some 
of the constituent materials, which you cannot do if they are very 
fine. Some of these contain scattered fluid cavities and enclosed 
crystals. If you leave the sections of sufficient thickness you can 
focus up and down with a proper object glass. You do not want a 
very thin section because the mineral is quite transparent. If you 
were to rub it down very fine you would let out the water from these 
fluid cavities by cutting them open. 
But if you want to examine the minute structure of some of 
the fine grained material that exists between these structures, then 
you must make it very thin ; and I should now be inclined to make 
two sections of each rock, so as to ascertain the constitution of 
each,—the larger and the finer grained material. 
As to the fluid contents of cavities in crystals, I thought it 
desirable to ascertain what the fluid was when the crystal was of 
considerable size. I obtained a crystal of quartz, which I was 
told had belonged to Francis Chantrey. It froze at 32° Fahrenheit. 
Then the next specimen I examined was a quartz from Ceylon, 
with fluid cavities of perhaps 4 of an inch in diameter. I reduced 

