Geology Reference
In-Depth Information
Holmes spent many tedious hours in the laboratory pains-
takingly separating these from the crushed rocks. He then per-
formed exquisitely delicate chemical preparations to isolate the
required elements for measurement. His own account of the
methods he used gives a good indication of just how laborious
the work was.
First of all the amount of uranium present was measured, not
directly, but by gauging the amount of radium emanation
(radon) being emitted from a piece of radium. Having crushed
up the mineral extracted from the rock, first in an iron mortar
and then more finely in an agate one, the powder was
'fused
with borax in a platinum crucible, and the resultant glass
dissolved in dilute hydrochloric acid. After boiling and stand-
ing for several days in a corked flask, the radium emanation
was boiled out, collected in a gas-holder, and ultimately trans-
ferred to an electroscope'
, which measured the amount of radon
present.
While waiting for the radon to accumulate the lead was meas-
ured by fusing the powder to a cake, whereupon it was broken
up by boiling with water, twice dissolved with hydrochloric acid
and twice evaporated to dryness. On the third addition of
hydrochloric acid a clear solution remained from which lead was
precipitated as a sulphide by heating and adding ammonium
sulphide. The precipitate was collected on a small filter, dried,
ignited, treated with nitric acid, boiled, treated with sulphuric
acid and heated again. Eventually
'A tiny white precipitate then
remained. This was collected on a very small filter . . . washed
with alcohol, dried, ignited, and weighed with the greatest
possible accuracy'.
Often the amount remaining was less than
a tenth of a gram.
These chemical methods of separation required the patience
of a saint, extraordinary dexterity and were incredibly time-
consuming. Not only that, but in order to verify the results, each
mineral analysis was repeated between two and five times,
depending on how much material was available. At one point
